[jira] [Created] (LUCENE-6096) Improve FilterCache.ramBytesUsed

["Adrien Grand (JIRA)" <ji...@apache.org>]

Adrien Grand created LUCENE-6096:
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             Summary: Improve FilterCache.ramBytesUsed
                 Key: LUCENE-6096
                 URL: https://issues.apache.org/jira/browse/LUCENE-6096
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Adrien Grand
            Assignee: Adrien Grand
            Priority: Minor


I worked on some tests to improve ram usage estimation of LRUFilterCache and found some issues:

1. It currently only takes into account filters and DocIdSets, not the internal overhead of the various hash tables and linked lists that are being maintained. If the filter cache mostly stores sparse doc id sets (or even worse DocIdSet.EMPTY instances that require 0 bytes since it's a singleton) then the memory usage can be significantly under-estimated.

2. It treats all filters as if they were singletons. The filter cache is essentially a Map<SegmentCoreReaders, Map<Filter, DocIdSet>> and it treats filters as if all filters that are equal are the same instance when it comes to memory usage. But this is not the case actually since the map on the second level uses the first-seen filter as a key, which might be different on different segments because of merging.



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