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Jason Lowe commented on MAPREDUCE-4842: --------------------------------------- Here's the sequence of events that I believe led to the hang during shuffle. See {{MergeManager}} for context of variable references. # Fetchers started fetching data # Enough data finishes transferring to reach the {{commitMemory}} threshold and an in-memory merge starts # While the merge takes place some of the output data is freed before the merge completes, lowering {{commitMemory}} and {{usedMemory}} which allows more data to be fetched # Eventually we try to fetch too much data because {{usedMemory}} exceeds {{memoryLimit}} and further fetchers are told to WAIT # All of the outstanding fetches complete and call {{closeInMemoryFile}}, but we don't start a merge because the previous merge is still marked in progress # Merge completes, allowing a new merge to be started on the next {{closeInMemoryFile}} call # With no outstanding fetches and no new fetches allowed, we never call {{closeInMemoryFile}} again and never start the next merge # With no merge in progress and therefore nothing to wait upon, fetcher threads proceed to pummel the {{MergeManager}} asking for merge data reservations that are never given, and the reducer log grows rather rapidly > Shuffle race can hang reducer > ----------------------------- > > Key: MAPREDUCE-4842 > URL: https://issues.apache.org/jira/browse/MAPREDUCE-4842 > Project: Hadoop Map/Reduce > Issue Type: Bug > Components: mrv2 > Affects Versions: 2.0.3-alpha, 0.23.5 > Reporter: Jason Lowe > > Saw an instance where the shuffle caused multiple reducers in a job to hang. > It looked similar to the problem described in MAPREDUCE-3721, where the > fetchers were all being told to WAIT by the MergeManager but no merge was > taking place. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira