MOAB Scheduler Policies on Guillimin


On any cluster, the  scheduler has policies and  limits imposed on accounts and users in order to use the available resources efficiently and fairly. This is implemented in a fairshare policy.  The limits also vary according to allocations. These limits are the fairshare targets for each group. Fairshare is based on the number of processors allocated to a PI and are assigned to their group's account in the scheduler's configuration. This is used to determine resources available for all the users under that particular account.

MOAB determines when and to which node  jobs are sent for processing. It monitors the entire job queue,  and assigns a priority to each job. The jobs are prioritized based on requested versus available resources and current usage versus fairshare target. It updates the status of the queue every 2 minutes. Parameters and policy settings can be tuned to efficiently handle a wide range of system workloads

The Limits imposed by the Scheduler

This refers to limits on the number of jobs in the queue which are enforced by the scheduler. The largest factors in determining limits  in numbers of jobs are the Maximum Processor Seconds (MAXPS) and the Maximum Processors (MaxPROC) for each account. The MAXPS is the total number of processor core seconds (ps) allocated for each group account. It is based on fairshare value times 2,592,000 seconds, which corresponds to 30 days (24/7). The MAXPS window on Guillimin is 30 days.

Explaining by an example, an account with a fairshare value of five processor cores has a MAXPS of 12,960,000 ps (5 proc * 2,592,000 sec). Such an account will exhaust the MAXPS window if 15 10-day single-core jobs are started. Any job sent to the queue after this will have the message "blocked due to MAXPS limit exceeded". Such jobs will have to wait until the outstanding scheduled processor seconds are less than the MAXPS value.

In fact, once jobs are started, the maximum ps in use decreases as the remaining time for the job to finish shortens. Finally, other jobs combinations can be 10 15-days jobs, 75 2-days jobs, etc. The usual scenario is to have a mixture of short and long jobs.

Users can submit as many jobs but they cannot be scheduled to run if their groups MAXPS value is exceeded. They instead enter into a "HOLD" state. If the MAXPS of the group is not reached but the resources are not available, the jobs enter into "IDLE" state and will run once the requested resources become available.

 Signifcant benefits of MAXPS

Other Limits

Limits can always be adjusted for work with deadlines that need resources beyond the current maximum assigned.  If this is the case, the PI or user should contact the Guillimin team and request a reservation.