Preemption of jobs
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|Preemption of jobs by high-priority jobs
Slurm supports job preemption_, the act of stopping one or more "low-priority" jobs to let a "high-priority" job run.
Job preemption_ is implemented as a variation of Slurm's Gang Scheduling logic.
When a high-priority job has been allocated resources that have already been allocated to one or more low priority jobs, the low priority job(s) are preempted.
The low priority job(s) can resume once the high priority job completes.
Alternately, the low priority job(s) can be requeued and started using other resources if so configured in newer versions of Slurm.
Preemption is configured in slurm.conf_.
.. _preemption: https://slurm.schedmd.com/preempt.html
Slurm job scheduler
- Scheduler configuration
- Multifactor Priority Plugin scheduler
- Scheduling commands
Jump to our Slurm top-level page.
The SchedulerType configuration parameter controls how queued jobs are executed. See the Scheduling_Configuration_Guide.
We use the backfill scheduler in slurm.conf:
but there are some backfill parameters that should be considered (see slurm.conf), for example:
The importance of bf_window is explained as:
- The default value is 1440 minutes (one day). A value at least as long as the highest allowed time limit is generally advisable to prevent job starvation. In order to limit the amount of data managed by the backfill scheduler, if the value of bf_window is increased, then it is generally advisable to also increase bf_resolution.
So you must configure bf_window according to the longest possible MaxTime in all partitions in slurm.conf:
PartitionName= ... MaxTime=XXX
The scontrol top job_list command is documented as:
Move the specified job IDs to the top of the queue of jobs belonging to the identical user ID, partition name, account, and QOS. The job_list argument is a comma separated ordered list of job IDs. Any job not matching all of those fields will not be effected. Only jobs submitted to a single partition will be effected. This operation changes the order of jobs by adjusting job nice values. The net effect on that user's throughput will be negligible to slightly negative. This operation is disabled by default for non-privileged (non-operator, admin, SlurmUser, or root) users. This operation may be enabled for non-privileged users by the system administrator by including the option "enable_user_top" in the SchedulerParameters configuration parameter.
While scontrol top job_list may be useful for the superuser to help with user requests, it is not recommended to configure SchedulerParameters=enable_user_top. The Slurm 17.11 news page (https://slurm.schedmd.com/news.html) highlights this change:
Regular user use of "scontrol top" command is now disabled. Use the configuration parameter "SchedulerParameters=enable_user_top" to enable that functionality. The configuration parameter "SchedulerParameters=disable_user_top" will be silently ignored.
There does not seem to be any documentation of why the scontrol top job_list is unwarranted, but we have made the following observations of a bad side effect:
- A user requests a high priority for a job, and the superuser grants a negative nice value with scontrol update jobid=10208 nice=-10000.
- The user can now assign a negative nice value to his other jobs with scontrol top jobid=10209,10210, thereby jumping ahead of normal jobs in the queue.
Slurm supports job preemption, the act of stopping one or more "low-priority" jobs to let a "high-priority" job run. Job preemption is implemented as a variation of Slurm's Gang Scheduling logic. When a high-priority job has been allocated resources that have already been allocated to one or more low priority jobs, the low priority job(s) are preempted. The low priority job(s) can resume once the high priority job completes. Alternately, the low priority job(s) can be requeued and started using other resources if so configured in newer versions of Slurm.
Preemption is configured in slurm.conf.
An example slurm.conf fairshare configuration may be:
PriorityType=priority/multifactor PriorityDecayHalfLife=7-0 PriorityFavorSmall=NO PriorityMaxAge=10-0 PriorityWeightAge=100000 PriorityWeightFairshare=1000000 PriorityWeightJobSize=100000 PriorityWeightPartition=100000 PriorityWeightQOS=100000 PropagateResourceLimitsExcept=MEMLOCK PriorityFlags=ACCRUE_ALWAYS,FAIR_TREE AccountingStorageEnforce=associations,limits,qos,safe
PriorityWeightXXX values are all 32-bit integers. The final Job Priority is a 32-bit integer.
IMPORTANT: Set PriorityWeight values high to generate wide range of job priorities.
sacctmgr show qos format=name,priority
To enforce user jobs to have a QOS you must (at least) have:
- associations - This will prevent users from running jobs if their association is not in the database. This option will prevent users from accessing invalid accounts.
- limits - This will enforce limits set to associations. By setting this option, the 'associations' option is also set.
- qos - This will require all jobs to specify (either overtly or by default) a valid qos (Quality of Service). QOS values are defined for each association in the database. By setting this option, the 'associations' option is also set.
- safe - limits and associations will automatically be set.
The Quality of Service (QOS) Factor is defined in the Multifactor_Priority_Plugin page as:
Each QOS can be assigned an integer priority. The larger the number, the greater the job priority will be for jobs that request this QOS. This priority value is then normalized to the highest priority of all the QOS's to become the QOS factor.
A non-zero weight must be defined in slurm.conf, for example:
To enable any limit enforcement you must at least have:
in your slurm.conf, otherwise, even if you have limits set, they will not be enforced. Other options for AccountingStorageEnforce and the explanation for each are found on the Resource_Limits document.
It is desirable to prevent individual users from flooding the queue with jobs, in case there are idle nodes available, because it may block future jobs by other users. Note:
With Slurm it appears that the only way to achieve user job throttling is the following:
The GrpTRESRunMins limits can be applied to associations (accounts or users) as well as QOS. Set the limit by:
sacctmgr modify association where name=XXX set GrpTRESRunMin=cpu=1000000 # For an account/user asociation sacctmgr modify qos where name=some_QOS set GrpTRESRunMin=cpu=1000000 # For a QOS
If some partition XXX (for example big memory nodes) should have a higher priority, this is explained in Multifactor_Priority_Plugin by:
(PriorityWeightPartition) * (partition_factor) +
The Partition factor is controlled in slurm.conf, for example:
PartitionName=XXX ... PriorityJobFactor=10 PriorityWeightPartition=1000
View scheduling information for the Multifactor_Priority_Plugin by the commands:
sprio - view the factors that comprise a job's scheduling priority:
sprio # List job priorities sprio -l # List job priorities including username etc. sprio -w # List weight factors used by the multifactor scheduler
sshare - Tool for listing the shares of associations to a cluster:
sshare sshare -l # Long listing with additional information sshare -a # Listing with also user information
sdiag - Scheduling diagnostic tool for Slurm