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https://issues.apache.org/jira/browse/LUCENE-10043?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Julie Tibshirani resolved LUCENE-10043.
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Resolution: Fixed
> Decrease default for LRUQueryCache#skipCacheFactor?
> ---------------------------------------------------
>
> Key: LUCENE-10043
> URL: https://issues.apache.org/jira/browse/LUCENE-10043
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Julie Tibshirani
> Priority: Minor
> Time Spent: 1h 20m
> Remaining Estimate: 0h
>
> In LUCENE-9002 we introduced logic to skip caching a clause if it would be
> too expensive compared to the usual query cost. Specifically, we avoid
> caching a clause if its cost is estimated to be a factor higher than the lead
> iterator's:
> {code}
> // skip cache operation which would slow query down too much
> if (cost / skipCacheFactor > leadCost) {
> return supplier.get(leadCost);
> }
> {code}
> Choosing good defaults is hard! We've seen some examples in Elasticsearch
> where caching a query clause causes a major slowdown, contributing to poor
> tail latencies. It made me think that the default 'skipCacheFactor' of 250
> may be too high -- interpreted simply, this means we'll cache a clause even
> if it is ~250 times more expensive than running the top-level query on its
> own. Would it make sense to decrease this to 10 or so? It seems okay to air
> on the side of less caching for individual clauses, especially since any
> parent 'BooleanQuery' is already eligible for caching?
> As a note, the interpretation "~250 times more expensive than running the
> top-level query on its own" isn't perfectly accurate. The true cost doesn't
> dependent on the number of matched documents, but also the cost of matching
> itself. Making it even more complex, some queries like
> 'IndexOrDocValuesQuery' have different matching strategies based on whether
> they're used as a lead iterator or verifier.
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