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https://issues.apache.org/jira/browse/MAPREDUCE-4946?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13559771#comment-13559771
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Jason Lowe commented on MAPREDUCE-4946:
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Ran test-patch manually on the branch-0.23 patch:
{noformat}
-1 overall.
+1 @author. The patch does not contain any @author tags.
+1 tests included. The patch appears to include 15 new or modified tests.
-1 javadoc. The javadoc tool appears to have generated 1 warning messages.
+1 javac. The applied patch does not increase the total number of javac
compiler warnings.
+1 eclipse:eclipse. The patch built with eclipse:eclipse.
+1 findbugs. The patch does not introduce any new Findbugs (version )
warnings.
+1 release audit. The applied patch does not increase the total number of
release audit warnings.
{noformat}
The javadoc warning is unrelated. I ran the unit tests in
hadoop-mapreduce-client-app and hadoop-mapreduce-client-hs and they all passed.
I also started up a pseudo-distributed cluster and ran wordcount and terasort
jobs successfully.
> Type conversion of map completion events leads to performance problems with
> large jobs
> --------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-4946
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4946
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mr-am
> Affects Versions: 2.0.2-alpha, 0.23.5
> Reporter: Jason Lowe
> Assignee: Jason Lowe
> Priority: Critical
> Attachments: MAPREDUCE-4946-branch-0.23.patch, MAPREDUCE-4946.patch
>
>
> We've seen issues with large jobs (e.g.: 13,000 maps and 3,500 reduces) where
> reducers fail to connect back to the AM after being launched due to
> connection timeout. Looking at stack traces of the AM during this time we
> see a lot of IPC servers stuck waiting for a lock to get the application ID
> while type converting the map completion events. What's odd is that normally
> getting the application ID should be very cheap, but in this case we're
> type-converting thousands of map completion events for *each* reducer
> connecting. That means we end up type-converting the map completion events
> over 45 million times during the lifetime of the example job (13,000 * 3,500).
> We either need to make the type conversion much cheaper (i.e.: lockless or at
> least read-write locked) or, even better, store the completion events in a
> form that does not require type conversion when serving them up to reducers.
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