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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Created: (MAPREDUCE-2205) FairScheduler preemption should only preempt tasks for pools/jobs that are up next for scheduling
Date Tue, 30 Nov 2010 02:20:12 GMT
FairScheduler preemption should only preempt tasks for pools/jobs that are up next for scheduling

                 Key: MAPREDUCE-2205
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2205
             Project: Hadoop Map/Reduce
          Issue Type: Bug
          Components: contrib/fair-share
            Reporter: Joydeep Sen Sarma

We have hit a problem with the preemption implementation in the FairScheduler where the following

# job X runs short of fair share or min share and requests/causes N tasks to be preempted
# when slots are then scheduled - tasks from some other job are actually scheduled
# after preemption_interval has passed, job X finds it's still underscheduled and requests
preemption. goto 1.

This has caused widespread preemption of tasks and the cluster going from high utilization
to low utilization in a few minutes.

Some of the problems are specific to our internal version of hadoop (still 0.20 and doesn't
have the hierarchical FairScheduler) - but i think the issue here is generic (just took a
look at the trunk assignTasks and tasksToPreempt routines). The basic problem seems to be
that the logic of assignTasks+FairShareComparator is not consistent with the logic in tasksToPreempt().
The latter can choose to preempt tasks on behalf of jobs that may not be first up for scheduling
based on the FairComparator. Understanding whether these two separate pieces of logic are
consistent and keeping it that way is difficult.

It seems that a much safer preemption implementation is to walk the jobs in the order they
would be scheduled on the next heartbeat - and only preempt for jobs that are at the head
of this sorted queue. In MAPREDUCE-2048 - we have already introduced a pre-sorted list of
jobs ordered by current scheduling priority. It seems much easier to preempt only jobs at
the head of this sorted list.

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