Platform LSF Administration Guide Version 6.2

Chapter 19
Reserving Resources
Administering Platform LSF
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To minimize the overhead in recalculating the predicted start times to include previously
skipped jobs, you should configure a small value for
LSB_TIME_RESERVE_NUMJOBS in
lsf.conf.
Reservation scenarios
Scenario 1 Even though no running jobs finish and no host status in cluster are changed, a job’s
future allocation may still change from time to time.
Why this happens
Each scheduling cycle, the scheduler recalculates a job’s reservation information,
estimated start time and opportunity for future allocation. The job candidate host list
may be reordered according to current load. This reordered candidate host list will be
used for the entire scheduling cycle, also including job future allocation calculation. So
different order of candidate hosts may lead to different result of job future allocation.
However, the job estimated start time should be the same.
For example, there are two hosts in cluster,
hostA and hostB. 4 CPUs per host. Job 1
is running and occupying 2 CPUs on
hostA and 2 CPUs on hostB. Job 2 requests 6
CPUs. If the order of hosts is
hostA and hostB, then the future allocation of job 2 will
be 4 CPUs on
hostA 2 CPUs on hostB. If the order of hosts changes in the next
scheduling cycle changes to
hostB and hostA, then the future allocation of job 2 will
be 4 CPUs on
hostB 2 CPUs on hostA.
Scenario 2:
If you set JOB_ACCEPT_INTERVAL to non-zero value, after job is dispatched, within
JOB_ACCEPT_INTERVAL period, pending job estimated start time and future
allocation may momentarily fluctuate.
Why this happens
The scheduler does a time-based reservation calculation each cycle. If
JOB_ACCEPT_INTERVAL is set to non-zero value. once a new job has been
dispatched to a host, this host will not accept new job within
JOB_ACCEPT_INTERVAL interval. Because the host will not be considered for the
entire scheduling cycle, no time-based reservation calculation is done, which may result
in slight change in job estimated start time and future allocation information. After
JOB_ACCEPT_INTERVAL has passed, host will become available for time-based
reservation calculation again, and the pending job estimated start time and future
allocation will be accurate again.