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Date: Fri, 1 Feb 2013 10:59:12 +0000 (UTC)
From: "Kristofer Karlsson (JIRA)" <jira@apache.org>
To: dev@lucene.apache.org
Message-ID: <JIRA.12630159.1359647384375.229010.1359716352525@arcas>
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Subject: [jira] [Commented] (LUCENE-4740) Weak references cause extreme GC
 churn
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    [ https://issues.apache.org/jira/browse/LUCENE-4740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568659#comment-13568659 ] 

Kristofer Karlsson commented on LUCENE-4740:
--------------------------------------------

freeBuffers in MMapIndexInput only looks at MMapDirectory.useUnmap, which is the thing that may change, unlike the trackClones / clones which is fixed once the master has been created.

I propose adding this to close():
{noformat}
if (clones != null) { freeBuffers(); }
{noformat}

                
> 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
>            Reporter: Kristofer Karlsson
>            Priority: Critical
>         Attachments: 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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