About processor reservation
When parallel jobs have to compete with sequential jobs for job slots, the slots that become available are likely to be taken immediately by a sequential job. Parallel jobs need multiple job slots to be available before they can be dispatched. If the cluster is always busy, a large parallel job could be pending indefinitely. The more processors a parallel job requires, the worse the problem is.
Processor reservation solves this problem by reserving job slots as they become available, until there are enough reserved job slots to run the parallel job.
You might want to configure processor reservation if your cluster has a lot of sequential jobs that compete for job slots with parallel jobs.
How processor reservation works
Processor reservation is disabled by default.
If processor reservation is enabled, and a parallel job cannot be dispatched because there are not enough job slots to satisfy its minimum processor requirements, the job slots that are currently available is reserved and accumulated.
A reserved job slot is unavailable to any other job. To avoid deadlock situations in which the system reserves job slots for multiple parallel jobs and none of them can acquire sufficient resources to start, a parallel job gives up all its reserved job slots if it has not accumulated enough to start within a specified time. The reservation time starts from the time the first slot is reserved. When the reservation time expires, the job cannot reserve any slots for one scheduling cycle, but then the reservation process can begin again.
If you specify first execution host candidates at the job or queue level, LSF tries to reserve a job slot on the first execution host. If LSF cannot reserve a first execution host job slot, it does not reserve slots on any other hosts.