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https://issues.apache.org/jira/browse/LUCENE-7868?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16052351#comment-16052351
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Simon Willnauer commented on LUCENE-7868:
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[~mikemccand] i did a first pass at the patch... good stuff but I think we need
to clean up some of the big ass loops :)
> Use multiple threads to apply deletes and DV updates
> ----------------------------------------------------
>
> Key: LUCENE-7868
> URL: https://issues.apache.org/jira/browse/LUCENE-7868
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Fix For: master (7.0)
>
> Attachments: cpu-after.png, cpu-before.png, LUCENE-7868.patch,
> LUCENE-7868.patch, LUCENE-7868.patch, LUCENE-7868.patch
>
>
> Today, when users delete documents or apply doc values updates, IndexWriter
> buffers them up into frozen packets and then eventually uses a single thread
> (BufferedUpdatesStream.applyDeletesAndUpdates) to resolve delete/update terms
> to docids. This thread also holds IW's monitor lock, so it also blocks
> refresh, merges starting/finishing, commits, etc.
> We have heavily optimized this part of Lucene over time, e.g. LUCENE-6161,
> LUCENE-2897, LUCENE-2680, LUCENE-3342, but still, it's a single thread so it
> can't use multiple CPU cores commonly available now.
> This doesn't affect append-only usage, but for update-heavy users (me!) this
> can be a big bottleneck, and causes long stop-the-world hangs during indexing.
> I have an initial exploratory patch to make these lookups concurrent, without
> holding IW's lock, so that when a new packet of deletes is pushed, which
> happens when we flush a new segment, we immediately use that same indexing
> thread to and resolve the deletions.
> This is analogous to when we made segment flushing concurrent (LUCENE-3023),
> just for deletes and DV updates as well.
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