Slurm job scheduler 

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Prerequisites 

Before configuring the Multifactor_Priority_Plugin scheduler, you must first configure Slurm accounting.

Scheduler configuration 

The SchedulerType configuration parameter controls how queued jobs are executed, see the Scheduling_Configuration_Guide.

SchedulerType options are sched/backfill, which performs backfill scheduling, and sched/builtin, which attempts to schedule jobs in a strict priority order within each partition/queue.

There is also a SchedulerParameters configuration parameter which can specify a wide range of parameters as described below. This first set of parameters applies to all scheduling configurations. See the slurm.conf man page for more details:

default_queue_depth=# - Specifies the number of jobs to consider for scheduling on each event that may result in a job being scheduled. Default value is 100 jobs. Since this happens frequently, a relatively small number is generally best.
defer - Do not attempt to schedule jobs individually at submit time. Can be useful for high-throughput computing.
max_switch_wait=# - Specifies the maximum time a job can wait for desired number of leaf switches. Default value is 300 seconds.
partition_job_depth=# - Specifies how many jobs are tested in any single partition, default value is 0 (no limit).
sched_interval=# - Specifies how frequently, in seconds, the main scheduling loop will execute and test all pending jobs. The default value is 60 seconds.

Backfill scheduler 

We use the backfill scheduler in slurm.conf:

SchedulerType=sched/backfill
SchedulerParameters=kill_invalid_depend,defer,bf_continue

but there are some backfill parameters that should be considered (see slurm.conf), for example:

...bf_interval=60,bf_max_job_start=20,bf_resolution=600,bf_window=11000

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

scontrol top command 

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.

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.

Multifactor Priority Plugin scheduler 

A sophisticated Multifactor_Priority_Plugin provides a very versatile facility for ordering the queue of jobs waiting to be scheduled. See the PriorityXXX parameters in the slurm.conf file.

Multifactor configuration 

The Fairshare is configured with PriorityX parameters in the Configuration section of the Multifactor_Priority_Plugin page, also documented in the slurm.conf page:

PriorityType
PriorityDecayHalfLife
PriorityCalcPeriod
PriorityUsageResetPeriod
PriorityFavorSmall
PriorityMaxAge
PriorityWeightAge
PriorityWeightFairshare
PriorityWeightJobSize
PriorityWeightPartition
PriorityWeightQOS
PriorityWeightTRES

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.

Fairshare 

See the page explaining the Classic_fairshare algorithm. A good page on Fairshare and Job Accounting explains many of the concepts used.

Several Fairshare Algorithms may be used:

Fair Tree Fairshare Algorithm fair_tree.
Depth-Oblivious Fair-share Factor priority_multifactor3.

The Fairshare algorithm is explained in the Multifactor_Priority_Plugin page. Note the meaning of the special value fairshare=parent:

If all users in an account are configured with:
```
FairShare=parent
```
the result is that all the jobs drawing from that account will get the same fairshare priority, based on the accounts total usage. No additional fairness is added based on users individual usage.

Use one of these commands to print the fairshare number of user xxx:

sacctmgr -nP list associations user=xxx format=fairshare
sshare -lU -u xxx

or get a complete fairshare listing with:

sacctmgr -sor show associations format=user,account,fairshare

Quality of Service (QOS)

One can specify a Quality of Service (QOS) for each job submitted to Slurm. A description and example are in the QOS page. Example QOS configurations are:

sacctmgr modify qos normal set priority=50
sacctmgr add qos high
sacctmgr modify qos high set priority=100

Example:

sacctmgr show qos format=name,priority

To enforce user jobs to have a QOS you must (at least) have:

AccountingStorageEnforce=qos

see the slurm.conf and Resource_Limits documents. The AccountingStorageEnforce options include:

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:

PriorityWeightQOS=100000

Resource Limits 

To enable any limit enforcement you must at least have:

AccountingStorageEnforce=limits

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.

Limiting (throttling) jobs in the queue 

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 the MAUI scheduler (an alternative product to Slurm) this is called Throttling_Policies.

With Slurm it appears that the only way to achieve user job throttling is the following:

Using the GrpTRESRunMins parameter defined in the Resource_Limits document. See also the TRES definition.

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
sacctmgr modify qos where name=some_QOS set MaxTRESPU=cpu=1000             # QOS Max TRES per user

Partition factor priority 

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

Scheduling commands 

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