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https://issues.apache.org/jira/browse/SPARK-11316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15131196#comment-15131196
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Apache Spark commented on SPARK-11316:
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User 'zhuoliu' has created a pull request for this issue:
https://github.com/apache/spark/pull/11060
> isEmpty before coalesce seems to cause huge performance issue in setupGroups
> ----------------------------------------------------------------------------
>
> Key: SPARK-11316
> URL: https://issues.apache.org/jira/browse/SPARK-11316
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 1.5.1
> Reporter: Thomas Graves
> Priority: Critical
>
> So I haven't fully debugged this yet but reporting what I'm seeing and think
> might be going on.
> I have a graph processing job that is seeing huge slow down in setupGroups in
> the location iterator where its getting the preferred locations for the
> coalesce. They are coalescing from 2400 down to 1200 and its taking 17+
> hours to do the calculation. Killed it at this point so don't know total
> time.
> It appears that the job is doing an isEmpty call, a bunch of other
> transformation, then a coalesce (where it takes so long), other
> transformations, then finally a count to trigger it.
> It appears that there is only one node that its finding in the setupGroup
> call and to get to that node it has to first to through the while loop:
> while (numCreated < targetLen && tries < expectedCoupons2) {
> where expectedCoupons2 is around 19000. It finds very few or none in this
> loop.
> Then it does the second loop:
> while (numCreated < targetLen) { // if we don't have enough partition
> groups, create duplicates
> var (nxt_replica, nxt_part) = rotIt.next()
> val pgroup = PartitionGroup(nxt_replica)
> groupArr += pgroup
> groupHash.getOrElseUpdate(nxt_replica, ArrayBuffer()) += pgroup
> var tries = 0
> while (!addPartToPGroup(nxt_part, pgroup) && tries < targetLen) { //
> ensure at least one part
> nxt_part = rotIt.next()._2
> tries += 1
> }
> numCreated += 1
> }
> Where it has an inner while loop and both of those are going 1200 times.
> 1200*1200 loops. This is taking a very long time.
> The user can work around the issue by adding in a count() call very close to
> after the isEmpty call before the coalesce is called. I also tried putting
> in a take(10000) right before the isEmpty call and it seems to work around
> the issue, took 1 hours with the take vs a few minutes with the count().
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