[jira] [Commented] (ACCUMULO-2766) Single walog operation may wait for multiple hsync calls

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    [ https://issues.apache.org/jira/browse/ACCUMULO-2766?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14016889#comment-14016889 ] 

ASF subversion and git services commented on ACCUMULO-2766:
-----------------------------------------------------------

Commit 1e16159cdb25121123a3ea64df8a47c0eda54709 in accumulo's branch refs/heads/1.5.2-SNAPSHOT from [~keith_turner]
[ https://git-wip-us.apache.org/repos/asf?p=accumulo.git;h=1e16159 ]

ACCUMULO-2766 fix wal group commit


> 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: 1.5.0, 1.5.1, 1.6.0
>            Reporter: Keith Turner
>            Assignee: Keith Turner
>            Priority: Critical
>              Labels: performance
>             Fix For: 1.5.2, 1.6.1
>
>         Attachments: ACCUMULO-2677-1.patch, ACCUMULO-2766-2.patch
>
>
> 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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