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https://issues.apache.org/jira/browse/LUCENE-6161?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14264815#comment-14264815
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ASF subversion and git services commented on LUCENE-6161:
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Commit 1649599 from [~mikemccand] in branch 'dev/trunk'
[ https://svn.apache.org/r1649599 ]
LUCENE-6161: reuse DocsEnum when resolving deleted terms/queries to doc id
> Applying deletes is sometimes dog slow
> --------------------------------------
>
> Key: LUCENE-6161
> URL: https://issues.apache.org/jira/browse/LUCENE-6161
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Michael McCandless
> Fix For: 5.0, Trunk
>
>
> I hit this while testing various use cases for LUCENE-6119 (adding
> auto-throttle to ConcurrentMergeScheduler).
> When I tested "always call updateDocument" (each add buffers a delete term),
> with many indexing threads, opening an NRT reader once per second (forcing
> all deleted terms to be applied), I see that
> BufferedUpdatesStream.applyDeletes sometimes seems to take a loooong time,
> e.g.:
> {noformat}
> BD 0 [2015-01-04 09:31:12.597; Lucene Merge Thread #69]: applyDeletes took
> 339 msec for 10 segments, 117 deleted docs, 607333 visited terms
> BD 0 [2015-01-04 09:31:18.148; Thread-4]: applyDeletes took 5533 msec for 62
> segments, 10989 deleted docs, 8517225 visited terms
> BD 0 [2015-01-04 09:31:21.463; Lucene Merge Thread #71]: applyDeletes took
> 1065 msec for 10 segments, 470 deleted docs, 1825649 visited terms
> BD 0 [2015-01-04 09:31:26.301; Thread-5]: applyDeletes took 4835 msec for 61
> segments, 14676 deleted docs, 9649860 visited terms
> BD 0 [2015-01-04 09:31:35.572; Thread-11]: applyDeletes took 6073 msec for 72
> segments, 13835 deleted docs, 11865319 visited terms
> BD 0 [2015-01-04 09:31:37.604; Lucene Merge Thread #75]: applyDeletes took
> 251 msec for 10 segments, 58 deleted docs, 240721 visited terms
> BD 0 [2015-01-04 09:31:44.641; Thread-11]: applyDeletes took 5956 msec for 64
> segments, 15109 deleted docs, 10599034 visited terms
> BD 0 [2015-01-04 09:31:47.814; Lucene Merge Thread #77]: applyDeletes took
> 396 msec for 10 segments, 137 deleted docs, 719914 visit
> {noformat}
> What this means is even though I want an NRT reader every second, often I
> don't get one for up to ~7 or more seconds.
> This is on an SSD, machine has 48 GB RAM, heap size is only 2 GB. 12
> indexing threads.
> As hideously complex as this code is, I think there are some inefficiencies,
> but fixing them could be hard / make code even hairier ...
> Also, this code is mega-locked: holds IW's lock, holds BD's lock. It blocks
> things like merges kicking off or finishing...
> E.g., we pull the MergedIterator many times on the same set of sub-iterators.
> Maybe we can create the sorted terms up front and reuse that?
> Maybe we should go "term stride" (one term visits all N segments) not
> "segment stride" (visit each segment, iterating all deleted terms for it).
> Just iterating the terms to be deleted takes a sizable part of the time, and
> we now do that once for every segment in the index.
> Also, the "isUnique" bit in LUCENE-6005 should help here, since if we know
> the field is unique, we can stop seekExact once we found a segment that has
> the deleted term, we can maybe pass false for removeDuplicates to
> MergedIterator...
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