Running jobs on ARCHER2
As with most HPC services, ARCHER2 uses a scheduler to manage access to resources and ensure that the thousands of different users of system are able to share the system and all get access to the resources they require. ARCHER2 uses the Slurm software to schedule jobs.
Writing a submission script is typically the most convenient way to submit your job to the scheduler. Example submission scripts (with explanations) for the most common job types are provided below.
Interactive jobs are also available and can be particularly useful for developing and debugging applications. More details are available below.
Hint
If you have any questions on how to run jobs on ARCHER2 do not hesitate to contact the ARCHER2 Service Desk.
You typically interact with Slurm by issuing Slurm commands from the login nodes (to submit, check and cancel jobs), and by specifying Slurm directives that describe the resources required for your jobs in job submission scripts.
Resources
CUs
Time used on ARCHER2 is measured in CUs. 1 CU = 1 Node Hour for a standard 128 core node.
The CU calculator will help you to calculate the CU cost for your jobs.
Checking available budget
You can check in SAFE by selecting Login accounts from the menu, select the login account you want to query.
Under Login account details you will see each of the budget codes you have access to listed e.g.
e123 resources and then under Resource Pool to the right of this, a note of the remaining budget in CUs.
When logged in to the machine you can also use the command
sacctmgr show assoc where user=$LOGNAME format=account,user,maxtresmins
This will list all the budget codes that you have access to e.g.
Account User MaxTRESMins
---------- ---------- -------------
e123 userx cpu=0
e123-test userx
This shows that userx is a member of budgets e123 and e123-test. However, the cpu=0 indicates that the e123 budget is empty or disabled. This user can submit jobs using the e123-test budget.
To see the number of CUs remaining you must check in SAFE.
Charging
Jobs run on ARCHER2 are charged for the time they use i.e. from the time the job begins to run until the time the job ends (not the full wall time requested).
Jobs are charged for the full number of nodes which are requested, even if they are not all used.
Charging takes place at the time the job ends, and the job is charged in full to the budget which is live at the end time.
Basic Slurm commands
There are four key commands used to interact with the Slurm on the command line:
sinfo- Get information on the partitions and resources availablesbatch jobscript.slurm- Submit a job submission script (in this case called:jobscript.slurm) to the schedulersqueue- Get the current status of jobs submitted to the schedulerscancel 12345- Cancel a job (in this case with the job ID12345)
We cover each of these commands in more detail below.
sinfo: information on resources
sinfo is used to query information about available resources and
partitions. Without any options, sinfo lists the status of all
resources and partitions, e.g.
auser@ln01:~> sinfo
PARTITION AVAIL TIMELIMIT NODES STATE NODELIST
standard up 1-00:00:00 105 down* nid[001006,...,002014]
standard up 1-00:00:00 12 drain nid[001016,...,001969]
standard up 1-00:00:00 5 resv nid[001000,001002-001004,001114]
standard up 1-00:00:00 683 alloc nid[001001,...,001970-001991]
standard up 1-00:00:00 214 idle nid[001022-001023,...,002015-002023]
standard up 1-00:00:00 2 down nid[001021,001050]
Here we see the number of nodes in different states. For example, 683 nodes are allocated (running jobs), and 214 are idle (available to run jobs).
Note
that long lists of node IDs have been abbreviated with ....
sbatch: submitting jobs
sbatch is used to submit a job script to the job submission system.
The script will typically contain one or more srun commands to launch
parallel tasks.
When you submit the job, the scheduler provides the job ID, which is used to identify this job in other Slurm commands and when looking at resource usage in SAFE.
auser@ln01:~> sbatch test-job.slurm
Submitted batch job 12345
squeue: monitoring jobs
squeue without any options or arguments shows the current status of
all jobs known to the scheduler. For example:
auser@ln01:~> squeue
will list all jobs on ARCHER2.
The output of this is often overwhelmingly large. You can restrict the
output to just your jobs by adding the -u $USER option:
auser@ln01:~> squeue -u $USER
scancel: deleting jobs
scancel is used to delete a jobs from the scheduler. If the job is
waiting to run it is simply cancelled, if it is a running job then it is
stopped immediately.
If you only want to cancel a specific job you need to provide the job ID of the job you wish to cancel/stop. For example:
auser@ln01:~> scancel 12345
will cancel (if waiting) or stop (if running) the job with ID 12345.
scancel can take other options. For example, if you want to cancel all
your pending (queued) jobs but leave the running jobs running, you could
use:
auser@ln01:~> scancel --state=PENDING --user=$USER
Resource Limits
The ARCHER2 resource limits for any given job are covered by three separate attributes.
- The amount of primary resource you require, i.e., number of compute nodes.
- The partition that you want to use - this specifies the nodes that are eligible to run your job.
- The Quality of Service (QoS) that you want to use - this specifies the job limits that apply.
Primary resource
The primary resource you can request for your job is the compute node.
Information
The --exclusive option is enforced on ARCHER2 which means you will
always have access to all of the memory on the compute node regardless
of how many processes are actually running on the node.
Note
You will not generally have access to the full amount of memory resource on the the node as some is retained for running the operating system and other system processes.
Partitions
On ARCHER2, compute nodes are grouped into partitions. You will have to
specify a partition using the --partition option in your Slurm
submission script. The following table has a list of active partitions
on ARCHER2.
| Partition | Description | Max nodes available |
|---|---|---|
| standard | CPU nodes with AMD EPYC 7742 64-core processor × 2, 256/512 GB memory | 5860 |
| highmem | CPU nodes with AMD EPYC 7742 64-core processor × 2, 512 GB memory | 584 |
| serial | CPU nodes with AMD EPYC 7742 64-core processor × 2, 512 GB memory | 2 |
| gpu | GPU nodes with AMD EPYC 32-core processor, 512 GB memory, 4×AMD Instinct MI210 GPU | 4 |
Note
The standard partition includes both the standard memory and high memory nodes but standard memory
nodes are preferentially chosen for jobs where possible. To guarantee access to high memory nodes
you should specify the highmem partition.
Quality of Service (QoS)
On ARCHER2, job limits are defined by the requested Quality of Service
(QoS), as specified by the --qos Slurm directive. The following table
lists the active QoS on ARCHER2.
| QoS | Max Nodes Per Job | Max Walltime | Jobs Queued | Jobs Running | Partition(s) | Notes |
|---|---|---|---|---|---|---|
| standard | 1024 | 24 hrs | 64 | 16 | standard | Maximum of 1024 nodes in use by any one user at any time |
| highmem | 256 | 24 hrs | 16 | 16 | highmem | Maximum of 512 nodes in use by any one user at any time |
| taskfarm | 16 | 24 hrs | 128 | 32 | standard | Maximum of 256 nodes in use by any one user at any time |
| short | 32 | 20 mins | 16 | 4 | standard | |
| long | 64 | 96 hrs | 16 | 16 | standard | Minimum walltime of 24 hrs, maximum 512 nodes in use by any one user at any time, maximum of 2048 nodes in use by QoS |
| largescale | 5860 | 12 hrs | 8 | 1 | standard | Minimum job size of 1025 nodes |
| lowpriority | 2048 | 24 hrs | 16 | 16 | standard | Jobs not charged but requires at least 1 CU in budget to use. |
| serial | 32 cores and/or 128 GB memory | 24 hrs | 32 | 4 | serial | Jobs not charged but requires at least 1 CU in budget to use. Maximum of 32 cores and/or 128 GB in use by any one user at any time. |
| reservation | Size of reservation | Length of reservation | No limit | no limit | standard | |
| capabilityday | At least 4096 nodes | 3 hrs | 8 | 2 | standard | Minimum job size of 512 nodes. Jobs only run during Capability Days |
| gpu-shd | 1 | 12 hrs | 2 | 1 | gpu | GPU nodes potentially shared with other users |
| gpu-exc | 2 | 12 hrs | 2 | 1 | gpu | GPU node exclusive node access |
You can find out the QoS that you can use by running the following command:
auser@ln01:~> sacctmgr show assoc user=$USER cluster=archer2 format=cluster,account,user,qos%50
Hint
If you have needs which do not fit within the current QoS, please contact the Service Desk and we can discuss how to accommodate your requirements.
E-mail notifications
E-mail notifications from the scheduler are not currently available on ARCHER2.
Priority
Job priority on ARCHER2 depends on a number of different factors:
- The QoS your job has specified
- The amount of time you have been queuing for
- The number of nodes you have requested (job size)
- Your current fairshare factor
Each of these factors is normalised to a value between 0 and 1, is multiplied with a weight and the resulting values combined to produce a priority for the job. The current job priority formula on ARCHER2 is:
Priority = [10000 * P(QoS)] + [500 * P(Age)] + [300 * P(Fairshare)] + [100 * P(size)]
The priority factors are:
- P(QoS) - The QoS priority normalised to a value between 0 and 1. The maximum raw
value is 10000 and the minimum is 0. Most QoS on ARCHER2 have a raw prioity of 500; the
lowpriorityQoS has a raw priority of 1. - P(Age) - The priority based on the job age normalised to a value between 0 and 1. The maximum raw value is 14 days (where P(Age) = 1).
- P(Fairshare) - The fairshare priority normalised to a value between 0 and 1. Your fairshare priority is determined by a combination of your budget code fairshare value and your user fairshare value within that budget code. The more use that the budget code you are using has made of the system recently relative to other budget codes on the system, the lower the budget code fairshare value will be; and the more use you have made of the system recently relative to other users within your budget code, the lower your user fairshare value will be. The decay half life for fairshare on ARCHER2 is set to 2 days. More information on the Slurm fairshare algorithm.
- P(Size) - The priority based on the job size normalised to a value between 0 and 1. The maximum size is the total number of ARCHER2 compute nodes.
You can view the priorities for current queued jobs on the system with the sprio
command:
auser@ln04:~> sprio -l
JOBID PARTITION PRIORITY SITE AGE FAIRSHARE JOBSIZE QOS
828764 standard 1049 0 45 0 4 1000
828765 standard 1049 0 45 0 4 1000
828770 standard 1049 0 45 0 4 1000
828771 standard 1012 0 8 0 4 1000
828773 standard 1012 0 8 0 4 1000
828791 standard 1012 0 8 0 4 1000
828797 standard 1118 0 115 0 4 1000
828800 standard 1154 0 150 0 4 1000
828801 standard 1154 0 150 0 4 1000
828805 standard 1118 0 115 0 4 1000
828806 standard 1154 0 150 0 4 1000
Troubleshooting
Slurm error messages
An incorrect submission will cause Slurm to return an error. Some common problems are listed below, with a suggestion about the likely cause:
-
sbatch: unrecognized option <text>One of your options is invalid or has a typo.
man sbatchto help. -
error: Batch job submission failed: No partition specified or system default partitionA
--partition=option is missing. You must specify the partition (see the list above). This is most often--partition=standard. -
error: invalid partition specified: <partition>error: Batch job submission failed: Invalid partition name specifiedCheck the partition exists and check the spelling is correct.
-
error: Batch job submission failed: Invalid account or account/partition combination specifiedThis probably means an invalid account has been given. Check the
--account=options against valid accounts in SAFE. -
error: Batch job submission failed: Invalid qos specificationA QoS option is either missing or invalid. Check the script has a
--qos=option and that the option is a valid one from the table above. (Check the spelling of the QoS is correct.) -
error: Your job has no time specification (--time=)...Add an option of the form
--time=hours:minutes:secondsto the submission script. E.g.,--time=01:30:00gives a time limit of 90 minutes. -
error: QOSMaxWallDurationPerJobLimiterror: Batch job submission failed: Job violates accounting/QOS policy(job submit limit, user's size and/or time limits)The script has probably specified a time limit which is too long for the corresponding QoS. E.g., the time limit for the short QoS is 20 minutes.
Slurm job state codes
The squeue command allows users to view information for jobs managed by Slurm. Jobs
typically go through the following states: PENDING, RUNNING, COMPLETING, and COMPLETED.
The first table provides a description of some job state codes. The second table provides a description
of the reasons that cause a job to be in a state.
| Status | Code | Description |
|---|---|---|
| PENDING | PD | Job is awaiting resource allocation. |
| RUNNING | R | Job currently has an allocation. |
| SUSPENDED | S | Job currently has an allocation. |
| COMPLETING | CG | Job is in the process of completing. Some processes on some nodes may still be active. |
| COMPLETED | CD | Job has terminated all processes on all nodes with an exit code of zero. |
| TIMEOUT | TO | Job terminated upon reaching its time limit. |
| STOPPED | ST | Job has an allocation, but execution has been stopped with SIGSTOP signal. CPUS have been retained by this job. |
| OUT_OF_MEMORY | OOM | Job experienced out of memory error. |
| FAILED | F | Job terminated with non-zero exit code or other failure condition. |
| NODE_FAIL | NF | Job terminated due to failure of one or more allocated nodes. |
| CANCELLED | CA | Job was explicitly cancelled by the user or system administrator. The job may or may not have been initiated. |
For a full list of see Job State Codes.
Slurm queued reasons
| Reason | Description |
|---|---|
| Priority | One or more higher priority jobs exist for this partition or advanced reservation. |
| Resources | The job is waiting for resources to become available. |
| BadConstraints | The job's constraints can not be satisfied. |
| BeginTime | The job's earliest start time has not yet been reached. |
| Dependency | This job is waiting for a dependent job to complete. |
| Licenses | The job is waiting for a license. |
| WaitingForScheduling | No reason has been set for this job yet. Waiting for the scheduler to determine the appropriate reason. |
| Prolog | Its PrologSlurmctld program is still running. |
| JobHeldAdmin | The job is held by a system administrator. |
| JobHeldUser | The job is held by the user. |
| JobLaunchFailure | The job could not be launched. This may be due to a file system problem, invalid program name, etc. |
| NonZeroExitCode | The job terminated with a non-zero exit code. |
| InvalidAccount | The job's account is invalid. |
| InvalidQOS | The job's QOS is invalid. |
| QOSUsageThreshold | Required QOS threshold has been breached. |
| QOSJobLimit | The job's QOS has reached its maximum job count. |
| QOSResourceLimit | The job's QOS has reached some resource limit. |
| QOSTimeLimit | The job's QOS has reached its time limit. |
| NodeDown | A node required by the job is down. |
| TimeLimit | The job exhausted its time limit. |
| ReqNodeNotAvail | Some node specifically required by the job is not currently available. The node may currently be in use, reserved for another job, in an advanced reservation, DOWN, DRAINED, or not responding. Nodes which are DOWN, DRAINED, or not responding will be identified as part of the job's "reason" field as "UnavailableNodes". Such nodes will typically require the intervention of a system administrator to make available. |
For a full list of see Job Reasons.
Output from Slurm jobs
Slurm places standard output (STDOUT) and standard error (STDERR) for
each job in the file slurm_<JobID>.out. This file appears in the job's
working directory once your job starts running.
Hint
Output may be buffered - to enable live output, e.g. for monitoring
job status, add --unbuffered to the srun command in your Slurm
script.
Specifying resources in job scripts
You specify the resources you require for your job using directives at
the top of your job submission script using lines that start with the
directive #SBATCH.
Hint
Most options provided using #SBATCH directives can also be specified as
command line options to srun.
If you do not specify any options, then the default for each option will be applied. As a minimum, all job submissions must specify the budget that they wish to charge the job too with the option:
--account=<budgetID>your budget ID is usually something liket01ort01-test. You can see which budget codes you can charge to in SAFE.
Important
You must specify an account code for your job otherwise it will
fail to submit with the error: sbatch: error: Batch job submission failed: Invalid account or account/partition combination specified.
(This error can also mean that you have specified a budget that
has run out of resources.)
Other common options that are used are:
--time=<hh:mm:ss>the maximum walltime for your job. e.g. For a 6.5 hour walltime, you would use--time=6:30:0.--job-name=<jobname>set a name for the job to help identify it in Slurm
To prevent the behaviour of batch scripts being dependent on the user environment at the point of submission, the option
--export=noneprevents the user environment from being exported to the batch system.
Using the --export=none means that the behaviour of batch submissions
should be repeatable. We strongly recommend its use.
Note
When submitting your job, the scheduler will check that the requested resources are available e.g. that your account
is a member of the requested budget, that the requested QoS exists.
If things change before the job starts and e.g. your account has been removed from the requested budget or the requested QoS has been deleted
then the job will not be able to start.
In such cases, the job will be removed from the pending queue by our systems team, as it will no longer be eligible to run.
Additional options for parallel jobs
Note
For parallel jobs, ARCHER2 operates in a node exclusive way. This means that you are assigned resources in the units of full compute nodes for your jobs (i.e. 128 cores) and that no other user can share those compute nodes with you. Hence, the minimum amount of resource you can request for a parallel job is 1 node (or 128 cores).
In addition, parallel jobs will also need to specify how many nodes, parallel processes and threads they require.
--nodes=<nodes>the number of nodes to use for the job.--ntasks-per-node=<processes per node>the number of parallel processes (e.g. MPI ranks) per node.--cpus-per-task=1if you are using parallel processes only with no threading and you want to use all 128 cores on the node then you should set the number of CPUs (cores) per parallel process to 1. Important: if you are using threading (e.g. with OpenMP) or you want to use less than 128 cores per node (e.g. to access more memory or memory bandwidth per core) then you will need to change this option as described below.--cpu-freq=<freq. in kHz>set the CPU frequency for the compute nodes. Valid values are2250000(2.25 GHz),2000000(2.0 GHz),1500000(1.5 GHz). For more information on CPU frequency settings and energy use see the Energy use section.
For parallel jobs that use threading (e.g. OpenMP) or when you want to
use less than 128 cores per node (e.g. to access more memory or memory
bandwidth per core), you will also need to change the --cpus-per-task
option.
For jobs using threading:
- --cpus-per-task=<threads per task> the number of threads per
parallel process (e.g. number of OpenMP threads per MPI task for
hybrid MPI/OpenMP jobs). Important: you must also set the
OMP_NUM_THREADS environment variable if using OpenMP in your
job.
For jobs using less than 128 cores per node:
- --cpus-per-task=<stride between placement of processes> the stride
between the parallel processes. For example, if you want to double the
memory and memory bandwidth per process on an ARCHER2 compute node you
would want to place 64 processes per node and leave an empty core between
each process you would set --cpus-per-task=2 and --ntasks-per-node=64.
Important
You must also add export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK to
your job submission script to pass the --cpus-per-task setting from
the job script to the srun command. (Alternatively, you could use
the --cpus-per-task option in the srun command itself.) If you do not do
this then the placement of processes/threads will be incorrect and you
will likely see poor performance of your application.
Options for jobs on the data analysis nodes
The data analysis nodes are shared between all users and can be used to run jobs that require small numbers of cores and/or access to an external network to transfer data. These jobs are often serial jobs that only require a single core.
To run jobs on the data analysis node you require the following options:
--partition=serialto select the data analysis nodes--qos=serialto select the data analysis QoS (see above for QoS limits)--ntasks=<number of cores>to select the number of cores you want to use in this job (up to the maximum defined in the QoS)--mem=<amount of memory>to select the amount of memory you require (up to the maximum defined in the QoS).
More information on using the data analysis nodes (including example job submission scripts) can be found in the Data Analysis section of the User and Best Practice Guide.
srun: Launching parallel jobs
If you are running parallel jobs, your job submission script should
contain one or more srun commands to launch the parallel executable
across the compute nodes. In most cases you will want to add the options
--distribution=block:block and --hint=nomultithread to your
srun command to ensure you get the correct pinning of processes to
cores on a compute node.
Warning
If you do not add the --distribution=block:block and --hint=nomultithread
options to your srun command the default process placement
may lead to a drop in performance for your jobs on ARCHER2.
A brief explanation of these options:
- --hint=nomultithread - do not use hyperthreads/SMP
- --distribution=block:block - the first block means use a block distribution
of processes across nodes (i.e. fill nodes before moving onto the next one) and
the second block means use a block distribution of processes across "sockets"
within a node (i.e. fill a "socket" before moving on to the next one).
Important
The Slurm definition of a "socket" does not correspond to a physical CPU socket. On ARCHER2 it corresponds to a 4-core CCX (Core CompleX).
Slurm definition of a "socket"
On ARCHER2, Slurm is configured with the following setting:
SlurmdParameters=l3cache_as_socket
The effect of this setting is to define a Slurm socket as a unit that has a shared L3 cache. On ARCHER2, this means that each Slurm "socket" corresponds to a 4-core CCX (Core CompleX). For a more detailed discussion on the hardware and the memory/cache layout see the Hardware section.
The effect of this setting can be illustrated by using the xthi program to report
placement when we select a cyclic distribution of processes across sockets from srun
(--distribution=block:cyclic). As you can see from the output from xthi included
below, the cyclic per-socket distribution results in sequential MPI processes being
placed on every 4th core (i.e. cyclic placement across CCX).
Node summary for 1 nodes:
Node 0, hostname nid000006, mpi 128, omp 1, executable xthi_mpi
MPI summary: 128 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 4)
Node 0, rank 2, thread 0, (affinity = 8)
Node 0, rank 3, thread 0, (affinity = 12)
Node 0, rank 4, thread 0, (affinity = 16)
Node 0, rank 5, thread 0, (affinity = 20)
Node 0, rank 6, thread 0, (affinity = 24)
Node 0, rank 7, thread 0, (affinity = 28)
Node 0, rank 8, thread 0, (affinity = 32)
Node 0, rank 9, thread 0, (affinity = 36)
Node 0, rank 10, thread 0, (affinity = 40)
Node 0, rank 11, thread 0, (affinity = 44)
Node 0, rank 12, thread 0, (affinity = 48)
Node 0, rank 13, thread 0, (affinity = 52)
Node 0, rank 14, thread 0, (affinity = 56)
Node 0, rank 15, thread 0, (affinity = 60)
Node 0, rank 16, thread 0, (affinity = 64)
Node 0, rank 17, thread 0, (affinity = 68)
Node 0, rank 18, thread 0, (affinity = 72)
Node 0, rank 19, thread 0, (affinity = 76)
Node 0, rank 20, thread 0, (affinity = 80)
Node 0, rank 21, thread 0, (affinity = 84)
Node 0, rank 22, thread 0, (affinity = 88)
Node 0, rank 23, thread 0, (affinity = 92)
Node 0, rank 24, thread 0, (affinity = 96)
Node 0, rank 25, thread 0, (affinity = 100)
Node 0, rank 26, thread 0, (affinity = 104)
Node 0, rank 27, thread 0, (affinity = 108)
Node 0, rank 28, thread 0, (affinity = 112)
Node 0, rank 29, thread 0, (affinity = 116)
Node 0, rank 30, thread 0, (affinity = 120)
Node 0, rank 31, thread 0, (affinity = 124)
Node 0, rank 32, thread 0, (affinity = 1)
Node 0, rank 33, thread 0, (affinity = 5)
Node 0, rank 34, thread 0, (affinity = 9)
Node 0, rank 35, thread 0, (affinity = 13)
Node 0, rank 36, thread 0, (affinity = 17)
Node 0, rank 37, thread 0, (affinity = 21)
Node 0, rank 38, thread 0, (affinity = 25)
...output trimmed...
bolt: Job submission script creation tool
The bolt job submission script creation tool has been written by EPCC to simplify the process of writing job submission scripts for modern multicore architectures. Based on the options you supply, bolt will generate a job submission script that uses ARCHER2 in a reasonable way.
MPI, OpenMP and hybrid MPI/OpenMP jobs are supported.
Warning
The tool will allow you to generate scripts for jobs that use the long
QoS but you will need to manually modify the resulting script to change
the QoS to long.
If there are problems or errors in your job parameter specifications then bolt will print warnings or errors. However, bolt cannot detect all problems.
Basic Usage
The basic syntax for using bolt is:
bolt -n [parallel tasks] -N [parallel tasks per node] -d [number of threads per task] \
-t [wallclock time (h:m:s)] -o [script name] -j [job name] -A [project code] [arguments...]
Example 1: to generate a job script to run an executable called
my_prog.x for 24 hours using 8192 parallel (MPI) processes and 128
(MPI) processes per compute node you would use something like:
bolt -n 8192 -N 128 -t 24:0:0 -o my_job.bolt -j my_job -A z01-budget my_prog.x arg1 arg2
(remember to substitute z01-budget for your actual budget code.)
Example 2: to generate a job script to run an executable called
my_prog.x for 3 hours using 2048 parallel (MPI) processes and 64 (MPI)
processes per compute node (i.e. using half of the cores on a compute
node), you would use:
bolt -n 2048 -N 64 -t 3:0:0 -o my_job.bolt -j my_job -A z01-budget my_prog.x arg1 arg2
These examples generate the job script my_job.bolt with the correct
options to run my_prog.x with command line arguments arg1 and
arg2. The project code against which the job will be charged is
specified with the ' -A ' option. As usual, the job script is submitted
as follows:
sbatch my_job.bolt
Hint
If you do not specify the script name with the '-o' option then your
script will be a file called a.bolt.
Hint
If you do not specify the number of parallel tasks then bolt will try to generate a serial job submission script (and throw an error on the ARCHER2 4 cabinet system as serial jobs are not supported).
Hint
If you do not specify a project code, bolt will use your default project code (set by your login account).
Hint
If you do not specify a job name, bolt will use either bolt_ser_job
(for serial jobs) or bolt_par_job (for parallel jobs).
Further help
You can access further help on using bolt on ARCHER2 with the ' -h ' option:
bolt -h
A selection of other useful options are:
-sWrite and submit the job script rather than just writing the job script.-pForce the job to be parallel even if it only uses a single parallel task.
checkScript job submission script validation tool
The checkScript tool has been written to allow users to validate their job submission scripts before submitting their jobs. The tool will read your job submission script and try to identify errors, problems or inconsistencies.
An example of the sort of output the tool can give would be:
auser@ln01:/work/t01/t01/auser> checkScript submit.slurm
===========================================================================
checkScript
---------------------------------------------------------------------------
Copyright 2011-2020 EPCC, The University of Edinburgh
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions.
===========================================================================
Script details
---------------
User: auser
Script file: submit.slurm
Directory: /work/t01/t01/auser (ok)
Job name: test (ok)
Partition: standard (ok)
QoS: standard (ok)
Combination: (ok)
Requested resources
-------------------
nodes = 3 (ok)
tasks per node = 16
cpus per task = 8
cores per node = 128 (ok)
OpenMP defined = True (ok)
walltime = 1:0:0 (ok)
CU Usage Estimate (if full job time used)
------------------------------------------
CU = 3.000
checkScript finished: 0 warning(s) and 0 error(s).
Checking scripts and estimating start time with --test-only
sbatch --test-only validates the batch script and returns an estimate of when the job would be scheduled to run given the current scheduler state. Please note that it is just an estimate, the actual start time may differ as the scheduler status when the start time was estimated may be different once the job is actually submitted and due to subsequent changes to the scheduler state. The job is not actually submitted.
auser@ln01:~> sbatch --test-only submit.slurm
sbatch: Job 1039497 to start at 2022-02-01T23:20:51 using 256 processors on nodes nid002836
in partition standard
Estimated start time for queued jobs
You can use the squeue command to show the current estimated start time for a job.
Please note that it is just an estimate, the actual start time may differ as the scheduler
status when the start time was estimated may be different due to subsequent changes to
the scheduler state. To return the estimated start time for a job you specify the
job ID with the --jobs=<jobid> and --Format=StartTime options.
For example, to show the estimated start time for job 123456, you would use:
squeue --jobs=123456 --Format=StartTime
The output from this command would look like:
START_TIME
2024-09-25T13:07:00
Example job submission scripts
A subset of example job submission scripts are included in full below. Examples are provided for both the full system and the 4-cabinet system.
Example: job submission script for MPI parallel job
A simple MPI job submission script to submit a job using 4 compute nodes and 128 MPI ranks per node for 20 minutes would look like:
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=Example_MPI_Job
#SBATCH --time=0:20:0
#SBATCH --nodes=4
#SBATCH --ntasks-per-node=128
#SBATCH --cpus-per-task=1
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Set the number of threads to 1
# This prevents any threaded system libraries from automatically
# using threading.
export OMP_NUM_THREADS=1
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Launch the parallel job
# Using 512 MPI processes and 128 MPI processes per node
# srun picks up the distribution from the sbatch options
srun --distribution=block:block --hint=nomultithread ./my_mpi_executable.x
This will run your executable "my_mpi_executable.x" in parallel on 512 MPI processes using 4 nodes (128 cores per node, i.e. not using hyper-threading). Slurm will allocate 4 nodes to your job and srun will place 128 MPI processes on each node (one per physical core).
See above for a more detailed discussion of the different sbatch
options
Example: job submission script for MPI+OpenMP (mixed mode) parallel job
Mixed mode codes that use both MPI (or another distributed memory parallel model) and OpenMP should take care to ensure that the shared memory portion of the process/thread placement does not span more than one NUMA region. Nodes on ARCHER2 are made up of two sockets each containing 4 NUMA regions of 16 cores, i.e. there are 8 NUMA regions in total. Therefore the total number of threads should ideally not be greater than 16, and also needs to be a factor of 16. Sensible choices for the number of threads are therefore 1 (single-threaded), 2, 4, 8, and 16. More information about using OpenMP and MPI+OpenMP can be found in the Tuning chapter.
To ensure correct placement of MPI processes the number of cpus-per-task needs to match the number of OpenMP threads, and the number of tasks-per-node should be set to ensure the entire node is filled with MPI tasks.
In the example below, we are using 4 nodes for 6 hours. There are 32 MPI processes in total (8 MPI processes per node) and 16 OpenMP threads per MPI process. This results in all 128 physical cores per node being used.
Hint
Note the use of the export OMP_PLACES=cores environment option to
generate the correct thread pinning.
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=Example_MPI_Job
#SBATCH --time=0:20:0
#SBATCH --nodes=4
#SBATCH --ntasks-per-node=8
#SBATCH --cpus-per-task=16
# Replace [budget code] below with your project code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Set the number of threads to 16 and specify placement
# There are 16 OpenMP threads per MPI process
# We want one thread per physical core
export OMP_NUM_THREADS=16
export OMP_PLACES=cores
# Launch the parallel job
# Using 32 MPI processes
# 8 MPI processes per node
# 16 OpenMP threads per MPI process
# Additional srun options to pin one thread per physical core
srun --hint=nomultithread --distribution=block:block ./my_mixed_executable.x arg1 arg2
Job arrays
The Slurm job scheduling system offers the job array concept, for running collections of almost-identical jobs. For example, running the same program several times with different arguments or input data.
Each job in a job array is called a subjob. The subjobs of a job array can be submitted and queried as a unit, making it easier and cleaner to handle the full set, compared to individual jobs.
All subjobs in a job array are started by running the same job script. The job script also contains information on the number of jobs to be started, and Slurm provides a subjob index which can be passed to the individual subjobs or used to select the input data per subjob.
Job script for a job array
As an example, the following script runs 56 subjobs, with the subjob index as the only argument to the executable. Each subjob requests a single node and uses all 128 cores on the node by placing 1 MPI process per core and specifies 4 hours maximum runtime per subjob:
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=Example_Array_Job
#SBATCH --time=04:00:00
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=128
#SBATCH --cpus-per-task=1
#SBATCH --array=0-55
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Set the number of threads to 1
# This prevents any threaded system libraries from automatically
# using threading.
export OMP_NUM_THREADS=1
srun --distribution=block:block --hint=nomultithread /path/to/exe $SLURM_ARRAY_TASK_ID
Submitting a job array
Job arrays are submitted using sbatch in the same way as for standard
jobs:
sbatch job_script.pbs
Expressing dependencies between jobs
SLURM allows one to express dependencies between jobs using the --dependency
(or -d) option. This allows the start of execution of the dependent job
to be delayed until some condition involving a current or previous job, or
set of jobs, has been satisfied. A simple example might be:
$ sbatch --dependency=4394150 myscript.sh
Submitted batch job 4394325
(Dependency) assuming 4394150 is still running.
A dependency may be of a different type, of which there are a number
of relevant possibilities. If we explicitly include the default type
afterany in the example above, we would have
$ sbatch --dependency=afterany:4394150 myscript.sh
Submitted batch job 4394325
afterok:
$ sbatch --dependency=afterok:4394150 myscript.sh
Submitted batch job 4394325
squeue as (DependencyNeverSatisfied) as the reason.
Such jobs will need to be cancelled.
The general form of the dependency list is <type:job_id[:job_id] [,type:job_id ...]> where a dependency may include one or more jobs, with one or more types.
If a list is comma-separated, all the dependencies must be satisfied before the
dependent job becomes eligible. The use of ? as the list separator implies
that any of the dependencies is sufficient.
Useful type options include afterany, afterok, and afternotok. For the
last case, the dependency is only satisfied if there is non-zero exit code
(the opposite of afterok). See the current SLURM documentation for a full
list of possibilities.
Chains of jobs
Fixed number of jobs
Job dependencies can be used to construct complex pipelines or chain together long simulations requiring multiple steps.
For example, if we have just two jobs, the following shell script extract will submit the second dependent on the first, irrespective of actual job ID:
jobid=$(sbatch --parsable first_job.sh)
sbatch --dependency=afterok:${jobid} second_job.sh
--parsable option to sbatch to return
just the new job ID (without the Submitted batch job).
This can be extended to a longer chain as required. E.g.:
jobid1=$(sbatch --parsable first_job.sh)
jobid2=$(sbatch --parsable --dependency=afterok:${jobid1} second_job.sh)
jobid3=$(sbatch --parsable --dependency=afterok:${jobid1} third_job.sh)
sbatch --dependency=afterok:${jobid2},afterok:${jobid3} last_job.sh
Number of jobs not known in advance
This automation may be taken a step further to a case where a submission script propagates itself. E.g., a script might include, schematically,
#SBATCH ...
# submit new job here ...
sbatch --dependency=afterok:${SLURM_JOB_ID} thisscript.sh
# perform work here...
srun ...
SLURM_JOB_ID which holds the id of the
current job. One could defer the sbatch until the end of the script to
avoid the dependency never being satisfied if the work associated with the
srun fails. This approach can be useful in situations
where, e.g., simulations with checkpoint/restart need to continue until
some criterion is met. Some care may be required to ensure the script
logic is correct in determining the criterion for stopping: it is
best to start with a small/short test example.
Incorrect logic and/or errors may lead to a rapid proliferation of
submitted jobs.
Termination of such chains needs to be arranged either via appropriate logic in the script, or manual intervention to cancel pending jobs when no longer required.
Using multiple srun commands in a single job script
You can use multiple srun commands within in a Slurm job submission script
to allow you to use the resource requested more flexibly. For example, you
could run a collection of smaller jobs within the requested resources or
you could even subdivide nodes if your individual calculations do not scale
up to use all 128 cores on a node.
In this guide we will cover two scenarios:
- Subdividing the job into multiple full-node or multi-node subjobs, e.g. requesting 100 nodes and running 100, 1-node subjobs or 50, 2-node subjobs.
- Subdividing the job into multiple subjobs that each use a fraction of a node, e.g. requesting 2 nodes and running 256, 1-core subjobs or 16, 16-core subjobs.
Running multiple, full-node subjobs within a larger job
When subdivding a larger job into smaller subjobs you typically need to
overwrite the --nodes option to srun and add the --ntasks option
to ensure that each subjob runs on the correct number of nodes and that
subjobs are placed correctly onto separate nodes.
For example, we will show how to request 100 nodes and then run 100
separate 1-node jobs, each of which use 128 MPI processes and which
run on a different compute node. We start by showing
the job script that would achieve this and then explain how this works
and the options used. In our case, we will run 100 copies of the xthi
program that prints the process placement on the node it is running on.
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=multi_xthi
#SBATCH --time=0:20:0
#SBATCH --nodes=100
#SBATCH --ntasks-per-node=128
#SBATCH --cpus-per-task=1
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Load the xthi module
module load xthi
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Set the number of threads to 1
# This prevents any threaded system libraries from automatically
# using threading.
export OMP_NUM_THREADS=1
# Loop over 100 subjobs starting each of them on a separate node
for i in $(seq 1 100)
do
# Launch this subjob on 1 node, note nodes and ntasks options and & to place subjob in the background
srun --nodes=1 --ntasks=128 --distribution=block:block --hint=nomultithread xthi > placement${i}.txt &
done
# Wait for all background subjobs to finish
wait
Key points from the example job script:
- The
#SBATCHoptions select 100 full nodes in the usual way. - Each subjob
sruncommand sets the following:--nodes=1We need override this setting from the main job so that each subjob only uses 1 node--ntasks=128For normal jobs, the number of parallel tasks (MPI processes) is calculated from the number of nodes you request and the number of tasks per node. We need to explicitly tellsrunhow many we require for this subjob.--distribution=block:block --hint=nomultithreadThese options ensure correct placement of processes within the compute nodes.&Each subjobsruncommand ends with an ampersand to place the process in the background and move on to the next loop iteration (and subjob submission). Without this, the script would wait for this subjob to complete before moving on to submit the next.
- Finally, there is the
waitcommand to tell the script to wait for all the background subjobs to complete before exiting. If we did not have this in place, the script would exit as soon as the last subjob was submitted and kill all running subjobs.
Running multiple subjobs that each use a fraction of a node
As the ARCHER2 nodes contain a large number of cores (128 per node) it
may sometimes be useful to be able to run multiple executables on a single
node. For example, you may want to run 128 copies of a serial executable or
Python script; or, you may want to run multiple copies of parallel executables
that use fewer than 128 cores each. This use model is possible using
multiple srun commands in a job script on ARCHER2
Note
You can never share a compute node with another user. Although you can
use srun to place multiple copies of an executable or script on a
compute node, you still have exclusive use of that node. The minimum
amount of resources you can reserve for your use on ARCHER2 is a single
node.
When using srun to place multiple executables or scripts on a compute
node you must be aware of a few things:
- The
sruncommand must specify any Slurm options that differ in value from those specified tosbatch. This typically means that you need to specify the--nodes,--ntasksand--ntasks-per-nodeoptions tosrun. - You will need to include the
--exactflag to yoursruncommand. With this flag on, Slurm will ensure that the resources you request are assigned to your subjob. Furthermore, if the resources are not currently available, Slurm will output a message letting you know that this is the case and stall the launch of this subjob until enough of your previous subjobs have completed to free up the resources for this subjob. - You will need to define the memory required by each subjob with the
--mem=<amount of memory>flag. The amount of memory is given in MiB by default but other units can be specified. If you do not know how much memory to specify, we recommend that you specify 1500M (1,500 MiB) per core being used. - You will need to place each
sruncommand into the background and then use thewaitcommand at the end of the submission script to make sure it does not exit before the commands are complete. - If you want to use more than one node in the job and use multiple
srunper node (e.g. 256 single core processes across 2 nodes) then you need to pass the node ID to thesruncommands otherwise Slurm will oversubscribe cores on the first node.
Below, we provide four examples or running multiple subjobs in a node: one that runs 128 serial processes across a single node; one that runs 8 subjobs each of which use 8 MPI processes with 2 OpenMP threads per MPI process; one that runs four inhomogeneous jobs, each of which requires a different number of MPI processes and OpenMP threads per process; and one that runs 256 serial processes across two nodes.
Example 1: 128 serial tasks running on a single node
For our first example, we will run 128 single-core copies of the xthi program (which
prints process/thread placement) on a single ARCHER2 compute node with each
copy of xthi pinned to a different core. The job submission script for
this example would look like:
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=MultiSerialOnCompute
#SBATCH --time=0:10:0
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=128
#SBATCH --cpus-per-task=1
#SBATCH --hint=nomultithread
#SBATCH --distribution=block:block
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Make xthi available
module load xthi
# Set the number of threads to 1
# This prevents any threaded system libraries from automatically
# using threading.
export OMP_NUM_THREADS=1
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Loop over 128 subjobs pinning each to a different core
for i in $(seq 1 128)
do
# Launch subjob overriding job settings as required and in the background
# Make sure to change the amount specified by the `--mem=` flag to the amount
# of memory required. The amount of memory is given in MiB by default but other
# units can be specified. If you do not know how much memory to specify, we
# recommend that you specify `--mem=1500M` (1,500 MiB).
srun --nodes=1 --ntasks=1 --ntasks-per-node=1 \
--exact --mem=1500M xthi > placement${i}.txt &
done
# Wait for all subjobs to finish
wait
Example 2: 8 subjobs on 1 node each with 8 MPI processes and 2 OpenMP threads per process
For our second example, we will run 8 subjobs, each running the xthi program (which
prints process/thread placement) across 1 node. Each subjob will use 8 MPI processes
and 2 OpenMP threads per process. The job submission script for
this example would look like:
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=MultiParallelOnCompute
#SBATCH --time=0:10:0
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=64
#SBATCH --cpus-per-task=2
#SBATCH --hint=nomultithread
#SBATCH --distribution=block:block
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Make xthi available
module load xthi
# Set the number of threads to 2 as required by all subjobs
export OMP_NUM_THREADS=2
# Loop over 8 subjobs
for i in $(seq 1 8)
do
echo $j $i
# Launch subjob overriding job settings as required and in the background
# Make sure to change the amount specified by the `--mem=` flag to the amount
# of memory required. The amount of memory is given in MiB by default but other
# units can be specified. If you do not know how much memory to specify, we
# recommend that you specify `--mem=12500M` (12,500 MiB).
srun --nodes=1 --ntasks=8 --ntasks-per-node=8 --cpus-per-task=2 \
--exact --mem=12500M xthi > placement${i}.txt &
done
# Wait for all subjobs to finish
wait
Example 3: Running inhomogeneous subjobs on one node
For our third example, we will run 4 subjobs, each running the xthi program
(which prints process/thread placement) across 1 node. Our subjobs will each
run with a different number of MPI processes and OpenMP threads. We will run:
one job with 64 MPI processes and 1 OpenMP process per thread; one job with
16 MPI processes and 2 threads per process; one job with 4 MPI processes and
4 OpenMP threads per job; and, one job with 1 MPI process and 16 OpenMP
threads per job.
To be able to change the number of MPI processes and OpenMP threads per
process, we will need to forgo using the #SBATCH --ntasks-per-node and the
#SBATCH cpus-per-task commands -- if you set these Slurm will not let you
alter the OMP_NUM_THREADS variable and you will not be able to change the
number of OpenMP threads per process between each job.
Before each srun command, you will need to define the number of OpenMP
threads per process you want by changing the OMP_NUM_THREADS variable.
Furthermore, for each srun command, you will need to set the --ntasks flag
to equal the number of MPI processes you want to use. You will also need to
set the --cpus-per-task flag to equal the number of OpenMP threads per
process you want to use.
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=MultiParallelOnCompute
#SBATCH --time=0:10:0
#SBATCH --nodes=1
#SBATCH --hint=nomultithread
#SBATCH --distribution=block:block
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Make xthi available
module load xthi
# Set the number of threads to value required by the first job
export OMP_NUM_THREADS=1
srun --ntasks=64 --cpus-per-task=${OMP_NUM_THREADS} \
--exact --mem=12500M xthi > placement${OMP_NUM_THREADS}.txt &
# Set the number of threads to the value required by the second job
export OMP_NUM_THREADS=2
srun --ntasks=16 --cpus-per-task=${OMP_NUM_THREADS} \
--exact --mem=12500M xthi > placement${OMP_NUM_THREADS}.txt &
# Set the number of threads to the value required by the second job
export OMP_NUM_THREADS=4
srun --ntasks=4 --cpus-per-task=${OMP_NUM_THREADS} \
--exact --mem=12500M xthi > placement${OMP_NUM_THREADS}.txt &
# Set the number of threads to the value required by the second job
export OMP_NUM_THREADS=16
srun --ntasks=1 --cpus-per-task=${OMP_NUM_THREADS} \
--exact --mem=12500M xthi > placement${OMP_NUM_THREADS}.txt &
# Wait for all subjobs to finish
wait
Example 4: 256 serial tasks running across two nodes
For our fourth example, we will run 256 single-core copies of the xthi program (which
prints process/thread placement) across two ARCHER2 compute nodes with each
copy of xthi pinned to a different core. We will illustrate a mechanism for getting the
node IDs to pass to srun as this is required to ensure that the individual subjobs are
assigned to the correct node. This mechanism uses the scontrol command to turn the
nodelist from sbatch into a format we can use as input to srun. The job submission
script for this example would look like:
#!/bin/bash
# Slurm job options (job-name, compute nodes, job time)
#SBATCH --job-name=MultiSerialOnComputes
#SBATCH --time=0:10:0
#SBATCH --nodes=2
#SBATCH --ntasks-per-node=128
#SBATCH --cpus-per-task=1
# Replace [budget code] below with your budget code (e.g. t01)
#SBATCH --account=[budget code]
#SBATCH --partition=standard
#SBATCH --qos=standard
# Make xthi available
module load xthi
# Set the number of threads to 1
# This prevents any threaded system libraries from automatically
# using threading.
export OMP_NUM_THREADS=1
# Propagate the cpus-per-task setting from script to srun commands
# By default, Slurm does not propagate this setting from the sbatch
# options to srun commands in the job script. If this is not done,
# process/thread pinning may be incorrect leading to poor performance
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Get a list of the nodes assigned to this job in a format we can use.
# scontrol converts the condensed node IDs in the sbatch environment
# variable into a list of full node IDs that we can use with srun to
# ensure the subjobs are placed on the correct node. e.g. this converts
# "nid[001234,002345]" to "nid001234 nid002345"
nodelist=$(scontrol show hostnames $SLURM_JOB_NODELIST)
# Loop over the nodes assigned to the job
for nodeid in $nodelist
do
# Loop over 128 subjobs on each node pinning each to a different core
for i in $(seq 1 128)
do
# Launch subjob overriding job settings as required and in the background
# Make sure to change the amount specified by the `--mem=` flag to the amount
# of memory required. The amount of memory is given in MiB by default but other
# units can be specified. If you do not know how much memory to specify, we
# recommend that you specify `--mem=1500M` (1,500 MiB).
srun --nodelist=${nodeid} --nodes=1 --ntasks=1 --ntasks-per-node=1 \
--exact --mem=1500M xthi > placement_${nodeid}_${i}.txt &
done
done
# Wait for all subjobs to finish
wait
Process placement
There are many occasions where you may want to control (usually, MPI) process placement and change it from the default, for example:
- You may want to place processes to different NUMA regions in a round-robin way rather than the default sequential placement
- You may be using fewer than 128 processes per node and want to ensure that processes are placed evenly across NUMA regions (16-core blocks) or core complexes (4-core blocks that share an L3 cache)
There are a number of different methods for defining process placement, below we cover two different
options: using Slurm options and using the MPICH_RANK_REORDER_METHOD environment variable. Most users will
likely use the Slurm options approach.
Standard process placement
The standard approach recommended on ARCHER2 is to place processes sequentially on nodes until the
maximum number of tasks is reached. You can use the xthi program
to verify this for MPI process placement:
auser@ln04:/work/t01/t01/auser> salloc --nodes=2 --ntasks-per-node=128 \
--cpus-per-task=1 --time=0:10:0 --partition=standard --qos=short \
--account=[your account]
salloc: Pending job allocation 1170365
salloc: job 1170365 queued and waiting for resources
salloc: job 1170365 has been allocated resources
salloc: Granted job allocation 1170365
salloc: Waiting for resource configuration
salloc: Nodes nid[002526-002527] are ready for job
auser@ln04:/work/t01/t01/auser> module load xthi
auser@ln04:/work/t01/t01/auser> export OMP_NUM_THREADS=1
auser@ln04:/work/t01/t01/auser> export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
auser@ln04:/work/t01/t01/auser> srun --distribution=block:block --hint=nomultithread xthi
Node summary for 2 nodes:
Node 0, hostname nid002526, mpi 128, omp 1, executable xthi
Node 1, hostname nid002527, mpi 128, omp 1, executable xthi
MPI summary: 256 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 1)
Node 0, rank 2, thread 0, (affinity = 2)
Node 0, rank 3, thread 0, (affinity = 3)
...output trimmed...
Node 0, rank 124, thread 0, (affinity = 124)
Node 0, rank 125, thread 0, (affinity = 125)
Node 0, rank 126, thread 0, (affinity = 126)
Node 0, rank 127, thread 0, (affinity = 127)
Node 1, rank 128, thread 0, (affinity = 0)
Node 1, rank 129, thread 0, (affinity = 1)
Node 1, rank 130, thread 0, (affinity = 2)
Node 1, rank 131, thread 0, (affinity = 3)
...output trimmed...
Note
For MPI programs on ARCHER2, each rank corresponds to a process.
Important
To get good performance out of MPI collective operations, MPI processes should be placed sequentially on cores as in the standard placement described above.
Setting process placement using Slurm options
For underpopulation of nodes with processes
When you are using fewer processes than cores on compute nodes (i.e. < 128 processes
per node) the basic Slurm options (usually supplied in your script as options to sbatch) for
process placement are:
--ntasks-per-node=XPlace X processes on each node--cpus-per-task=YSet a stride of Y cores between each placed process. If you specify this option in a job submission script (queued usingsbatch) or viasallocthey you will also need to setexport SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASKto ensure the setting is passed tosruncommands in the script or allocation.
In addition, the following options are added to your srun commands in your job
submission script:
--hint=nomultithreadOnly use physical cores (avoids use of SMT/hyperthreads)--distribution=block:blockAllocate processes to cores in a sequential fashion
For example, to place 32 processes per node and have 1 process per 4-core block (corresponding to a CCX, Core CompleX, that shares an L3 cache), you would set:
--ntasks-per-node=32Place 32 processes on each node--cpus-per-task=4Set a stride of 4 cores between each placed process
Here is the output from xthi:
auser@ln04:/work/t01/t01/auser> salloc --nodes=2 --ntasks-per-node=32 \
--cpus-per-task=4 --time=0:10:0 --partition=standard --qos=short \
--account=[your account]
salloc: Pending job allocation 1170383
salloc: job 1170383 queued and waiting for resources
salloc: job 1170383 has been allocated resources
salloc: Granted job allocation 1170383
salloc: Waiting for resource configuration
salloc: Nodes nid[002526-002527] are ready for job
auser@ln04:/work/t01/t01/auser> module load xthi
auser@ln04:/work/t01/t01/auser> export OMP_NUM_THREADS=1
auser@ln04:/work/t01/t01/auser> export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
auser@ln04:/work/t01/t01/auser> srun --distribution=block:block --hint=nomultithread xthi
Node summary for 2 nodes:
Node 0, hostname nid002526, mpi 32, omp 1, executable xthi
Node 1, hostname nid002527, mpi 32, omp 1, executable xthi
MPI summary: 64 ranks
Node 0, rank 0, thread 0, (affinity = 0-3)
Node 0, rank 1, thread 0, (affinity = 4-7)
Node 0, rank 2, thread 0, (affinity = 8-11)
Node 0, rank 3, thread 0, (affinity = 12-15)
Node 0, rank 4, thread 0, (affinity = 16-19)
Node 0, rank 5, thread 0, (affinity = 20-23)
Node 0, rank 6, thread 0, (affinity = 24-27)
Node 0, rank 7, thread 0, (affinity = 28-31)
Node 0, rank 8, thread 0, (affinity = 32-35)
Node 0, rank 9, thread 0, (affinity = 36-39)
Node 0, rank 10, thread 0, (affinity = 40-43)
Node 0, rank 11, thread 0, (affinity = 44-47)
Node 0, rank 12, thread 0, (affinity = 48-51)
Node 0, rank 13, thread 0, (affinity = 52-55)
Node 0, rank 14, thread 0, (affinity = 56-59)
Node 0, rank 15, thread 0, (affinity = 60-63)
Node 0, rank 16, thread 0, (affinity = 64-67)
Node 0, rank 17, thread 0, (affinity = 68-71)
Node 0, rank 18, thread 0, (affinity = 72-75)
Node 0, rank 19, thread 0, (affinity = 76-79)
Node 0, rank 20, thread 0, (affinity = 80-83)
Node 0, rank 21, thread 0, (affinity = 84-87)
Node 0, rank 22, thread 0, (affinity = 88-91)
Node 0, rank 23, thread 0, (affinity = 92-95)
Node 0, rank 24, thread 0, (affinity = 96-99)
Node 0, rank 25, thread 0, (affinity = 100-103)
Node 0, rank 26, thread 0, (affinity = 104-107)
Node 0, rank 27, thread 0, (affinity = 108-111)
Node 0, rank 28, thread 0, (affinity = 112-115)
Node 0, rank 29, thread 0, (affinity = 116-119)
Node 0, rank 30, thread 0, (affinity = 120-123)
Node 0, rank 31, thread 0, (affinity = 124-127)
Node 1, rank 32, thread 0, (affinity = 0-3)
Node 1, rank 33, thread 0, (affinity = 4-7)
Node 1, rank 34, thread 0, (affinity = 8-11)
Node 1, rank 35, thread 0, (affinity = 12-15)
Node 1, rank 36, thread 0, (affinity = 16-19)
Node 1, rank 37, thread 0, (affinity = 20-23)
Node 1, rank 38, thread 0, (affinity = 24-27)
Node 1, rank 39, thread 0, (affinity = 28-31)
Node 1, rank 40, thread 0, (affinity = 32-35)
Node 1, rank 41, thread 0, (affinity = 36-39)
Node 1, rank 42, thread 0, (affinity = 40-43)
Node 1, rank 43, thread 0, (affinity = 44-47)
Node 1, rank 44, thread 0, (affinity = 48-51)
Node 1, rank 45, thread 0, (affinity = 52-55)
Node 1, rank 46, thread 0, (affinity = 56-59)
Node 1, rank 47, thread 0, (affinity = 60-63)
Node 1, rank 48, thread 0, (affinity = 64-67)
Node 1, rank 49, thread 0, (affinity = 68-71)
Node 1, rank 50, thread 0, (affinity = 72-75)
Node 1, rank 51, thread 0, (affinity = 76-79)
Node 1, rank 52, thread 0, (affinity = 80-83)
Node 1, rank 53, thread 0, (affinity = 84-87)
Node 1, rank 54, thread 0, (affinity = 88-91)
Node 1, rank 55, thread 0, (affinity = 92-95)
Node 1, rank 56, thread 0, (affinity = 96-99)
Node 1, rank 57, thread 0, (affinity = 100-103)
Node 1, rank 58, thread 0, (affinity = 104-107)
Node 1, rank 59, thread 0, (affinity = 108-111)
Node 1, rank 60, thread 0, (affinity = 112-115)
Node 1, rank 61, thread 0, (affinity = 116-119)
Node 1, rank 62, thread 0, (affinity = 120-123)
Node 1, rank 63, thread 0, (affinity = 124-127)
Tip
You usually only want to use physical cores on ARCHER2, so
(ntasks-per-node) × (cpus-per-task) should generally be equal to 128.
Full node population with non-sequential process placement
If you want to change the order processes are placed on nodes and cores using Slurm options
then you should use the --distribution option to srun to change this.
For example, to place processes sequentially on nodes but round-robin on the 16-core NUMA regions in a
single node, you would use the --distribution=block:cyclic option to srun. This type
of process placement can be beneficial when a code is memory bound.
auser@ln04:/work/t01/t01/auser> salloc --nodes=2 --ntasks-per-node=128 \
--cpus-per-task=1 --time=0:10:0 --partition=standard --qos=short \
--account=[your account]
salloc: Pending job allocation 1170594
salloc: job 1170594 queued and waiting for resources
salloc: job 1170594 has been allocated resources
salloc: Granted job allocation 1170594
salloc: Waiting for resource configuration
salloc: Nodes nid[002616,002621] are ready for job
auser@ln04:/work/t01/t01/auser> module load xthi
auser@ln04:/work/t01/t01/auser> export OMP_NUM_THREADS=1
auser@ln04:/work/t01/t01/auser> srun --distribution=block:cyclic --hint=nomultithread xthi
Node summary for 2 nodes:
Node 0, hostname nid002616, mpi 128, omp 1, executable xthi
Node 1, hostname nid002621, mpi 128, omp 1, executable xthi
MPI summary: 256 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 16)
Node 0, rank 2, thread 0, (affinity = 32)
Node 0, rank 3, thread 0, (affinity = 48)
Node 0, rank 4, thread 0, (affinity = 64)
Node 0, rank 5, thread 0, (affinity = 80)
Node 0, rank 6, thread 0, (affinity = 96)
Node 0, rank 7, thread 0, (affinity = 112)
Node 0, rank 8, thread 0, (affinity = 1)
Node 0, rank 9, thread 0, (affinity = 17)
Node 0, rank 10, thread 0, (affinity = 33)
Node 0, rank 11, thread 0, (affinity = 49)
Node 0, rank 12, thread 0, (affinity = 65)
Node 0, rank 13, thread 0, (affinity = 81)
Node 0, rank 14, thread 0, (affinity = 97)
Node 0, rank 15, thread 0, (affinity = 113
...output trimmed...
Node 0, rank 120, thread 0, (affinity = 15)
Node 0, rank 121, thread 0, (affinity = 31)
Node 0, rank 122, thread 0, (affinity = 47)
Node 0, rank 123, thread 0, (affinity = 63)
Node 0, rank 124, thread 0, (affinity = 79)
Node 0, rank 125, thread 0, (affinity = 95)
Node 0, rank 126, thread 0, (affinity = 111)
Node 0, rank 127, thread 0, (affinity = 127)
Node 1, rank 128, thread 0, (affinity = 0)
Node 1, rank 129, thread 0, (affinity = 16)
Node 1, rank 130, thread 0, (affinity = 32)
Node 1, rank 131, thread 0, (affinity = 48)
Node 1, rank 132, thread 0, (affinity = 64)
Node 1, rank 133, thread 0, (affinity = 80)
Node 1, rank 134, thread 0, (affinity = 96)
Node 1, rank 135, thread 0, (affinity = 112)
...output trimmed...
If you wish to place processes round robin on both nodes and 16-core regions
(cores that share access to a DRAM single memory controller) within in a node
you would use --distribution=cyclic:cyclic:
auser@ln04:/work/t01/t01/auser> salloc --nodes=2 --ntasks-per-node=128 \
--cpus-per-task=1 --time=0:10:0 --partition=standard --qos=short \
--account=[your account]
salloc: Pending job allocation 1170594
salloc: job 1170594 queued and waiting for resources
salloc: job 1170594 has been allocated resources
salloc: Granted job allocation 1170594
salloc: Waiting for resource configuration
salloc: Nodes nid[002616,002621] are ready for job
auser@ln04:/work/t01/t01/auser> module load xthi
auser@ln04:/work/t01/t01/auser> export OMP_NUM_THREADS=1
auser@ln04:/work/t01/t01/auser> export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
auser@ln04:/work/t01/t01/auser> srun --distribution=cyclic:cyclic --hint=nomultithread xthi
Node summary for 2 nodes:
Node 0, hostname nid002616, mpi 128, omp 1, executable xthi
Node 1, hostname nid002621, mpi 128, omp 1, executable xthi
MPI summary: 256 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 2, thread 0, (affinity = 16)
Node 0, rank 4, thread 0, (affinity = 32)
Node 0, rank 6, thread 0, (affinity = 48)
Node 0, rank 8, thread 0, (affinity = 64)
Node 0, rank 10, thread 0, (affinity = 80)
Node 0, rank 12, thread 0, (affinity = 96)
Node 0, rank 14, thread 0, (affinity = 112)
Node 0, rank 16, thread 0, (affinity = 1)
Node 0, rank 18, thread 0, (affinity = 17)
Node 0, rank 20, thread 0, (affinity = 33)
Node 0, rank 22, thread 0, (affinity = 49)
Node 0, rank 24, thread 0, (affinity = 65)
Node 0, rank 26, thread 0, (affinity = 81)
Node 0, rank 28, thread 0, (affinity = 97)
Node 0, rank 30, thread 0, (affinity = 113)
...output trimmed...
Node 1, rank 1, thread 0, (affinity = 0)
Node 1, rank 3, thread 0, (affinity = 16)
Node 1, rank 5, thread 0, (affinity = 32)
Node 1, rank 7, thread 0, (affinity = 48)
Node 1, rank 9, thread 0, (affinity = 64)
Node 1, rank 11, thread 0, (affinity = 80)
Node 1, rank 13, thread 0, (affinity = 96)
Node 1, rank 15, thread 0, (affinity = 112)
Node 1, rank 17, thread 0, (affinity = 1)
Node 1, rank 19, thread 0, (affinity = 17)
Node 1, rank 21, thread 0, (affinity = 33)
Node 1, rank 23, thread 0, (affinity = 49)
Node 1, rank 25, thread 0, (affinity = 65)
Node 1, rank 27, thread 0, (affinity = 81)
Node 1, rank 29, thread 0, (affinity = 97)
Node 1, rank 31, thread 0, (affinity = 113)
...output trimmed...
Remember, MPI collective performance is generally much worse if processes are not placed sequentially on a node (so adjacent MPI ranks are as close to each other as possible). This is the reason that the default recommended placement on ARCHER2 is sequential rather than round-robin.
MPICH_RANK_REORDER_METHOD for MPI process placement
The MPICH_RANK_REORDER_METHOD environment variable can also be used to specify
other types of MPI task placement. For example, setting it to "0" results
in a round-robin placement on both nodes and NUMA regions in a node (equivalent to
the --distribution=cyclic:cyclic option to srun). Note, we do not specify the --distribution
option to srun in this case as the environment variable is controlling placement:
salloc --nodes=8 --ntasks-per-node=2 --cpus-per-task=1 --time=0:10:0 --account=t01
salloc: Granted job allocation 24236
salloc: Waiting for resource configuration
salloc: Nodes cn13 are ready for job
module load xthi
export OMP_NUM_THREADS=1
export MPICH_RANK_REORDER_METHOD=0
srun --hint=nomultithread xthi
Node summary for 2 nodes:
Node 0, hostname nid002616, mpi 128, omp 1, executable xthi
Node 1, hostname nid002621, mpi 128, omp 1, executable xthi
MPI summary: 256 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 2, thread 0, (affinity = 16)
Node 0, rank 4, thread 0, (affinity = 32)
Node 0, rank 6, thread 0, (affinity = 48)
Node 0, rank 8, thread 0, (affinity = 64)
Node 0, rank 10, thread 0, (affinity = 80)
Node 0, rank 12, thread 0, (affinity = 96)
Node 0, rank 14, thread 0, (affinity = 112)
Node 0, rank 16, thread 0, (affinity = 1)
Node 0, rank 18, thread 0, (affinity = 17)
Node 0, rank 20, thread 0, (affinity = 33)
Node 0, rank 22, thread 0, (affinity = 49)
Node 0, rank 24, thread 0, (affinity = 65)
Node 0, rank 26, thread 0, (affinity = 81)
Node 0, rank 28, thread 0, (affinity = 97)
Node 0, rank 30, thread 0, (affinity = 113)
...output trimmed...
There are other modes available with the MPICH_RANK_REORDER_METHOD
environment variable, including one which lets the user provide a file
called MPICH_RANK_ORDER which contains a list of each task's placement
on each node. These options are described in detail in the intro_mpi
man page.
grid_order
For MPI applications which perform a large amount of nearest-neighbor
communication, e.g., stencil-based applications on structured grids,
HPE provide a tool in the perftools-base module (Loaded by default for
all users) called grid_order
which can generate a MPICH_RANK_ORDER file automatically by taking as
parameters the dimensions of the grid, core count, etc. For example, to
place 256 MPI parameters in row-major order on a Cartesian grid of size $(8, 8,
4)$, using 128 cores per node:
grid_order -R -c 128 -g 8,8,4
# grid_order -R -Z -c 128 -g 8,8,4
# Region 3: 0,0,1 (0..255)
0,1,2,3,32,33,34,35,64,65,66,67,96,97,98,99,128,129,130,131,160,161,162,163,192,193,194,195,224,225,226,227,4,5,6,7,36,37,38,39,68,69,70,71,100,101,102,103,132,133,134,135,164,165,166,167,196,197,198,199,228,229,230,231,8,9,10,11,40,41,42,43,72,73,74,75,104,105,106,107,136,137,138,139,168,169,170,171,200,201,202,203,232,233,234,235,12,13,14,15,44,45,46,47,76,77,78,79,108,109,110,111,140,141,142,143,172,173,174,175,204,205,206,207,236,237,238,239
16,17,18,19,48,49,50,51,80,81,82,83,112,113,114,115,144,145,146,147,176,177,178,179,208,209,210,211,240,241,242,243,20,21,22,23,52,53,54,55,84,85,86,87,116,117,118,119,148,149,150,151,180,181,182,183,212,213,214,215,244,245,246,247,24,25,26,27,56,57,58,59,88,89,90,91,120,121,122,123,152,153,154,155,184,185,186,187,216,217,218,219,248,249,250,251,28,29,30,31,60,61,62,63,92,93,94,95,124,125,126,127,156,157,158,159,188,189,190,191,220,221,222,223,252,253,254,255
One can then save this output to a file called MPICH_RANK_ORDER and
then set MPICH_RANK_REORDER_METHOD=3 before running the job, which
tells Cray MPI to read the MPICH_RANK_ORDER file to set the MPI task
placement. For more information, please see the man page man grid_order.
Interactive Jobs
Using salloc to reserve resources
When you are developing or debugging code you often want to run many short jobs with a small amount of editing the code between runs. This can be achieved by using the login nodes to run MPI but you may want to test on the compute nodes (e.g. you may want to test running on multiple nodes across the high performance interconnect). One of the best ways to achieve this on ARCHER2 is to use interactive jobs.
An interactive job allows you to issue srun commands directly from the
command line without using a job submission script, and to see the
output from your program directly in the terminal.
You use the salloc command to reserve compute nodes for interactive
jobs.
To submit a request for an interactive job reserving 8 nodes (1024 physical cores) for 20 minutes on the short QoS you would issue the following command from the command line:
auser@ln01:> salloc --nodes=8 --ntasks-per-node=128 --cpus-per-task=1 \
--time=00:20:00 --partition=standard --qos=short \
--account=[budget code]
When you submit this job your terminal will display something like:
salloc: Granted job allocation 24236
salloc: Waiting for resource configuration
salloc: Nodes nid000002 are ready for job
auser@ln01:>
It may take some time for your interactive job to start. Once it runs
you will enter a standard interactive terminal session (a new shell).
Note that this shell is still on the front end (the prompt has not
change). Whilst the interactive session lasts you will be able to run
parallel jobs on the compute nodes by issuing the srun
--distribution=block:block --hint=nomultithread command directly at
your command prompt using the same syntax as you would inside a job
script. The maximum number of nodes you can use is limited by resources
requested in the salloc command.
Important
If you wish the cpus-per-task option to salloc to propagate to srun
commands in the allocation, you will need to use the command
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK before you issue any
srun commands.
If you know you will be doing a lot of intensive debugging you may find it useful to request an interactive session lasting the expected length of your working session, say a full day.
Your session will end when you hit the requested walltime. If you wish
to finish before this you should use the exit command - this will
return you to your prompt before you issued the salloc command.
Using srun directly
A second way to run an interactive job is to use srun directly in the
following way (here using the short QoS):
auser@ln01:/work/t01/t01/auser> srun --nodes=1 --exclusive --time=00:20:00 \
--partition=standard --qos=short --account=[budget code] \
--pty /bin/bash
auser@nid001261:/work/t01/t01/auser> hostname
nid001261
The --pty /bin/bash will cause a new shell to be started on the first
node of a new allocation . This is perhaps closer to what
many people consider an 'interactive' job than the method using salloc
appears.
One can now issue shell commands in the usual way. A further invocation
of srun is required to launch a parallel job in the allocation.
Note
When using srun within an interactive srun session, you will need to
include both the --overlap and --oversubscribe flags, and specify the number of cores you want
to use:
auser@nid001261:/work/t01/t01/auser> srun --overlap --oversubscribe --distribution=block:block \
--hint=nomultithread --ntasks=128 ./my_mpi_executable.x
Without --overlap the second srun will block until the first one
has completed. Since your interactive session was launched with srun
this means it will never actually start -- you will get repeated
warnings that "Requested nodes are busy".
When finished, type exit to relinquish the allocation and control will
be returned to the front end.
By default, the interactive shell will retain the environment of the
parent. If you want a clean shell, remember to specify --export=none.
Heterogeneous jobs
Most of the Slurm submissions discussed above involve running a single executable. However, there are situations where two or more distinct executables are coupled and need to be run at the same time, potentially using the same MPI communicator. This is most easily handled via the Slurm heterogeneous job mechanism.
Two common cases are discussed below: first, a client server model in
which client and server each have a different MPI_COMM_WORLD, and second
the case were two or more executables share MPI_COMM_WORLD.
Heterogeneous jobs for a client/server model: distinct MPI_COMM_WORLDs
The essential feature of a heterogeneous job here is to create a single batch submission which specifies the resource requirements for the individual components. Schematically, we would use
#!/bin/bash
# Slurm specifications for the first component
#SBATCH --partition=standard
...
#SBATCH hetjob
# Slurm specifications for the second component
#SBATCH --partition=standard
...
#SBATCH hetjob (note this is not a normal option and is not
--hetjob). Each component must specify a partition.
Such a job will appear in the scheduler as, e.g.,
50098+0 standard qscript- user PD 0:00 1 (None)
50098+1 standard qscript- user PD 0:00 2 (None)
Consider a case where we have two executables which may both be parallel (in
that they use MPI), both run at the same time, and communicate with each
other using MPI or by some other means. In the following example, we run two
different executables, xthi-a and xthi-b, both of which must finish
before the jobs completes.
#!/bin/bash
#SBATCH --time=00:20:00
#SBATCH --exclusive
#SBATCH --export=none
#SBATCH --partition=standard
#SBATCH --qos=standard
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=8
#SBATCH hetjob
#SBATCH --partition=standard
#SBATCH --qos=standard
#SBATCH --nodes=2
#SBATCH --ntasks-per-node=4
# Run two executables with separate MPI_COMM_WORLD
srun --distribution=block:block --hint=nomultithread --het-group=0 ./xthi-a &
srun --distribution=block:block --hint=nomultithread --het-group=1 ./xthi-b &
wait
srun but specifies a different heterogeneous group via the
--het-group option. The first group is --het-group=0.
Both are run in the background with & and the wait is required
to ensure both executables have completed before the job submission
exits.
The above is a rather artificial example using two executables which
are in fact just symbolic links in the job directory to xthi,
used without loading the module. You can test this script yourself
by creating symbolic links to the original executable before
submitting the job:
auser@ln04:/work/t01/t01/auser/job-dir> module load xthi
auser@ln04:/work/t01/t01/auser/job-dir> which xthi
/work/y07/shared/utils/core/xthi/1.2/CRAYCLANG/11.0/bin/xthi
auser@ln04:/work/t01/t01/auser/job-dir> ln -s /work/y07/shared/utils/core/xthi/1.2/CRAYCLANG/11.0/bin/xthi xthi-a
auser@ln04:/work/t01/t01/auser/job-dir> ln -s /work/y07/shared/utils/core/xthi/1.2/CRAYCLANG/11.0/bin/xthi xthi-b
The example job will produce two reports showing the placement of the
MPI tasks from the two instances of xthi running in each of the
heterogeneous groups. For example, the output might be
Node summary for 1 nodes:
Node 0, hostname nid002400, mpi 8, omp 1, executable xthi-a
MPI summary: 8 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 1)
Node 0, rank 2, thread 0, (affinity = 2)
Node 0, rank 3, thread 0, (affinity = 3)
Node 0, rank 4, thread 0, (affinity = 4)
Node 0, rank 5, thread 0, (affinity = 5)
Node 0, rank 6, thread 0, (affinity = 6)
Node 0, rank 7, thread 0, (affinity = 7)
Node summary for 2 nodes:
Node 0, hostname nid002146, mpi 4, omp 1, executable xthi-b
Node 1, hostname nid002149, mpi 4, omp 1, executable xthi-b
MPI summary: 8 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 1)
Node 0, rank 2, thread 0, (affinity = 2)
Node 0, rank 3, thread 0, (affinity = 3)
Node 1, rank 4, thread 0, (affinity = 0)
Node 1, rank 5, thread 0, (affinity = 1)
Node 1, rank 6, thread 0, (affinity = 2)
Node 1, rank 7, thread 0, (affinity = 3)
Finally, if your workflow requires the different heterogeneous jobs to
communicate via MPI, but without sharing their MPI_COM_WORLD, you will need
to export two new variables before your srun commands as defined below:
export PMI_UNIVERSE_SIZE=3
export MPICH_SINGLE_HOST_ENABLED=0
Heterogeneous jobs for a shared MPI_COM_WORLD
Note
The directive SBATCH hetjob can no longer be used for jobs requiring
a shared MPI_COMM_WORLD
Note
In this approach, each hetjob component must be on its own set of nodes.
You cannot use this approach to place different hetjob components on
the same node.
If two or more heterogeneous components need to share a unique
MPI_COMM_WORLD, a single srun invocation with the different
components separated by a colon : should be used. Arguments
to the individual components of the srun control the placement of
the tasks and threads for each component. For example, running the
same xthi-a and xthi-b executables as above but now in a shared
communicator, we might run:
#!/bin/bash
#SBATCH --time=00:20:00
#SBATCH --export=none
#SBATCH --account=[...]
#SBATCH --partition=standard
#SBATCH --qos=standard
# We must specify correctly the total number of nodes required.
#SBATCH --nodes=3
SHARED_ARGS="--distribution=block:block --hint=nomultithread"
srun --het-group=0 --nodes=1 --ntasks-per-node=8 ${SHARED_ARGS} ./xthi-a : \
--het-group=1 --nodes=2 --ntasks-per-node=4 ${SHARED_ARGS} ./xthi-b
The output should confirm we have a single MPI_COMM_WORLD with
a total of three nodes, xthi-a running on one and xthi-b on two,
with ranks 0-15 extending across both executables.
Node summary for 3 nodes:
Node 0, hostname nid002668, mpi 8, omp 1, executable xthi-a
Node 1, hostname nid002669, mpi 4, omp 1, executable xthi-b
Node 2, hostname nid002670, mpi 4, omp 1, executable xthi-b
MPI summary: 16 ranks
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 1, thread 0, (affinity = 1)
Node 0, rank 2, thread 0, (affinity = 2)
Node 0, rank 3, thread 0, (affinity = 3)
Node 0, rank 4, thread 0, (affinity = 4)
Node 0, rank 5, thread 0, (affinity = 5)
Node 0, rank 6, thread 0, (affinity = 6)
Node 0, rank 7, thread 0, (affinity = 7)
Node 1, rank 8, thread 0, (affinity = 0)
Node 1, rank 9, thread 0, (affinity = 1)
Node 1, rank 10, thread 0, (affinity = 2)
Node 1, rank 11, thread 0, (affinity = 3)
Node 2, rank 12, thread 0, (affinity = 0)
Node 2, rank 13, thread 0, (affinity = 1)
Node 2, rank 14, thread 0, (affinity = 2)
Node 2, rank 15, thread 0, (affinity = 3)
Heterogeneous placement for mixed MPI/OpenMP work
Some care may be required for placement of tasks/threads in heterogeneous jobs in which the number of threads needs to be specified differently for different components.
In the following we have two components, again using xthi-a and
xthi-b as our two separate executables. The first component runs
8 MPI tasks each with 16 OpenMP threads on one node. The second component
runs 8 MPI tasks with one task per NUMA region on a second node; each
task has one thread. An appropriate Slurm submission might be:
#!/bin/bash
#SBATCH --time=00:20:00
#SBATCH --export=none
#SBATCH --account=[...]
#SBATCH --partition=standard
#SBATCH --qos=standard
#SBATCH --nodes=2
SHARED_ARGS="--distribution=block:block --hint=nomultithread \
--nodes=1 --ntasks-per-node=8 --cpus-per-task=16"
# Do not set OMP_NUM_THREADS in the calling environment
unset OMP_NUM_THREADS
export OMP_PROC_BIND=spread
srun --het-group=0 ${SHARED_ARGS} --export=all,OMP_NUM_THREADS=16 ./xthi-a : \
--het-group=1 ${SHARED_ARGS} --export=all,OMP_NUM_THREADS=1 ./xthi-b
The important point here is that OMP_NUM_THREADS must not be set
in the environment that calls srun in order that the different
specifications for the separate groups via --export on the srun
command line take effect. If OMP_NUM_THREADS is set in the calling
environment, then that value takes precedence, and each component will
see the same value of OMP_NUM_THREADS.
The output might then be:
Node 0, hostname nid001111, mpi 8, omp 16, executable xthi-a
Node 1, hostname nid001126, mpi 8, omp 1, executable xthi-b
Node 0, rank 0, thread 0, (affinity = 0)
Node 0, rank 0, thread 1, (affinity = 1)
Node 0, rank 0, thread 2, (affinity = 2)
Node 0, rank 0, thread 3, (affinity = 3)
Node 0, rank 0, thread 4, (affinity = 4)
Node 0, rank 0, thread 5, (affinity = 5)
Node 0, rank 0, thread 6, (affinity = 6)
Node 0, rank 0, thread 7, (affinity = 7)
Node 0, rank 0, thread 8, (affinity = 8)
Node 0, rank 0, thread 9, (affinity = 9)
Node 0, rank 0, thread 10, (affinity = 10)
Node 0, rank 0, thread 11, (affinity = 11)
Node 0, rank 0, thread 12, (affinity = 12)
Node 0, rank 0, thread 13, (affinity = 13)
Node 0, rank 0, thread 14, (affinity = 14)
Node 0, rank 0, thread 15, (affinity = 15)
Node 0, rank 1, thread 0, (affinity = 16)
Node 0, rank 1, thread 1, (affinity = 17)
...
Node 0, rank 7, thread 14, (affinity = 126)
Node 0, rank 7, thread 15, (affinity = 127)
Node 1, rank 8, thread 0, (affinity = 0)
Node 1, rank 9, thread 0, (affinity = 16)
Node 1, rank 10, thread 0, (affinity = 32)
Node 1, rank 11, thread 0, (affinity = 48)
Node 1, rank 12, thread 0, (affinity = 64)
Node 1, rank 13, thread 0, (affinity = 80)
Node 1, rank 14, thread 0, (affinity = 96)
Node 1, rank 15, thread 0, (affinity = 112)
Here we can see the eight MPI tasks from xthi-a each running with
sixteen OpenMP threads. Then the 8 MPI tasks with no threading from
xthi-b are spaced across the cores on the second node, one per NUMA region.
Low priority access
Low priority jobs are not charged against your allocation but will only run when other, higher-priority, jobs cannot be run. Although low priority jobs are not charged, you do need a valid, positive budget to be able to submit and run low priority jobs, i.e. you need at least 1 CU in your budget.
Low priority access is always available and has the following limits:
- 1024 node maximum job size
- Maximum 16 low priority jobs submitted (including running) per user
- Maximum 16 low priority job running per user
- Maximum runtime of 24 hours
You submit a low priority job on ARCHER2 by using the lowpriority QoS. For example,
you would usually have the following line in your job submission script sbatch
options:
#SBATCH --qos=lowpriority
Reservations
Reservations are available on ARCHER2. These allow users to reserve a number of nodes for a specified length of time starting at a particular time on the system.
Reservations require justification. They will only be approved if the request could not be fulfilled with the normal QoS's. For instance, you require a job/jobs to run at a particular time e.g. for a demonstration or course.
Note
Reservation requests must be submitted at least 60 hours in advance of the reservation start time. If requesting a reservation for a Monday at 18:00, please ensure this is received by the Friday at 12:00 the latest. The same applies over Service Holidays.
Note
Reservations are only valid for standard compute nodes, high memory compute nodes and/or PP nodes cannot be included in reservations.
Reservations will be charged at 1.5 times the usual CU rate and our policy is that they will be charged the full rate for the entire reservation at the time of booking, whether or not you use the nodes for the full time. In addition, you will not be refunded the CUs if you fail to use them due to a job issue unless this issue is due to a system failure.
To request a reservation you complete a form on SAFE:
- Log into SAFE
- Under the "Apply" menu, choose the "Request reservation" option
On the first page, you need to provide the following:
- The start time and date of the reservation.
- The end time and date of the reservation.
- Your justification for the reservation -- this must be provided or the request will be rejected.
- The number of nodes required.
- The machine name (ARCHER2) under resource type.
On the second page, you will need to specify which username you wish the reservation to be charged against and, once the username has been selected, the budget you want to charge the reservation to. (The selected username will be charged for the reservation but the reservation can be used by all members of the selected budget.)
Your request will be checked by the ARCHER2 User Administration team and, if approved, you will be provided a reservation ID which can be used on the system. To submit jobs to a reservation, you need to add --reservation=<reservation ID> and --qos=reservation options to your job submission script or command.
Important
You must have at least 1 CU in the budget to submit a job on ARCHER2, even to a pre-paid reservation.
Tip
You can submit jobs to a reservation as soon as the reservation has been set up; jobs will remain queued until the reservation starts.
Capability Days
Important
The next Capability Days session has not been scheduled yet
ARCHER2 Capability Days are a mechanism to allow users to run large scale (512 node or more) tests on the system free of charge. The motivations behind Capability Days are:
- Enhancing world-leading science from ARCHER2 by enabling modelling and simulation at scales that are not otherwise possible.
- Enabling capability use cases that are not possible on other UK HPC services.
- Providing a facility that can be used to test scaling to help prepare software and communities for future exascale resources.
To enable this, a period will be made available regularly where users can run jobs at large scale free of charge.
Capability Days are made up of different parts:
- pre-Capability Day session (
pre-capabilitydayQoS) to allow users to test scaling and job setup ahead of full Capability Day - NERC Capability reservation (
NERCcapabilityreservation) to allow NERC users to test at large scale - Capability Day session (
capabilitydayQoS)
Tip
Any jobs left in the queues when Capability Days finish will be deleted.
pre-Capability Day session
The pre-Capability Day session is typically available directly before the full Capability Day session and allows short test jobs to prepare for Capability Day.
Submit to the pre-capabilityday QoS. Jobs can be submitted ahead of time and will start when the pre-Capability Day
session starts.
pre-capabilityday QoS limits:
- Available for 12 hours
- 1024 nodes available
- Minimum job size: 256 nodes, maximum job size: 1024 nodes
- Individual jobs steps (i.e.
sruncommands) within job scripts should also be a minimum of 256 nodes - Jobs that do not meet these limits will be killed
- Individual jobs steps (i.e.
- Maximum walltime: 20 minutes
- Job numbers: 8 jobs maximum per budget code in the QoS
- 1 job maximum running per budget code
- High memory nodes are not available
- Users must have a valid, positive CU budget to be able to run jobs in the pre-Capability Day session
- Jobs are free
Example pre-Capability Day session job submission script
#!/bin/bash
#SBATCH --job-name=test_capability_job
#SBATCH --nodes=256
#SBATCH --ntasks-per-node=8
#SBATCH --cpus-per-task=16
#SBATCH --time=1:0:0
#SBATCH --partition=standard
#SBATCH --qos=pre-capabilityday
#SBATCH --account=t01
export OMP_NUM_THREADS=16
export OMP_PLACES=cores
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Check process/thread placement
module load xthi
srun --hint=multithread --distribution=block:block xthi > placement-${SLURM_JOBID}.out
srun --hint=multithread --distribution=block:block my_app.x
NERC Capability reservation
The NERC Capability reservation is typically available directly before the full Capability Day session and allows short test jobs to prepare for Capability Day.
Submit to the NERCcapability reservation. Jobs can be submitted ahead of time and will start when the NERC Capability
reservatoin starts.
NERCcapability reservation limits:
- Only available to users in NERC projects
- Available for 8 hours
- 1024 nodes available
- Maximum job size: 1024 nodes
- Maximum walltime: 8 hours (reservation length)
- We will monitor use of the reservation to ensure multiple users get a chance to run
- Any long jobs blocking access for other users will be killed
- High memory nodes are not available
- Jobs are free
Example NERC Capability reservation job submission script
#!/bin/bash
#SBATCH --job-name=NERC_capability_job
#SBATCH --nodes=256
#SBATCH --ntasks-per-node=8
#SBATCH --cpus-per-task=16
#SBATCH --time=1:0:0
#SBATCH --partition=standard
#SBATCH --reservation=NERCcapability
#SBATCH --qos=reservation
#SBATCH --account=t01
export OMP_NUM_THREADS=16
export OMP_PLACES=cores
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Check process/thread placement
module load xthi
srun --hint=multithread --distribution=block:block xthi > placement-${SLURM_JOBID}.out
srun --hint=multithread --distribution=block:block my_app.x
Capability Day session
The Capability Day session is typically available directly after the pre-Capability Day session.
Submit to the capability QoS. Jobs can be submitted ahead of time and will start when the Capability Day
session starts.
capabilityday QoS limits:
- Available for 42 hours
- 4096 nodes available
- Minimum job size: 512 nodes, maximum job size: 4096 nodes
- Individual jobs steps (i.e.
sruncommands) within job scripts should also be a minimum of 512 nodes - Jobs that do not meet these limits will be killed
- Individual jobs steps (i.e.
- Maximum walltime: 1 hour
- Job numbers: 16 jobs maximum per budget code in the QoS
- 2 jobs maximum running per budget code
- High memory nodes are not available
- Users must have a valid, positive CU budget to be able to run jobs in the pre-Capability Day session
- Jobs are free
Example Capability Day job submission script
#!/bin/bash
#SBATCH --job-name=capability_job
#SBATCH --nodes=1024
#SBATCH --ntasks-per-node=8
#SBATCH --cpus-per-task=16
#SBATCH --time=1:0:0
#SBATCH --partition=standard
#SBATCH --qos=capabilityday
#SBATCH --account=t01
export OMP_NUM_THREADS=16
export OMP_PLACES=cores
export SRUN_CPUS_PER_TASK=$SLURM_CPUS_PER_TASK
# Check process/thread placement
module load xthi
srun --hint=multithread --distribution=block:block xthi > placement-${SLURM_JOBID}.out
srun --hint=multithread --distribution=block:block my_app.x
Capability Day tips
- OFI communications protocol seems to work more reliably at capability scale than UCX protocol
- UCX often sees memory/timeout errors
- All-to-all collective patterns do not generally scale well to large MPI process counts, particularly when there are high MPI process counts per node
- c.f. On the Frontier exascale system there are typically a maximum of 8 MPI processes per node (1 per GPU). 9,408 compute nodes gives a maximum of 75,264 MPI processes for a whole system job.
- 4096 ARCHER2 compute nodes, 1 MPI process per core is 524,488 MPI processes!
- MPI-IO does not generally scale well to high process counts unless the IO pattern is very simple
- Same for IO libraries based on MPI-IO: parallel HDF5, NetCDF
- Consider a different parallel IO approach, e.g. ADIOS2
- Make use of the scratch, solid state file system so you do not hit unexpected storage quota issues
- With very high MPI process counts, you may see long MPI startup times, take this into account in wall times in your job scripts
Serial jobs
You can run serial jobs on the shared data analysis nodes. More information on using the data analysis nodes (including example job submission scripts) can be found in the Data Analysis section of the User and Best Practice Guide.
GPU jobs
You can run on the ARCHER2 GPU nodes and full guidance can be found on the GPU development platform page
Best practices for job submission
This guidance is adapted from the advice provided by NERSC
Time Limits
Due to backfill scheduling, short and variable-length jobs generally
start quickly resulting in much better job throughput. You can specify a
minimum time for your job with the --time-min option to SBATCH:
#SBATCH --time-min=<lower_bound>
#SBATCH --time=<upper_bound>
Within your job script, you can get the time remaining in the job with
squeue -h -j ${Slurm_JOBID} -o %L to allow you to deal with
potentially varying runtimes when using this option.
Long Running Jobs
Simulations which must run for a long period of time achieve the best throughput when composed of many small jobs using a checkpoint and restart method chained together (see above for how to chain jobs together). However, this method does occur a startup and shutdown overhead for each job as the state is saved and loaded so you should experiment to find the best balance between runtime (long runtimes minimise the checkpoint/restart overheads) and throughput (short runtimes maximise throughput).
Interconnect locality
For jobs which are sensitive to interconnect (MPI) performance and utilise 128 nodes or less it is possible to request that all nodes are in a single Slingshot dragonfly group. The maximum number of nodes in a group on ARCHER2 is 128.
Slurm has a concept of "switches" which on ARCHER2 are configured to map to Slingshot electrical groups; where all compute nodes have all-to-all electrical connections which minimises latency. Since this places an additional constraint on the scheduler a maximum time to wait for the requested topology can be specified - after this time, the job will be placed without the constraint.
For example, to specify that all requested nodes should come from one electrical group and to wait for up to 6 hours (360 minutes) for that placement, you would use the following option in your job:
#SBATCH --switches=1@360
You can request multiple groups using this option if you are using more nodes than are in a single group to maximise the number of nodes that share electrical connetions in the job. For example, to request 4 groups (maximum of 512 nodes) and have this as an absolute constraint with no timeout, you would use:
#SBATCH --switches=4
Danger
When specifying the number of groups take care to request enough groups to satisfy the requested number of nodes. If the number is too low then an unnecessary delay will be added due to the unsatisfiable request.
A useful heuristic to ensure this is the case is to ensure that the total nodes requested is less than or equal to the number of groups multiplied by 128.
Large Jobs
Large jobs may take longer to start up. The sbcast command is
recommended for large jobs requesting over 1500 MPI tasks. By default,
Slurm reads the executable on the allocated compute nodes from the
location where it is installed; this may take long time when the file
system (where the executable resides) is slow or busy. The sbcast
command, the executable can be copied to the /tmp directory on each of
the compute nodes. Since /tmp is part of the memory on the compute
nodes, it can speed up the job startup time.
sbcast --compress=none /path/to/exe /tmp/exe
srun /tmp/exe
Huge pages
Huge pages are virtual memory pages which are bigger than the default page size of 4K bytes. Huge pages can improve memory performance for common access patterns on large data sets since it helps to reduce the number of virtual to physical address translations when compared to using the default 4KB.
To use huge pages for an application (with the 2 MB huge pages as an example):
module load craype-hugepages2M
cc -o mycode.exe mycode.c
And also load the same huge pages module at runtime.
Warning
Due to the huge pages memory fragmentation issue, applications may get Cannot allocate memory warnings or errors when there are not enough hugepages on the compute node, such as:
libhugetlbfs [nid0000xx:xxxxx]: WARNING: New heap segment map at 0x10000000 failed: Cannot allocate memory``
By default, The verbosity level of libhugetlbfs HUGETLB_VERBOSE is set
to 0 on ARCHER2 to suppress debugging messages. Users can adjust this
value to obtain more information on huge pages use.
When to Use Huge Pages
- For MPI applications, map the static data and/or heap onto huge pages.
- For an application which uses shared memory, which needs to be concurrently registered with the high speed network drivers for remote communication.
- For SHMEM applications, map the static data and/or private heap onto huge pages.
- For applications written in Unified Parallel C, Coarray Fortran, and other languages based on the PGAS programming model, map the static data and/or private heap onto huge pages.
- For an application doing heavy I/O.
- To improve memory performance for common access patterns on large data sets.
When to Avoid Huge Pages
- Applications sometimes consist of many steering programs in addition
to the core application. Applying huge page behavior to all
processes would not provide any benefit and would consume huge pages
that would otherwise benefit the core application. The runtime
environment variable
HUGETLB_RESTRICT_EXEcan be used to specify the susbset of the programs to use hugepages. - For certain applications if using hugepages either causes issues or slows down performance. One such example is that when an application forks more subprocesses (such as pthreads) and these threads allocate memory, the newly allocated memory are the default 4 KB pages.