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https://issues.apache.org/jira/browse/LUCENE-4930?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13630105#comment-13630105
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Karl Wright commented on LUCENE-4930:
-------------------------------------
bq. During normal indexing this should have no contention at all. What Java
version are you using, and is there really a visible slowdown caused by this -
you gave no numbers! The contention here may be displayed in stack traces
requested from threads, but its unlikely to have an effect on indexing (because
the map is mostly static).
The way this shows up is on a 16-core machine I only see 700-800% utilization.
So I'm losing 1/2 the available processing power available.
The JDK is the amd64 openjdk 1.6.0. I'll try to chase down a patch level for
you if that seems warranted.
> Lucene's use of WeakHashMap at index time prevents full use of cores on some
> multi-core machines, due to contention
> -------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-4930
> URL: https://issues.apache.org/jira/browse/LUCENE-4930
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/index
> Affects Versions: 4.2
> Environment: Dell blade system with 16 cores
> Reporter: Karl Wright
>
> Our project is not optimally using full processing power during under
> indexing load on Lucene 4.2.0. The reason is the
> AttributeSource.addAttribute() method, which goes through a WeakHashMap
> synchronizer, which is apparently single-threaded for a significant amount of
> time. Have a look at the following trace:
> "pool-1-thread-28" prio=10 tid=0x00007f47fc104800 nid=0x672b waiting for
> monitor entry [0x00007f47d19ed000]
> java.lang.Thread.State: BLOCKED (on object monitor)
> at java.lang.ref.ReferenceQueue.poll(ReferenceQueue.java:98)
> - waiting to lock <0x00000005c5cd9988> (a
> java.lang.ref.ReferenceQueue$Lock)
> at
> org.apache.lucene.util.WeakIdentityMap.reap(WeakIdentityMap.java:189)
> at org.apache.lucene.util.WeakIdentityMap.get(WeakIdentityMap.java:82)
> at
> org.apache.lucene.util.AttributeSource$AttributeFactory$DefaultAttributeFactory.getClassForInterface(AttributeSource.java:74)
> at
> org.apache.lucene.util.AttributeSource$AttributeFactory$DefaultAttributeFactory.createAttributeInstance(AttributeSource.java:65)
> at
> org.apache.lucene.util.AttributeSource.addAttribute(AttributeSource.java:271)
> at
> org.apache.lucene.index.DocInverterPerField.processFields(DocInverterPerField.java:107)
> at
> org.apache.lucene.index.DocFieldProcessor.processDocument(DocFieldProcessor.java:254)
> at
> org.apache.lucene.index.DocumentsWriterPerThread.updateDocument(DocumentsWriterPerThread.java:256)
> at
> org.apache.lucene.index.DocumentsWriter.updateDocument(DocumentsWriter.java:376)
> at
> org.apache.lucene.index.IndexWriter.updateDocument(IndexWriter.java:1473)
> at
> org.apache.lucene.index.IndexWriter.addDocument(IndexWriter.java:1148)
> at
> org.apache.lucene.index.IndexWriter.addDocument(IndexWriter.java:1129)
> …
> We’ve had to make significant changes to the way we were indexing in order to
> not hit this issue as much, such as indexing using TokenStreams which we
> reuse, when it would have been more convenient to index with just tokens.
> (The reason is that Lucene internally creates TokenStream objects when you
> pass a token array to IndexableField, and doesn’t reuse them, and the
> addAttribute() causes massive contention as a result.) However, as you can
> see from the trace above, we’re still running into contention due to other
> addAttribute() method calls that are buried deep inside Lucene.
> I can see two ways forward. Either not use WeakHashMap or use it in a more
> efficient way, or make darned sure no addAttribute() calls are done in the
> main code indexing execution path. (I think it would be easy to fix
> DocInverterPerField in that way, FWIW. I just don’t know what we’ll run into
> next.)
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