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From "Jason Lowe (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-4946) Type conversion of map completion events leads to performance problems with large jobs
Date Thu, 17 Jan 2013 17:28:13 GMT
Jason Lowe created MAPREDUCE-4946:
-------------------------------------

             Summary: 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: 0.23.5, 2.0.2-alpha
            Reporter: Jason Lowe
            Priority: Critical


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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