How memory reservation for pending jobs works

Amount of memory reserved

The amount of memory reserved is based on the currently available memory when the job is pending. For example, if LIM reports that a host has 300 MB of memory available, the job submitted by the following command:

bsub -R "rusage[mem=400]" -q reservation my_job

will be pending and reserve the 300 MB of available memory. As other jobs finish, the memory that becomes available is added to the reserved memory until 400 MB accumulates, and the job starts.

No memory is reserved if no job slots are available for the job because the job could not run anyway, so reserving memory would waste the resource.

Only memory is accumulated while the job is pending; other resources specified on the rusage string are only reserved when the job is running. Duration and decay have no effect on memory reservation while the job is pending.

How long memory is reserved (MAX_RESERVE_TIME)

Reserved memory expires at the end of the time period represented by the number of dispatch cycles specified by the value of MAX_RESERVE_TIME set on the RESOURCE_RESERVE parameter. If a job has not accumulated enough memory to start by the time MAX_RESERVE_TIME expires, it releases all its reserved memory so that other pending jobs can run. After the reservation time expires, the job cannot reserve slots or memory for one scheduling session, so other jobs have a chance to be dispatched. After one scheduling session, the job can reserve available resources again for another period that is specified by MAX_RESERVE_TIME.



The following queues are defined in lsb.queues:

Begin Queue 
QUEUE_NAME = reservation 
DESCRIPTION = For resource reservation 
End Queue


Assume one host in the cluster with 10 CPUs and 1 GB of free memory currently available.

Sequential jobs

Each of the following sequential jobs requires 400 MB of memory and runs for 300 minutes.

Job 1:

bsub -W 300 -R "rusage[mem=400]" -q reservation myjob1

The job starts running, using 400M of memory and one job slot.

Job 2:

Submitting a second job with same requirements yields the same result.

Job 3:

Submitting a third job with same requirements reserves one job slot, and reserves all free memory, if the amount of free memory is between 20 MB and 200 MB (some free memory may be used by the operating system or other software.)