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Posted to dev@lucene.apache.org by "Uwe Schindler (JIRA)" <ji...@apache.org> on 2013/06/14 19:48:22 UTC
[jira] [Commented] (LUCENE-4740) Weak references cause extreme GC
churn
[ https://issues.apache.org/jira/browse/LUCENE-4740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13683594#comment-13683594 ]
Uwe Schindler commented on LUCENE-4740:
---------------------------------------
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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