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From "Todd Feak (JIRA)" <>
Subject [jira] Updated: (LUCENE-1316) Avoidable synchronization bottleneck in MatchAlldocsQuery$MatchAllScorer
Date Fri, 27 Jun 2008 20:23:45 GMT


Todd Feak updated LUCENE-1316:

I applied the patch to the 2.3.0 file. I ran against an optimized and non-optimized (12 segment)
index with 4700 entries.

2.3.0 non-optimized index  *104 tps*
2.3.0 patched non-optimized index *482 tps*

2.3.0 optimized index *21 tps*
2.3.0 patched optimized index *718 tps*

The patch provided improvements in both optimized and unoptimized indexes. Thanks again Yonik.

> Avoidable synchronization bottleneck in MatchAlldocsQuery$MatchAllScorer
> ------------------------------------------------------------------------
>                 Key: LUCENE-1316
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Query/Scoring
>    Affects Versions: 2.3
>         Environment: All
>            Reporter: Todd Feak
>            Priority: Minor
>         Attachments: LUCENE_1316.patch, LUCENE_1316.patch, LUCENE_1316.patch,
>   Original Estimate: 1h
>  Remaining Estimate: 1h
> The isDeleted() method on IndexReader has been mentioned a number of times as a potential
synchronization bottleneck. However, the reason this  bottleneck occurs is actually at a higher
level that wasn't focused on (at least in the threads I read).
> In every case I saw where a stack trace was provided to show the lock/block, higher in
the stack you see the method. In Solr paricularly, this scorer is used
for "NOT" queries. We saw incredibly poor performance (order of magnitude) on our load tests
for NOT queries, due to this bottleneck. The problem is that every single document is run
through this isDeleted() method, which is synchronized. Having an optimized index exacerbates
this issues, as there is only a single SegmentReader to synchronize on, causing a major thread
pileup waiting for the lock.
> By simply having the MatchAllScorer see if there have been any deletions in the reader,
much of this can be avoided. Especially in a read-only environment for production where you
have slaves doing all the high load searching.
> I modified line 67 in the MatchAllDocsQuery
>   if (!reader.isDeleted(id)) {
> TO:
>   if (!reader.hasDeletions() || !reader.isDeleted(id)) {
> In our micro load test for NOT queries only, this was a major performance improvement.
 We also got the same query results. I don't believe this will improve the situation for indexes
that have deletions. 
> Please consider making this adjustment for a future bug fix release.

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