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From "Paul Cowan (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-806) Synchronization bottleneck in FieldSortedHitQueue with many concurrent readers
Date Sat, 17 Feb 2007 03:35:05 GMT
Synchronization bottleneck in FieldSortedHitQueue with many concurrent readers
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                 Key: LUCENE-806
                 URL: https://issues.apache.org/jira/browse/LUCENE-806
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Search
    Affects Versions: 2.0.0
            Reporter: Paul Cowan
            Priority: Minor


The below is from a post by (my colleague) Paul Smith to the java-users list:

---

Hi ho peoples.

We have an application that is internationalized, and stores data from many languages (each
project has it's own index, mostly aligned with a single language, maybe 2).

Anyway, I've noticed during some thread dumps diagnosing some performance issues, that there
appears to be a _potential_ synchronization bottleneck using Locale-based sorting of Strings.
 I don't think this problem is the root cause of our performance problem, but I thought I'd
mention it here.  Here's the stack dump of a thread waiting:

"http-1001-Processor245" daemon prio=1 tid=0x31434da0 nid=0x3744 waiting for monitor entry
[0x2cd44000..0x2cd45f30]
        at java.text.RuleBasedCollator.compare(RuleBasedCollator.java)
        - waiting to lock <0x6b1e8c68> (a java.text.RuleBasedCollator)
        at org.apache.lucene.search.FieldSortedHitQueue$4.compare(FieldSortedHitQueue.java:320)
        at org.apache.lucene.search.FieldSortedHitQueue.lessThan(FieldSortedHitQueue.java:114)
        at org.apache.lucene.util.PriorityQueue.upHeap(PriorityQueue.java:120)
        at org.apache.lucene.util.PriorityQueue.put(PriorityQueue.java:47)
        at org.apache.lucene.util.PriorityQueue.insert(PriorityQueue.java:58)
        at org.apache.lucene.search.FieldSortedHitQueue.insert(FieldSortedHitQueue.java:90)
        at org.apache.lucene.search.FieldSortedHitQueue.insert(FieldSortedHitQueue.java:97)
        at org.apache.lucene.search.TopFieldDocCollector.collect(TopFieldDocCollector.java:47)
        at org.apache.lucene.search.BooleanScorer2.score(BooleanScorer2.java:291)
        at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:132)
        at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:110)
        at com.aconex.index.search.FastLocaleSortIndexSearcher.search(FastLocaleSortIndexSearcher.java:90)
.....

In our case we had 12 threads waiting like this, while one thread had the lock on the RuleBasedCollator.
 Turns out RuleBasedCollator's.compare(...) method is synchronized.  I wonder if a ThreadLocal
based collator would be better here... ?  There doesn't appear to be a reason for other threads
searching the same index to wait on this sort.  Be just as easy to use their own.  (Is RuleBasedCollator
a "heavy" object memory wise?  Wouldn't have thought so, per thread)

Thoughts?

---

I've investigated this somewhat, and agree that this is a potential problem with a series
of possible workarounds. Further discussion (including proof-of-concept patch) to follow.

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