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From "Commit Tag Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4740) Weak references cause extreme GC churn
Date Sat, 02 Feb 2013 10:46:14 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13569499#comment-13569499
] 

Commit Tag Bot commented on LUCENE-4740:
----------------------------------------

[branch_4x commit] Uwe Schindler
http://svn.apache.org/viewvc?view=revision&revision=1441727

Merged revision(s) 1441726 from lucene/dev/trunk:
LUCENE-4740: Don't track clones of MMapIndexInput if unmapping is disabled. This reduces GC
overhead.

                
> Weak references cause extreme GC churn
> --------------------------------------
>
>                 Key: LUCENE-4740
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4740
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/store
>    Affects Versions: 3.6.1
>         Environment: Linux debian squeeze 64 bit, Oracle JDK 6, 32 GB RAM, 16 cores
> -Xmx16G
>            Reporter: Kristofer Karlsson
>             Fix For: 4.2, 5.0
>
>         Attachments: LUCENE-4740.patch, LUCENE-4740.patch
>
>
> We are running a set of independent search machines, running our custom software using
lucene as a search library. We recently upgraded from lucene 3.0.3 to 3.6.1 and noticed a
severe degradation of performance.
> After doing some heap dump digging, it turns out the process is stalling because it's
spending so much time in GC. We noticed about 212 million WeakReference, originating from
WeakIdentityMap, originating from MMapIndexInput.
> Our problem completely went away after removing the clones weakhashmap from MMapIndexInput,
and as a side-effect, disabling support for explictly unmapping the mmapped data.

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