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From "Kristofer Karlsson (JIRA)" <>
Subject [jira] [Created] (LUCENE-4740) Weak references cause extreme GC churn
Date Thu, 31 Jan 2013 15:51:12 GMT
Kristofer Karlsson created LUCENE-4740:

             Summary: Weak references cause extreme GC churn
                 Key: LUCENE-4740
             Project: Lucene - Core
          Issue Type: Bug
          Components: core/index
    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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