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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-3055) Improve segment cache in SegmentTracker
Date Wed, 15 Jul 2015 14:24:04 GMT

    [ https://issues.apache.org/jira/browse/OAK-3055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14628126#comment-14628126
] 

Michael Dürig commented on OAK-3055:
------------------------------------

I went with an implementation along the lines of this \[1]. Although in my case I *do have*
concurrent threads accessing the rng, I figured this is not too problematic as the race in
updating {{x}} will only create a slight tilt in the random numbers, which should not be problematic
for the given use case. 

An alternative implementation could use {{AtomicInteger.compareAndSet}} for {{x}}:

{code}
    int i = x.get();  
    int j = 0xc3e157c1 - rotateLeft(i, 19);
    while (!x.compareAndSet(i, j)) {
        i = x.get();
        j = 0xc3e157c1 - rotateLeft(i, 19);
    }
{code}

Please go ahead if you think this could be further optimised.

\[1] http://svn.apache.org/viewvc/jackrabbit/oak/trunk/oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/SegmentId.java?r1=1691220&r2=1691219&pathrev=1691220

> Improve segment cache in SegmentTracker
> ---------------------------------------
>
>                 Key: OAK-3055
>                 URL: https://issues.apache.org/jira/browse/OAK-3055
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: segmentmk
>            Reporter: Michael Dürig
>            Assignee: Michael Dürig
>              Labels: doc-impacting, resilience, scalability
>             Fix For: 1.3.3
>
>         Attachments: OAK-3055.patch
>
>
> The hand crafted segment cache in {{SegmentTracker}} is prone to lock contentions in
concurrent access scenarios. As {{SegmentNodeStore#merge}} might also end up acquiring this
lock while holding the commit semaphore the situation can easily lead to many threads being
blocked on the commit semaphore. The {{SegmentTracker}} cache doesn't differentiate between
read and write access, which means that reader threads can block writer threads. 



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