[LUCENE-7868] Use multiple threads to apply deletes and DV updates

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 7.0
• Lucene Fields: New
• Assignee: Michael McCandless
• Reporter: Michael McCandless
• Created: 07/Jun/17 14:27
• Updated: 28/Aug/22 15:16
• Resolved: 21/Jun/17 17:47

Description

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.