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https://issues.apache.org/jira/browse/LUCENE-4740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568007#comment-13568007
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Kristofer Karlsson commented on LUCENE-4740:
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Agree that the weakreferences for classes is probably a very minor part of it,
and very unlikely part of the problem here.
The unmap option is nice, and you could make it less complicated by simply
disallowing the option to be changed after the mmapdirectory has ever been
cloned. In practice it will always be set immediately after construction.
> 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
>
> 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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