Slurm Workload Manager - Quality of Service (QOS)

# Quality of Service (QOS)

One can specify a Quality of Service (QOS) for each job submitted to Slurm. The quality of service associated with a job will affect the job in three ways:

- Job Scheduling Priority
- Job Preemption
- Job Limits
- Partition QOS
- Other QOS Options
- Configuration
- Examples

The QOSs are defined in the Slurm database using the sacctmgr utility.

Jobs request a QOS using the "--qos=" option to the sbatch, salloc, and srun commands.

## Job Scheduling Priority

Job scheduling priority is made up of a number of factors as described in the priority/multifactor plugin. One of the factors is the QOS priority. Each QOS is defined in the Slurm database and includes an associated priority. Jobs that request and are permitted a QOS will incorporate the priority associated with that QOS in the job's multi-factor priority calculation.

To enable the QOS priority component of the multi-factor priority calculation, the "PriorityWeightQOS" configuration parameter must be defined in the slurm.conf file and assigned an integer value greater than zero.

A job's QOS only affects is scheduling priority when the multi-factor plugin is loaded.

## Job Preemption

Slurm offers two ways for a queued job to preempt a running job, free-up the running job's resources and allocate them to the queued job. See the Preemption description for details.

The preemption method is determined by the "PreemptType" configuration parameter defined in slurm.conf. When the "PreemptType" is set to "preempt/qos", a queued job's QOS will be used to determine whether it can preempt a running job. It is important to note that the QOS used to determine if a job is eligible for preemption is the QOS associated with the job and not a Partition QOS.

The QOS can be assigned (using sacctmgr) a list of other QOSs that it can preempt. When there is a queued job with a QOS that is allowed to preempt a running job of another QOS, the Slurm scheduler will preempt the running job.

The QOS option PreemptExemptTime specifies the minimum run time before the job is considered for preemption. The QOS option takes precedence over the global option of the same name. A Partition QOS with PreemptExemptTime takes precedence over a job QOS with PreemptExemptTime, unless the job QOS has the OverPartQOS flag enabled.

## Job Limits

Each QOS is assigned a set of limits which will be applied to the job. The limits mirror the limits imposed by the user/account/cluster/partition association defined in the Slurm database and described in the Resource Limits section. When limits for a QOS have been defined, they will take precedence over the association's limits.

## Partition QOS

A QOS can be attached to a partition. This means a partition will have all the same limits as a QOS. This also gives the ability of a true 'floating' partition, meaning if you assign all the nodes to a partition and then in the Partition's QOS limit the number of GrpTRES the partition will have access to all the nodes, but only be able to run on the number of resources in it.

The Partition QOS will override the job's QOS. If the opposite is desired you need to have the job's QOS have the 'OverPartQOS' flag which will reverse the order of precedence.

## Other QOS Options

- Flags Used by the slurmctld to override or enforce certain characteristics. Valid options are:
  - DenyOnLimit If set, jobs using this QOS will be rejected at submission time if they do not conform to the QOS 'Max' limits as stand-alone jobs. Jobs that go over these limits when other jobs are considered, but conform to the limits when considered individually will not be rejected. Instead they will pend until resources are available (as by default without DenyOnLimit). Group limits (e.g. GrpTRES) will also be treated like 'Max' limits (e.g. MaxTRESPerNode) and jobs will be denied if they would violate the limit as stand-alone jobs. This currently only applies to QOS and Association limits.
  - EnforceUsageThreshold If set, and the QOS also has a UsageThreshold, any jobs submitted with this QOS that fall below the UsageThreshold will be held until their Fairshare Usage goes above the Threshold.
  - NoReserve If this flag is set and backfill scheduling is used, jobs using this QOS will not reserve resources in the backfill schedule's map of resources allocated through time. This flag is intended for use with a QOS that may be preempted by jobs associated with all other QOS (e.g use with a "standby" QOS). If this flag is used with a QOS which can not be preempted by all other QOS, it could result in starvation of larger jobs.
  - PartitionMaxNodes If set, jobs using this QOS will be able to override the requested partition's MaxNodes limit.
  - PartitionMinNodes If set, jobs using this QOS will be able to override the requested partition's MinNodes limit.
  - OverPartQOS If set, jobs using this QOS will be able to override any limits used by the requested partition's QOS limits.
  - PartitionTimeLimit If set, jobs using this QOS will be able to override the requested partition's TimeLimit.
  - RequiresReservation If set, jobs using this QOS must designate a reservation when submitting a job. This option can be useful in restricting usage of a QOS that may have greater preemptive capability or additional resources to be allowed only within a reservation.
  - NoDecay If set, this QOS will not have its GrpTRESMins, GrpWall and UsageRaw decayed by the slurm.conf PriorityDecayHalfLife or PriorityUsageResetPeriod settings. This allows a QOS to provide aggregate limits that, once consumed, will not be replenished automatically. Such a QOS will act as a time-limited quota of resources for an association that has access to it. Account/user usage will still be decayed for associations using the QOS. The QOS GrpTRESMins and GrpWall limits can be increased or the QOS RawUsage value reset to 0 (zero) to again allow jobs submitted with this QOS to run (if pending with QOSGrp{TRES}MinutesLimit or QOSGrpWallLimit reasons, where {TRES} is some type of trackable resource).
  - UsageFactorSafe If set, and AccountingStorageEnforce includes Safe, jobs will only be able to run if the job can run to completion with the UsageFactor applied.
- GraceTime Preemption grace time to be extended to a job which has been selected for preemption.

- UsageFactor A float that is factored into a job’s TRES usage (e.g. RawUsage, TRESMins, TRESRunMins). For example, if the usagefactor was 2, for every TRESBillingUnit second a job ran it would count for 2. If the usagefactor was .5, every second would only count for half of the time. A setting of 0 would add no timed usage from the job.

  The usage factor only applies to the job's QOS and not the partition QOS.

  If the UsageFactorSafe flag is set and AccountingStorageEnforce includes Safe, jobs will only be able to run if the job can run to completion with the UsageFactor applied.

  If the UsageFactorSafe flag is not set and AccountingStorageEnforce includes Safe, a job will be able to be scheduled without the UsageFactor applied and will be able to run without being killed due to limits.

  If the UsageFactorSafe flag is not set and AccountingStorageEnforce does not include Safe, a job will be able to be scheduled without the UsageFactor applied and could be killed due to limits.

  See AccountingStorageEnforce in slurm.conf man page.

  Default is 1. To clear a previously set value use the modify command with a new value of -1.

- UsageThreshold A float representing the lowest fairshare of an association allowable to run a job. If an association falls below this threshold and has pending jobs or submits new jobs those jobs will be held until the usage goes back above the threshold. Use sshare to see current shares on the system.

## Configuration

To summarize the above, the QOSs and their associated limits are defined in the Slurm database using the sacctmgr utility. The QOS will only influence job scheduling priority when the multi-factor priority plugin is loaded and a non-zero "PriorityWeightQOS" has been defined in the slurm.conf file. The QOS will only determine job preemption when the "PreemptType" is defined as "preempt/qos" in the slurm.conf file. Limits defined for a QOS (and described above) will override the limits of the user/account/cluster/partition association.

## QOS examples

QOS manipulation examples. All QOS operations are done using the sacctmgr command. The default output of 'sacctmgr show qos' is very long given the large number of limits and options available so it is best to use the format option which filters the display.

By default when a cluster is added to the database a default qos named normal is created.

    $ sacctmgr show qos format=name,priority
          Name   Priority
    ---------- ----------
        normal          0

Add a new QOS

    $ sacctmgr add qos zebra
     Adding QOS(s)
      zebra
     Settings
      Description    = QOS Name

    $ sacctmgr show qos format=name,priority
          Name   Priority
    ---------- ----------
        normal          0
         zebra          0

Set QOS priority

    $ sacctmgr modify qos zebra set priority=10
     Modified qos...
      zebra

    $ sacctmgr show qos format=name,priority
          Name   Priority
    ---------- ----------
        normal          0
         zebra         10

Set some other limits

    $ sacctmgr modify qos zebra set GrpTRES=cpu=24
     Modified qos...
      zebra

    $ sacctmgr show qos format=name,priority,GrpTRES
          Name   Priority       GrpTRES
    ---------- ---------- -------------
        normal          0
         zebra         10        cpu=24

Add a QOS to a user account

    $ sacctmgr modify user crock set qos=zebra

    $ sacctmgr show assoc format=cluster,user,qos
       Cluster       User                  QOS
    ---------- ---------- --------------------
    canis_major                          normal
    canis_major      root                normal
    canis_major                          normal
    canis_major     crock                zebra

Users can belong to multiple QOSs

    $ sacctmgr modify user crock set qos+=alligator
    $ sacctmgr show assoc format=cluster,user,qos
       Cluster       User                  QOS
    ---------- ---------- --------------------
    canis_major                          normal
    canis_major      root                normal
    canis_major                          normal
    canis_major     crock       alligator,zebra

Finally, delete a QOS

    $ sacctmgr delete qos alligator
     Deleting QOS(s)...
      alligator

Last modified 07 April 2023