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https://issues.apache.org/jira/browse/TEZ-4580?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17907811#comment-17907811
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László Bodor commented on TEZ-4580:
-----------------------------------
merged to master, thanks [~himanshum] for the patch and [~simhadri-g] for the
review!
> Slow preemption of new containers when re-use is enabled
> --------------------------------------------------------
>
> Key: TEZ-4580
> URL: https://issues.apache.org/jira/browse/TEZ-4580
> Project: Apache Tez
> Issue Type: Improvement
> Reporter: Himanshu Mishra
> Assignee: Himanshu Mishra
> Priority: Major
> Time Spent: 2h 40m
> Remaining Estimate: 0h
>
> I observed intermittent high runtime of a TPCDS query with running with YARN
> async scheduler
> `{_}yarn.scheduler.capacity.schedule-asynchronously.enable=true` along with
> container reuse.{_}
>
> I found that preemption of lower priority containers was taking very long
> time in such cases. Tez AM log had warning {{{}Expected delayed containers to
> be empty.{}}}, followed by another {{Held container expected to be not null
> for a non-AM-released container}} and after this only 1 container was getting
> released, even when {{tez.am.preemption.percentage}} is high.
> Further investigation lead to following conclusion:
> 1. [Warn log / Assertion
> error|https://github.com/apache/tez/blob/master/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java#L1335]
> thrown because in
> [preemptIfNeeded()|https://github.com/apache/tez/blob/master/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java#L1314],
> when releasing new containers, the loop counter is being decremented with
> each `{{{}releaseUnassignedContainers{}}}`, leading to looping only half
> number of times. By using another counter, assertion passes because of
> condition method returns with check `{{{}if (numPendingRequestsToService < 1)
> {{}}}`.
> 2. In
> [releaseContainer()|https://github.com/apache/tez/blob/master/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java#L1566],
> the container is not getting removed from `{{{}delayedContainers{}}}` queue
> and only from `{{{}heldContainers{}}}` map, hence same container is being
> picked up for release in every iteration till next cycle of
> `{{{}DelayedContainerManager{}}}` finds out that the container is not in
> `{{{}heldContainers{}}}` and skips it [with log
> |https://github.com/apache/tez/blob/master/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java#L2095]`{{{}Skipping
> delayed container as container is no longer running, containerId=...{}}}`
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