# Resource Limits

Familiarity with Slurm's Accounting web page is strongly recommended before use of this document.

Note for users of Maui or Moab schedulers:
Maui and Moab are not integrated with Slurm's resource limits, but should use their own resource limits mechanisms.

## Hierarchy

Slurm's hierarchical limits are enforced in the following order with Job QOS and Partition QOS order being reversible by using the QOS flag 'OverPartQOS':

1. Partition QOS limit
2. Job QOS limit
3. User association
4. Account association(s), ascending the hierarchy
5. Root/Cluster association
6. Partition limit
7. None

Note: If limits are defined at multiple points in this hierarchy, the point in this list where the limit is first defined will be used. Consider the following example:

- MaxJobs=20 and MaxSubmitJobs is undefined in the partition QOS
- No limits are set in the job QOS and
- MaxJobs=4 and MaxSubmitJobs=50 in the user association

The limits in effect will be MaxJobs=20 and MaxSubmitJobs=50.

## Configuration

Scheduling policy information must be stored in a database as specified by the AccountingStorageType configuration parameter in the slurm.conf configuration file. Information can be recorded in a MySQL or MariaDB database. For security and performance reasons, the use of SlurmDBD (Slurm Database Daemon) as a front-end to the database is strongly recommended. SlurmDBD uses a Slurm authentication plugin (e.g. MUNGE). SlurmDBD also uses an existing Slurm accounting storage plugin to maximize code reuse. SlurmDBD uses data caching and prioritization of pending requests in order to optimize performance. While SlurmDBD relies upon existing Slurm plugins for authentication and database use, the other Slurm commands and daemons are not required on the host where SlurmDBD is installed. Only the slurmdbd and slurm-plugins RPMs are required for SlurmDBD execution.

Both accounting and scheduling policies are configured based upon an association. An association is a 4-tuple consisting of the cluster name, bank account, user and (optionally) the Slurm partition. In order to enforce scheduling policy, set the value of AccountingStorageEnforce. This option contains a comma separated list of options you may want to enforce. The valid options are:

- 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 - This will ensure a job will only be launched when using an association or qos that has a GrpCPUMins limit set if the job will be able to run to completion. Without this option set, jobs will be launched as long as their usage hasn't reached the cpu-minutes limit which can lead to jobs being launched but then killed when the limit is reached. By setting this option, both the 'associations' option and the 'limits' option are set automatically.
- wckeys - This will prevent users from running jobs under a wckey that they don't have access to. By using this option, the 'associations' option is also set. The 'TrackWCKey' option is also set to true.

(NOTE: The association is a combination of cluster, account, user names and optional partition name.)
Without AccountingStorageEnforce being set (the default behavior) jobs will be executed based upon policies configured in Slurm on each cluster.
It is advisable to run without the 'limits' option set when running a scheduler on top of Slurm, like Moab, that does not update in real time its limits per association.

## Tools

The tool used to manage accounting policy is sacctmgr. It can be used to create and delete cluster, user, bank account, and partition records plus their combined association record. See man sacctmgr for details on this tools and examples of its use.

Changes made to the scheduling policy are uploaded to the Slurm control daemons on the various clusters and take effect immediately. When an association is deleted, all running or pending jobs which belong to that association are immediately canceled. When limits are lowered, running jobs will not be canceled to satisfy the new limits, but the new lower limits will be enforced.

## Limits in both Associations and QOS

When dealing with Associations, most of these limits are available not only for a user association, but also for each cluster and account. If a new association is created for some user and a scheduling policy option is not specified the default will be: the option for the cluster/account pair, and if both are not specified then the option for the cluster, and if that also is not specified then no limit will apply.

- GrpTRESMins= The total number of TRES minutes that can possibly be used by past, present and future jobs running from an association and its children or QOS. If this limit is reached all jobs running in this group will be killed, and no new jobs will be allowed to run. This usage is decayed (at a rate of PriorityDecayHalfLife). It can also be reset (according to PriorityUsageResetPeriod) in order to allow jobs to run against the association tree or QOS again. This limit only applies when using the Priority Multifactor plugin.
- GrpTRESRunMins= Used to limit the combined total number of TRES minutes used by all jobs running with an association and its children or QOS. This takes into consideration time limit of running jobs and consumes it, if the limit is reached no new jobs are started until other jobs finish to allow time to free up.
- GrpTRES= The total count of TRES able to be used at any given time from jobs running from an association and its children or QOS. If this limit is reached new jobs will be queued but only allowed to run after resources have been relinquished from this group.
- GrpJobs= The total number of jobs able to run at any given time from an association and its children QOS. If this limit is reached new jobs will be queued but only allowed to run after previous jobs complete from this group.
- GrpSubmitJobs= The total number of jobs able to be submitted to the system at any given time from an association and its children or QOS. If this limit is reached new submission requests will be denied until previous jobs complete from this group.
- GrpWall= The maximum wall clock time any job submitted to this group can run for. If this limit is reached submission requests will be denied.
- MaxTRESMinsPerJob= A limit of TRES minutes to be used by a job. If this limit is reached the job will be killed if not running in Safe mode, otherwise the job will pend until enough time is given to complete the job.
- MaxTRESPerJob= The maximum size in TRES any given job can have from the association/QOS. If this limit is reached the job will be denied at submission if the QOS running the job has the DenyOnLimit flag set, otherwise the job will pend.
- MaxTRESPerNode= The maximum size in TRES each node in a job allocation can use. If this limit is reached the job will be denied at submission if the QOS running the job has the DenyOnLimit flag set, otherwise the job will pend.
- MaxJobs= The total number of jobs able to run at any given time for the given association/QOS. If this limit is reached new jobs will be queued but only allowed to run after previous jobs complete from the association/QOS.
- MaxSubmitJobs= The maximum number of jobs able to be submitted to the system at any given time from the given association/QOS. If this limit is reached new submission requests will be denied until previous jobs complete from this association.
- MaxWallDurationPerJob= The maximum wall clock time any job submitted can run for the given association/QOS. If this limit is reached the job will be denied at submission.

## Association specific scheduling policies supported

These represent the scheduling policies unique to associations. Shared policies and limits QOS has in common are listed later.

- Fairshare= Integer value used for determining priority. Essentially this is the amount of claim this association and it's children have to the above system. Can also be the string "parent", when used on a user this means that the parent association is used for fairshare. If Fairshare=parent is set on an account, that account's children will be effectively re-parented for fairshare calculations to the first parent of their parent that is not Fairshare=parent. Limits remain the same, only it's fairshare value is affected.
- QOS= comma separated list of QOS's an association is able to run.

## QOS specific limits supported

- MaxTRESPerUser= The maximum number of limited TRES a user can request (combo of all jobs running or pending). If this limit is breached the job will pend unless the QOS running the job has the DenyOnLimit flag set, which will cause the job to be denied at submission.
- MinTRESPerJob= The minimum size in TRES any given job can have from when using the requested QOS. If this limit is breached the job will pend unless the QOS running the job has the DenyOnLimit flag set, which will cause the job to be denied at submission.

The MaxNodes and MaxWall options already exist in Slurm's configuration on a per-partition basis, but the above options provide the ability to impose limits on a per-user basis. The MaxJobs option provides an entirely new mechanism for Slurm to control the workload any individual may place on a cluster in order to achieve some balance between users.

Fair-share scheduling is based upon the hierarchical bank account data maintained in the Slurm database. More information can be found in the priority/multifactor plugin description.

Last modified 19 October 2015