[jira] [Created] (YARN-4134) FairScheduler preemption stops at queue level that all child queues are not over their fairshare

["Xianyin Xin (JIRA)" <ji...@apache.org>]

Xianyin Xin created YARN-4134:
---------------------------------

             Summary: FairScheduler preemption stops at queue level that all child queues are not over their fairshare
                 Key: YARN-4134
                 URL: https://issues.apache.org/jira/browse/YARN-4134
             Project: Hadoop YARN
          Issue Type: Bug
          Components: fairscheduler
            Reporter: Xianyin Xin


Now FairScheudler uses a choose-a-candidate method to select a container from leaf queues that to be preempted, in {{FSParentQueue.preemptContainer()}},
{code}
    readLock.lock();
    try {
      for (FSQueue queue : childQueues) {
        if (candidateQueue == null ||
            comparator.compare(queue, candidateQueue) > 0) {
          candidateQueue = queue;
        }
      }
    } finally {
      readLock.unlock();
    }

    // Let the selected queue choose which of its container to preempt
    if (candidateQueue != null) {
      toBePreempted = candidateQueue.preemptContainer();
    }
{code}
a candidate child queue is selected. However, if the queue's usage isn't over it's fairshare, preemption will not happen:
{code}
    if (!preemptContainerPreCheck()) {
      return toBePreempted;
    }
{code}
 A scenario:
{code}
            root
           /    \
      queue1   queue2
      /    \
queue1.3, (  queue1.4  )
{code}
suppose there're 8 containers, and queues at any level have the same weight. queue1.3 takes 4 and queue2 takes 4, so both queue1 and queue2 are at their fairshare. Now we submit an app in queue1.4 with 4 containers needs, it should preempt 2 from queue1.3, but the candidate-containers selection procedure will stop at level that all of the child queues are not over their fairshare, and none of the containers will be preempted.



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