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From "Shawn Heisey (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4740) Weak references cause extreme GC churn
Date Sat, 02 Feb 2013 20:30:13 GMT


Shawn Heisey commented on LUCENE-4740:

I've been watching the activity on this issue because I have occasional extreme GC pauses
in Solr with an 8GB heap.  GC tuning has reduced them somewhat so that my load balancer hasn't
marked the service offline in a few days, but I think that things still aren't ideal.

I will admit that I find most of the comments baffling.  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.

> Weak references cause extreme GC churn
> --------------------------------------
>                 Key: LUCENE-4740
>                 URL:
>             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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