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https://issues.apache.org/jira/browse/LUCENE-4740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13683594#comment-13683594
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Uwe Schindler commented on LUCENE-4740:
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bq. Therefore I have a simple question. If I run Solr branch_4x with this patch
applied, will I benefit? I can see from the commit log that unmmapping must
disabled to benefit, but I don't know if this is how Solr operates.
If you have a version with this patch enabled and you are using an index that
changes not too often, it is better to not unmap. In that case you can pass
unmap="false" as parameter to your MMapDirectoryFactory in Solr. If the index
does not change too often, the overhead by a delay in unmapping the files at a
lter stage does not matter. GC has less to do then.
> 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: 4.4
>
> 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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