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https://issues.apache.org/jira/browse/LUCENE-1316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12608147#action_12608147
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Yonik Seeley commented on LUCENE-1316:
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bq. why would deletes be stop being instantly visible
It's minor, but before, if thread A deleted a document, and then thread B
checked if it was deleted, thread B was guaranteed to see that it was in fact
deleted.
If the check for deletedDocs == null were moved outside of the synchronized,
there's no guarantee when thread B will see (if ever) that deletedDocs has been
set (no memory barrier).
It's not a major issue since client code shouldn't be written that way IMO, but
it's worth factoring into the decision.
> 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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