[LUCENE-4930] Lucene's use of WeakHashMap at index time prevents full use of cores on some multi-core machines, due to contention

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.2
• Fix Version/s: 4.3, 6.0
• Component/s: core/index

Environment

Dell blade system with 16 cores

• Lucene Fields: New
• Assignee: Uwe Schindler
• Reporter: Karl Wright
• Created: 12/Apr/13 14:04
• Updated: 28/Aug/22 13:44
• Resolved: 13/Apr/13 09:39

Description

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.)

Issue Links

• is superceded by: LUCENE-4931 Make oal.document.Field reuse its internal StringTokenStream