Affects Version/s: None
Fix Version/s: 7.0
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.