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Posted to dev@lucene.apache.org by "Uwe Schindler (JIRA)" <ji...@apache.org> on 2013/04/12 16:48:16 UTC
[jira] [Comment Edited] (LUCENE-4930) Lucene's use of WeakHashMap
at index time prevents full use of cores on some multi-core machines, due
to contention
[ https://issues.apache.org/jira/browse/LUCENE-4930?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13630112#comment-13630112 ]
Uwe Schindler edited comment on LUCENE-4930 at 4/12/13 2:46 PM:
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bq. 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.
Are you sure that this is caused by the lock? :-)
As said before the code will never reach the lock, because the double-checked locking idiom (see my above link to ReferenceQueue) will exit before the lock (line 98 in the above source code link). As classes in JDK are never unloaded unless you unload a web application, the reference queue will be always empty (means head==null in ReferenceQueue).
Are you using special JVM flags like ConcMarkSweepGC's ClassUnloading feature?
was (Author: thetaphi):
bq. 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.
Are you sure that this is caused by the lock? :-)
As said before the code will never reach the lock, because the double-checked locking idiom (see my above link to ReferenceQueue) will exit before the lock (line 98 in the above source code link). As classes in JDK are never unloaded unless you unload a web application, the reference queue will be always empty.
Are you using special JVM flags like ConcMarkSweepGC's ClassUnloading feature?
> 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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