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https://issues.apache.org/jira/browse/MAPREDUCE-6678?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15276694#comment-15276694
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Eric Payne commented on MAPREDUCE-6678:
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Thanks, [~nroberts], for raising this issue and providing a patch.
bq. Tested this patch on a 10-node cluster using terasort. Verified using
strace that nodemanager is issuing correct WILLNEED without DONTNEED.
I recognize that it's difficult to produce a unit test for the patch. Would it
be possible for you to post a very brief justification of that?
Otherwise, patch looks good to me.
+1
> Allow ShuffleHandler readahead without drop-behind
> --------------------------------------------------
>
> Key: MAPREDUCE-6678
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6678
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: nodemanager
> Affects Versions: 3.0.0, 2.7.2
> Reporter: Nathan Roberts
> Assignee: Nathan Roberts
> Attachments: YARN-4964.001.patch
>
>
> Currently mapreduce.shuffle.manage.os.cache enables/disables both readahead
> (POSIX_FADV_WILLNEED) and drop-behind (POSIX_FADV_DONTNEED) logic within the
> ShuffleHandler.
> It would be beneficial if these were separately configurable.
> - Running without readahead can lead to significant seek storms caused by
> large numbers of sendfiles() competing with one another.
> - However, running with drop-behind can also lead to seek storms because
> there are cases where the server can successfully write the shuffle bytes to
> the network, BUT the client doesn't want the bytes right now (MergeManager
> wants to WAIT is an example) so it ignores them and asks for them again a bit
> later. This causes repeated reads of the same data from disk.
> I'll attach a simple patch that enables/disables readahead based on
> mapreduce.shuffle.readahead.bytes==0, leaving
> mapreduce.shuffle.manage.os.cache controlling only the drop-behind.
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