Adrien Grand created LUCENE-7235:
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Summary: Avoid taking the lock in LRUQueryCache when not necessary
Key: LUCENE-7235
URL: https://issues.apache.org/jira/browse/LUCENE-7235
Project: Lucene - Core
Issue Type: Improvement
Reporter: Adrien Grand
Assignee: Adrien Grand
Priority: Minor
LRUQueryCache's CachingWeightWrapper works this way:
- first it looks up the cache to see if there is an entry for the query in the
current leaf
- if yes, it returns it
- otherwise it checks whether the query should be cached on this leaf
- if yes, it builds a cache entry and returns it
- otherwise it returns a scorer built from the wrapped weight
The potential issue is that this first step always takes the lock, and I have
seen a couple cases where indices were small and/or queries were very cheap and
this showed up as a bottleneck. On the other hand, we have checks in step 3
that tell the cache to not cache on a particular segment regardless of the
query. So I would like to move that part before 1 so that we do not even take
the lock in that case.
For instance right now we require that segments have at least 10k documents and
3% of all docs in the index to be cached. I just looked at a random index that
contains 1.7m documents, and only 4 segments out of 29 met this criterion (yet
they contain 1.1m documents: 65% of the total index size). So in the case of
that index, we would take the lock 7x less often.
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