Administering Platform LSF - SAS

How Preemptive Scheduling Works
Chapter 12
Preemptive Scheduling
Preemptive scheduling occurs when two jobs compete for the same resource.
If a high-priority job is pending, LSF can suspend a lower priority job that is
running, and then start the high-priority job using the job slot that becomes
available. For this to happen, the high-priority job must be pending in a
preemptive queue, or the low-priority job must belong to a preemptable
queue.
By default, when multiple preemptable jobs exist (low-priority jobs holding the
required resource), LSF preempts a job from the least-loaded host.
The preempted job is resumed as soon as more resources become available; it
does not necessarily have to wait for the preempting job to finish.
Queues that can preempt others are more aggressive in scheduling jobs
because a resource that is not available to a low-priority queue might be
available (by preemption) to a high-priority queue.
By default, job slot limits are enforced based on the number of job slots taken
by running and suspended jobs. With preemptive scheduling, the suspended
jobs don’t count against certain job slot limits. This means that when one job
is suspended, another job can be started in its place.
LSF makes sure that the number of running jobs never exceeds the job slot
limits. When LSF tries to place a preemptive job, LSF considers each job slot
limit, but for certain job slot limits, LSF only counts the job slots used by
running jobs that are not preemptable. Then, if starting the preemptive job
would violate job slot limits, LSF suspends one or more low-priority jobs.
Job slot limits affected by preemptive scheduling
When you enable preemptive scheduling, you automatically affect the
following job slot limits:
◆ Total job slot limit for hosts, specified at the host level (SLOTS and HOSTS
in lsb.resources)
◆ Total job slot limit for individual users, specified at the user level (SLOTS
and USERS in lsb.resources); by default, suspended jobs still count
against the limit for user groups
You can configure preemptive scheduling to affect following limits:
◆ Total job slot limit for user groups, specified at the user level (SLOTS and
USERS in lsb.resources); if preemptive scheduling is enabled,
suspended jobs never count against the limit for individual users
◆ Total number of jobs for users and user groups, specified at the host level
(SLOTS, PER_USER=all, and HOSTS in lsb.resources)
◆ Per-processor job slot limit for individual users, specified at the user level
(SLOTS_PER_PROCESSOR, USERS, and PER_HOST=all in
lsb.resources)
◆ Per-processor job slot limit for user groups, specified at the user level
(SLOTS_PER_PROCESSOR, USERS, and PER_HOST=all in
lsb.resources)
Administering Platform LSF 179