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From "Keith Turner (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ACCUMULO-2766) Single walog operation may wait for multiple hsync calls
Date Wed, 30 Apr 2014 22:11:14 GMT
Keith Turner created ACCUMULO-2766:
--------------------------------------

             Summary: Single walog operation may wait for multiple hsync calls
                 Key: ACCUMULO-2766
                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2766
             Project: Accumulo
          Issue Type: Bug
          Components: tserver
    Affects Versions: 1.5.1, 1.5.0, 1.6.0
            Reporter: Keith Turner


While looking into slow {{hsync}} calls, I noticed an oddity in the way Accumulo processes
syncs.  Specifically the way {{closeLock}} is used in {{DfsLogger}}, it seems like the following
situation could occur. 
 
 # thread B starts executing DfsLogger.LogSyncingTask.run()
 # thread 1 enters DfsLogger.logFileData()
 # thread 1 writes to walog
 # thread 1 locks _closeLock_ 
 # thread 1 adds sync work to workQueue
 # thread 1 unlocks _closeLock_
 # thread B takes sync work off of workQueue
 # thread B locks _closeLock_
 # thread B calls sync
 # thread 3 enters DfsLogger.logFileData()
 # thread 3 writes to walog
 # thread 3 blocks locking _closeLock_
 # thread 4 enters DfsLogger.logFileData()
 # thread 4 writes to walog
 # thread 4 blocks locking _closeLock_
 # thread B unlocks _closeLock_
 # thread 4 locks _closeLock_ 
 # thread 4 adds sync work to workQueue
 # thread B takes sync work off of workQueue
 # thread B blocks locking _closeLock_
 # thread 4 unlocks _closeLock_
 # thread B locks _closeLock_
 # thread B calls sync
 # thread B unlocks _closeLock_
 # thread 3 locks _closeLock_
 # thread 3 adds sync work to workQueue
 # thread 3 unlocks _closeLock_

In this situation thread 3 unnecessarily has to wait for an extra {{hsync}} call.  Not sure
if this situation actually occurs, or if it occurs very frequently.  Looking at the code it
seems like it would be nice if sync operations could be queued w/o synchronizing w/ sync operations.




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