Administering Platform LSF - SAS

Allowing Jobs to Use Reserved Job Slots
In this scenario, assume the cluster consists of a 4-CPU multiprocessor host.
1 A sequential job (job1) with a run limit of 2 hours is submitted and gets
started at 8:00 am (figure a).
2 Shortly afterwards, a parallel job (job2) requiring all 4 CPUs is submitted.
It cannot start right away because job1 is using one CPU, so it reserves the
remaining 3 processors (figure b).
3 At 8:30 am, another parallel job (job3) is submitted requiring only two
processors and with a run limit of 1 hour. Since job2 cannot start until
10:00am (when job1 finishes), its reserved processors can be backfilled by
job3 (figure c). Therefore job3 can complete before job2's start time,
making use of the idle processors.
4 Job3 will finish at 9:30am and job1 at 10:00am, allowing job2 to start
shortly after 10:00am.
In this example, if job3's run limit was 2 hours, it would not be able to backfill
job2's reserved slots, and would have to run after job2 finishes.
Limitations ◆ A job will not have an estimated start time immediately after mbatchd is
reconfigured.
◆ Jobs in a backfill queue cannot be preempted (a job in a backfill queue
might be running in a reserved job slot, and starting a new job in that slot
might delay the start of the big parallel job):
❖ A backfill queue cannot be preemptable.
❖ A preemptive queue whose priority is higher than the backfill queue
cannot preempt the jobs in backfill queue.
Backfilling and job
slot limits
A backfill job borrows a job slot that is already taken by another job. The
backfill job will not run at the same time as the job that reserved the job slot
first. Backfilling can take place even if the job slot limits for a host or processor
have been reached. Backfilling cannot take place if the job slot limits for users
or queues have been reached.
Configuring backfill scheduling
Configuring a
backfill queue
Example
Backfill scheduling is enabled at the queue level. Only jobs in a backfill queue
can backfill reserved job slots. If the backfill queue also allows processor
reservation, then backfilling can occur among jobs within the same queue.
To configure a backfill queue, define BACKFILL in lsb.queues.
Specify Y to enable backfilling. To disable backfilling, specify N or blank space.
BACKFILL=Y
Enforcing run limits
Backfill scheduling works most efficiently when all the jobs in a cluster have a
run limit specified at the job level (bsub -W). You can use the external
submission executable, esub, to make sure that all users specify a job-level run
limit.
Otherwise, you can specify ceiling and default run limits at the queue level
(RUNLIMIT in lsb.queues ).
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Administering Platform LSF