Factors Affecting Job Scheduling on Quest

Jobs submitted to the Slurm scheduler on Quest are given a "priority" score that affects when they run.

If your job is waiting on the queue, the reason is most probably one of the following:

  • Your job's score is lower compared to others
  • Unavailable/occupied compute resources at that moment.


Total priority score is combination of several factors. These factors are the following:

i. Fair Share Quest's job scheduler uses a fair share mechanism to dynamically determine a score. The calculation is based on the comparison between your share of the resources and your actual usage of these resources. If you or other members of your allocation used large amounts of resources in the recent past, the priority of current jobs will be lower. Accordingly, your jobs will wait longer before they start if the scheduler queue is busy.

On the other hand, if the job queue is empty and the compute resources are idling, regardless of the priority, your jobs will run. You will never run out of compute hours in this model.

Fairshare also includes a recovery mechanism for job priority. The contribution of past resource usage to priority calculations decays over time. Without new usage, the job scores will be restored significantly within a month.

If you are using a general access allocation, the fair share scores of your jobs will be affected from the overall resource usage by all members of the allocation. If you are using a buy-in allocation that has its own compute nodes, your own usage of the those nodes will be the determining factor for your job's fair share score.

ii. Allocation Type There are two types of allocations for research projects on Quest. Research I allocations are ideal for small to moderate computational needs whereas Research II allocations require considerably more resources. Due to this difference in computational needs, a Research II allocation has a higher share of the system resources (or initial fairshare score) compared to a Research I allocation. This is equivalent to receiving more compute hours with a Research II than a Research I.

iii. Job Age Age is length of time an eligible job has been waiting in the queue. Jobs will accumulate priority proportional to their age. This can help overcome starting priority differences between jobs coming from other actors.

iv. Partition A priority score is associated with partition that the job uses. This is only applicable for certain buy-in allocations which have multiple partitions.

More detailed information about Slurm's multi-factor priority system, please see this page.

Backfill Scheduling

There is a secondary mechanism that starts lower priority jobs on slots reserved by higher priority jobs while these jobs are acquiring their full set of resources. This is called "backfill" scheduling which helps to increase the utilization of the compute nodes and guarantees no delay in starting the higher priority jobs. To benefit from this mechanism, it is important to accurately request resources (wall time, core, memory) so that the scheduler can find appropriate space on the resource map. Please review resource utilization page for different methods you can use to identify your job's needs.

From time to time, compute resources cannot be backfilled effectively and nodes/cores may appear idle while jobs are waiting on the queue.

See Also:

Keywords:quest, slurm, batch, hpc, queue   Doc ID:90274
Owner:Research Computing .Group:Northwestern
Created:2019-03-08 17:12 CDTUpdated:2019-04-15 13:16 CDT
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