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https://issues.apache.org/jira/browse/LUCENE-1316?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Todd Feak updated LUCENE-1316:
------------------------------
10 threads in JMeter throwing load at the Tomcat as fast as possible. The
Tomcat was on a separate machine with more then 10 worker threads, though only
10 were in use at any one time.
To eliminate any differences, the tests were run back to back. The only
difference was the lucene-core JAR and a Tomcat bounce between the tests.
Otherwise, the same exact test is run in both cases. What you have is threads
all trying to synchronize on isDeleted() 4700+ times per request. Lock
contention goes through the roof. At any point during the test I can take a
thread stack snapshot and they are all blocked waiting for isDeleted().
> Avoidable synchronization bottleneck in MatchAlldocsQuery$MatchAllScorer
> ------------------------------------------------------------------------
>
> Key: LUCENE-1316
> URL: https://issues.apache.org/jira/browse/LUCENE-1316
> Project: Lucene - Java
> Issue Type: Bug
> Components: Query/Scoring
> Affects Versions: 2.3
> Environment: All
> Reporter: Todd Feak
> Priority: Minor
> Attachments: MatchAllDocsQuery.java
>
> 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 MatchAllScorer.next() 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
> FROM:
> 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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