[jira] [Resolved] (OAK-3330) FileStore lock contention with concurrent writers

["Michael Dürig (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/OAK-3330?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Michael Dürig resolved OAK-3330.
--------------------------------
    Resolution: Fixed

* {{Segment.readString()}} loads the segment even when the string is in the string cache. Fixed at http://svn.apache.org/viewvc?rev=1708401&view=rev

* Use read/write lock in {{FileStore}} instead of synchronize: http://svn.apache.org/viewvc?rev=1708402&view=rev

* Read from {{FileChannel}} instead of {{RandomAccessFile}} to reduce concurrency: http://svn.apache.org/viewvc?rev=1708403&view=rev

With above changes I see an increase of just above 50% in write throughput with {{SegmenCompactionIT}} and above parameters.

> FileStore lock contention with concurrent writers
> -------------------------------------------------
>
>                 Key: OAK-3330
>                 URL: https://issues.apache.org/jira/browse/OAK-3330
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segmentmk
>            Reporter: Michael Dürig
>            Assignee: Michael Dürig
>              Labels: compaction
>             Fix For: 1.4
>
>         Attachments: OAK-3330.patch
>
>
> Concurrently writing to the file store can lead to a sever lock contention in {{FileStore#readSegment}}. That method searches the current {{TarWriter}} instance for the segment once it could not be found in any of the {{TarReader}} instances. This is the point where synchronizes on the {{FileStore}} instance, which leads to  the contention. 
> The effect is only observable once the segment cache becomes full and reads actually need to go to the file store. Thus a possible improvement could be to pin segments from the current tar writer to the cache. Alternatively we could try to ease locking by employing read/write locks where possible. 



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