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From "Uwe Schindler (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4740) Weak references cause extreme GC churn
Date Fri, 01 Feb 2013 09:53:13 GMT


Uwe Schindler commented on LUCENE-4740:

bq. After doing some more thinking and micro-benchmarking, I think the problem occurs when
you create clones at a faster rate than the GC can cope with.

I agree that might be aproblem and you may be facing it. How mayn requests per second do you
have on your server?

This behaviour is Java's weak reference GC behaviour, it has nothing to do with WeakIdentityMap.
The default WeakHashMap from JDK has the same problems.

bq. Agree that the weakreferences for classes is probably a very minor part of it, and very
unlikely part of the problem here.

That is very common, the JDK uses the same mechanism like in AttributeSource at several places.
It is definitely not part of the problem.

The problem here is the weak map that has a very high throughput of puts (every query produces
at least one IndexInput clone, possibly more). The high throughput already lead to the change
to WeakIdentityMap recently, because a synchronized WeakHashMap was not able to handle the
large number of concurrent puts (Lucene 3.6.0 regression).

I am currently thinking of making the whole thing work without weak references and instead
have some "hard reference" from the clone to master (it is already there, MappedByteBuffer.duplicate()
returns a duplicate buffer that has a reference to the master). The problem with this is,
that you need a check on every access of the IndexInput if the buffer is still valid. If it
is only some null check, we may add it, but its risky for performance too.

My ide was that the master creates some boolean[1] and passes this boolen[1] array to all
childs. When the master closes, it does set the b[0] to false. All childs would do a check
on b[0]... Not sure how this affects performance.
> 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
>            Reporter: Kristofer Karlsson
>            Priority: Critical
>         Attachments: 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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