# Prolog and Epilog Guide

Slurm supports a multitude of prolog and epilog programs. Note that for security reasons, these programs do not have a search path set. Either specify fully qualified path names in the program or set the PATH environment variable. The first table below identifies what prologs and epilogs are available for job allocations, when and where they run.

Parameter | Location | Invoked by | User | When executed
Prolog (from slurm.conf) | Compute or front end node | slurmd daemon | SlurmdUser (normally user root) | First job or job step initiation on that node (by default); PrologFlags=Alloc will force the script to be executed at job allocation
PrologSlurmctld (from slurm.conf) | Head node (where slurmctld daemon runs) | slurmctld daemon | SlurmctldUser | At job allocation
Epilog (from slurm.conf) | Compute or front end node | slurmd daemon | SlurmdUser (normally user root) | At job termination
EpilogSlurmctld (from slurm.conf) | Head node (where slurmctld daemon runs) | slurmctld daemon | SlurmctldUser | At job termination

This second table below identifies what prologs and epilogs are available for job step allocations, when and where they run.

Parameter | Location | Invoked by | User | When executed
SrunProlog (from slurm.conf) or srun --prolog | srun invocation node | srun command | User invoking srun command | Prior to launching job step
TaskProlog (from slurm.conf) | Compute node | slurmstepd daemon | User invoking srun command | Prior to launching job step
srun --task-prolog | Compute node | slurmstepd daemon | User invoking srun command | Prior to launching job step
TaskEpilog (from slurm.conf) | Compute node | slurmstepd daemon | User invoking srun command | Completion job step
srun --task-epilog | Compute node | slurmstepd daemon | User invoking srun command | Completion job step
SrunEpilog (from slurm.conf) or srun --epilog | srun invocation node | srun command | User invoking srun command | Completion job step

By default the Prolog script is only run on any individual node when it first sees a job step from a new allocation; it does not run the Prolog immediately when an allocation is granted. If no job steps from an allocation are run on a node, it will never run the Prolog for that allocation. This Prolog behaviour can be changed by the PrologFlags parameter. The Epilog, on the other hand, always runs on every node of an allocation when the allocation is released.

Prolog and Epilog scripts should be designed to be as short as possible and should not call Slurm commands (e.g. squeue, scontrol, sacctmgr, etc). Long running scripts can cause scheduling problems when jobs take a long time to start or finish. Slurm commands in these scripts can potentially lead to performance issues and should not be used.

The task prolog is executed with the same environment as the user tasks to be initiated. The standard output of that program is read and processed as follows:
export name=value sets an environment variable for the user task
unset name clears an environment variable from the user task
print ... writes to the task's standard output.
Special treatment is given to the SLURM_PROLOG_CPU_MASK variable when set in the task prolog. The variable is interpreted as a coma separated list of hex maps. It allows you to specify the CPU(s) that will be bound to a task and is applied using sched_setaffinity. The above functionality is limited to the task prolog script.

Unless otherwise specified, these environment variables are available to all of the programs.

- CUDA_MPS_ACTIVE_THREAD_PERCENTAGE Specifies the percentage of a GPU that should be allocated to the job. The value is set only if the gres/mps plugin is configured and the job requests those resources. Available in Prolog and Epilog only.
- CUDA_VISIBLE_DEVICES Specifies the GPU devices that should be allocated to the job. The value is set only if the gres/gpu or gres/mps plugin is configured and the job requests those resources. Note that the environment variable set for the job may differ from that set for the Prolog and Epilog if Slurm is configured to constrain the device files visible to a job using Linux cgroup. This is because the Prolog and Epilog programs run outside of any Linux cgroup while the job runs inside of the cgroup and may thus have a different set of visible devices. For example, if a job is allocated the device "/dev/nvidia1", then CUDA_VISIBLE_DEVICES will be set to a value of "1" in the Prolog and Epilog while the job's value of CUDA_VISIBLE_DEVICES will be set to a value of "0" (i.e. the first GPU device visible to the job). Available in Prolog and Epilog only.
- SLURM_ARRAY_JOB_ID If this job is part of a job array, this will be set to the job ID. Otherwise it will not be set. To reference this specific task of a job array, combine SLURM_ARRAY_JOB_ID with SLURM_ARRAY_TASK_ID (e.g. scontrol update ${SLURM_ARRAY_JOB_ID}_{$SLURM_ARRAY_TASK_ID} ...); Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_ARRAY_TASK_COUNT If this job is part of a job array, this will be set to the number of tasks in the array. Otherwise it will not be set. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_ARRAY_TASK_ID If this job is part of a job array, this will be set to the task ID. Otherwise it will not be set. To reference this specific task of a job array, combine SLURM_ARRAY_JOB_ID with SLURM_ARRAY_TASK_ID (e.g. scontrol update ${SLURM_ARRAY_JOB_ID}_{$SLURM_ARRAY_TASK_ID} ...); Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_ARRAY_TASK_MAX If this job is part of a job array, this will be set to the maximum task ID. Otherwise it will not be set. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_ARRAY_TASK_MIN If this job is part of a job array, this will be set to the minimum task ID. Otherwise it will not be set. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_ARRAY_TASK_STEP If this job is part of a job array, this will be set to the step size of task IDs. Otherwise it will not be set. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_CLUSTER_NAME Name of the cluster executing the job.
- SLURM_CONF Location of the slurm.conf file. Available in Prolog, SrunProlog, TaskProlog, Epilog, SrunEpilog and TaskEpilog.
- SLURM_CPUS_ON_NODE Count of processors available to the job on current node. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_DISTRIBUTION Distribution type for the job. Available in SrunProlog and SrunEpilog.
- SLURMD_NODENAME Name of the node running the task. In the case of a parallel job executing on multiple compute nodes, the various tasks will have this environment variable set to different values on each compute node. Available in Prolog, SrunProlog, TaskProlog, Epilog, SrunEpilog and TaskEpilog.
- SLURM_GPUS Count of the GPUs available to the job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_GTID Global Task IDs running on this node. Zero origin and comma separated. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_JOB_CPUS_PER_NODE Count of processors available per node. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_JOB_ACCOUNT Account name used for the job. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld SrunEpilog and TaskEpilog.
- SLURM_JOB_CONSTRAINTS Features required to run the job. Available in Prolog, PrologSlurmctld and EpilogSlurmctld only.
- SLURM_JOB_DERIVED_EC The highest exit code of all of the job steps. Available in EpilogSlurmctld only.
- SLURM_JOB_EXIT_CODE The exit code of the job script (or salloc). The value is the status as returned by the wait() system call (See wait(2)). Available in EpilogSlurmctld only.
- SLURM_JOB_EXIT_CODE2 The exit code of the job script (or salloc). The value has the format <exit>:<sig>. The first number is the exit code, typically as set by the exit() function. The second number is the signal that caused the process to terminate if it was terminated by a signal. Available in EpilogSlurmctld only.
- SLURM_JOB_GID Group ID of the job's owner.
- SLURM_JOB_GPUS GPU IDs allocated to the job (if any). Available in the Prolog only.
- SLURM_JOB_GROUP Group name of the job's owner. Available in PrologSlurmctld and EpilogSlurmctld only.
- SLURM_JOB_ID Job ID.
- SLURM_JOBID Job ID.
- SLURM_JOB_NAME Name of the job. Available in PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_JOB_NODELIST Nodes assigned to job. A Slurm hostlist expression. scontrol show hostnames can be used to convert this to a list of individual host names. Available in PrologSlurmctld SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_JOB_NUM_NODES Number of nodes assigned to a job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_JOB_PARTITION Partition that job runs in. Available in Prolog, PrologSlurmctld, SrunProlog, TaskProlog, EpilogSlurmctld, SrunEpilog and TaskEpilog.
- SLURM_JOB_QOS QOS assigned to job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_JOB_UID User ID of the job's owner.
- SLURM_JOB_USER User name of the job's owner.
- SLURM_LOCAL_GLOBALS_FILE Globals file used to set up the environment for the testsuite. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_LOCALID Node local task ID for the process within a job. Available in SrunProlog, Taskprolog, SrunEpilog and TaskEpilog.
- SLURM_NNODES Number of nodes assigned to a job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_NODE_ALIASES Contains the node name, communication address and hostname of a node. Used for cloud environments. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_NODEID ID of current node relative to other nodes in a multi-node job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_PRIO_PROCESS Scheduling priority (nice value) at the time of submission. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_PROCID The MPI rank (or relative process ID) of the current process. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_AS Resource limit on the job's address space. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_CORE Resource limit on the size of a core file the job is able to produce. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_CPU Resource limit on the amount of CPU time a job is able to use. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_DATA Resource limit on the size of a job's data segment. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_FSIZE Resource limit on the maximum size of files a job may create. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_MEMLOCK Resource limit on the bytes of data that may be locked into RAM. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_NOFILE Resource limit on the number of file descriptors that can be opened by the job. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_NPROC Resource limit on the number of processes that can be opened by the calling process. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_RSS Resource limit on the job's resident set size. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_RLIMIT_STACK Resource limit on the job's process stack. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_SCRIPT_CONTEXT Identifies which epilog or prolog program is currently running. The value is one of the following:
  - prolog_slurmctld
  - epilog_slurmctld
  - prolog_slurmd
  - epilog_slurmd
  - prolog_task
  - epilog_task
  - prolog_srun
  - epilog_srun
- SLURM_STEP_ID Step ID of the current job. Available in SrunProlog and SrunEpilog.
- SLURM_STEPID Step ID of the current job. Available in SrunProlog and SrunEpilog.
- SLURM_SUBMIT_DIR Directory from which the job was submitted or, if applicable, the directory specified by the -D, --chdir option. Available in SrunProlog, Taskprolog, SrunEpilog and TaskEpilog.
- SLURM_SUBMIT_HOST Host from which the job was submitted. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_TASK_PID Process ID of the process started for the task. Available in TaskProlog.
- SLURM_TASKS_PER_NODE Number of tasks per node. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_TOPOLOGY_ADDR Set to the names of network switches or nodes that may be involved in the job's communications. Starts with the top level switch down to the node name. A period is used to separate each hardware component name. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_TOPOLOGY_ADDR_PATTERN Set to the network component types that corresponds with the list of names from SLURM_TOPOLOGY_ADDR. Each component will be identified as either switch or node. A period is used to separate each component type. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.
- SLURM_WCKEY User name of the job's wckey (if any). Available in PrologSlurmctld and EpilogSlurmctld only.
- SLURM_WORKING_CLUSTER For use when using a Federation. Set to the remote sibling cluster's name, hostname, port, rpc version and plugin ID. Available in SrunProlog, TaskProlog, SrunEpilog and TaskEpilog.

Plugin functions may also be useful to execute logic at various well-defined points.

SPANK is another mechanism that may be useful to invoke logic in the user commands, slurmd daemon, and slurmstepd daemon.

### Failure Handling

If the Epilog fails (returns a non-zero exit code), this will result in the node being set to a DRAIN state. If the EpilogSlurmctld fails (returns a non-zero exit code), this will only be logged. If the Prolog fails (returns a non-zero exit code), this will result in the node being set to a DRAIN state and the job requeued in a held state (unless nohold_on_prolog_fail is configured in SchedulerParameters). If the PrologSlurmctld fails (returns a non-zero exit code), this will cause the job to be requeued. Only batch jobs can be requeued. Interactive jobs (salloc and srun) will be cancelled if the PrologSlurmctld fails.

Based upon work by Jason Sollom, Cray Inc. and used by permission.

Last modified 8 June 2021