Administering Platform LSF - SAS

Chapter 34
Running Parallel Jobs
Allowing Jobs to Use Reserved Job Slots
About backfill scheduling
How backfilling works
Example
By default, a reserved job slot cannot be used by another job. To make better
use of resources and improve performance of LSF, you can configure backfill
scheduling. Backfill scheduling allows other jobs to use the reserved job slots,
as long as the other jobs will not delay the start of another job. Backfilling,
together with processor reservation, allows large parallel jobs to run while not
underutilizing resources.
In a busy cluster, processor reservation helps to schedule large parallel jobs
sooner. However, by default, reserved processors remain idle until the large
job starts. This degrades the performance of LSF because the reserved
resources are idle while jobs are waiting in the queue.
Backfill scheduling allows the reserved job slots to be used by small jobs that
can run and finish before the large job starts. This improves the performance
of LSF because it increases the utilization of resources.
For backfill scheduling, LSF assumes that a job will run until its run limit
expires. Backfill scheduling works most efficiently when all the jobs in the
cluster have a run limit.
Since jobs with a shorter run limit have more chance of being scheduled as
backfill jobs, users who specify appropriate run limits in a backfill queue will
be rewarded by improved turnaround time.
Once the big parallel job has reserved sufficient job slots, LSF calculates the
start time of the big job, based on the run limits of the jobs currently running
in the reserved slots. LSF cannot backfill if the big job is waiting for a job that
has no run limit defined.
If LSF can backfill the idle job slots, only jobs with run limits that expire before
the start time of the big job will be allowed to use the reserved job slots. LSF
cannot backfill with a job that has no run limit.
Administering Platform LSF 453