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