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From "Rajesh Balamohan (JIRA)" <j...@apache.org>
Subject [jira] Created: (MAPREDUCE-1904) Reducing locking contention in TaskTracker.MapOutputServlet's LocalDirAllocator
Date Thu, 01 Jul 2010 04:11:50 GMT
Reducing locking contention in TaskTracker.MapOutputServlet's LocalDirAllocator

                 Key: MAPREDUCE-1904
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1904
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: tasktracker
    Affects Versions: 0.20.1
            Reporter: Rajesh Balamohan

While profiling tasktracker with Sort benchmark, it was observed that threads block on LocalDirAllocator.getLocalPathToRead()
in order to get the index file and temporary map output file.

As LocalDirAllocator is tied up with ServetContext,  only one instance would be available
per tasktracker httpserver.  Given the jobid & mapid, LocalDirAllocator retrieves index
file path and temporary map output file path. getLocalPathToRead() is internally synchronized.

Introducing a LRUCache for this lookup reduces the contention heavily (LRUCache with key =jobid
+mapid and value=PATH to the file). Size of the LRUCache can be varied based on the environment
and I observed a throughput improvement in the order of 4-7% with the introduction of LRUCache.

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