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https://issues.apache.org/jira/browse/LUCENE-1316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12664404#action_12664404
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Yonik Seeley commented on LUCENE-1316:
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In a way, this is a mirror image of Jason's request in LUCENE-1476 for a
getDeletedDocs() that returned a DocIdSet... provided it also worked on a
MultiReader, etc. MatchAllDocs could be efficiently implemented with that.
It does seem like having some sort of iterator over existing docs is useful to
avoid the binary search cost of associating ids with segments, but there was
never any feedback on what the API should be. Instead of adding new
functionality to termDocs(), we could also add a getAllDocs() that returns
either a DocIdSet or an interator.
> 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
> Fix For: 2.9
>
> Attachments: LUCENE-1316.patch, LUCENE_1316.patch, LUCENE_1316.patch,
> LUCENE_1316.patch, 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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