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https://issues.apache.org/jira/browse/LUCENE-10449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17500204#comment-17500204
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Renaud Delbru commented on LUCENE-10449:
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We will, thanks Robert.
> Performance regression due to LZ4 compression of TermsDict in
> SortedSetDocValues
> --------------------------------------------------------------------------------
>
> Key: LUCENE-10449
> URL: https://issues.apache.org/jira/browse/LUCENE-10449
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/codecs
> Affects Versions: 9.0
> Reporter: Renaud Delbru
> Priority: Major
> Attachments: lucene-8.11-no-compression.png, lucene-9.png
>
>
> LUCENE-9843 removed the compression option for SortedSetDocValues term
> dictionaries and enabled LZ4 compression all the time. This has quite an
> impact on our workloads which heavily uses sorted set doc values. It may lead
> to perf regression from 2x up to 5x. See samples below.
>
> {code:java}
> ❯ times_tasks Elasticsearch 7.10.2 (Lucene 8.7) - no terms dict compression
> name type time_min time_max
> time_p50 time_p90
> 7.10.2-22.6-SNAPSHOT.json total 42 90 45
> 66
> 7.10.2-22.6-SNAPSHOT.json SearchJoinRequest1 14 32 15
> 18
> 7.10.2-22.6-SNAPSHOT.json SearchTaskBroadcastRequest2 23 53 27
> 43
> ❯ times_tasks Elasticsearch 7.17.1 (Lucene 8.11) - with terms dict compression
> name type time_min time_max
> time_p50 time_p90
> 7.17.0-27.1-SNAPSHOT.json total 253 327 285
> 310
> 7.17.0-27.1-SNAPSHOT.json SearchJoinRequest1 121 154 142
> 152
> 7.17.0-27.1-SNAPSHOT.json SearchTaskBroadcastRequest2 122 173 140
> 152
> ❯ times_tasks Elasticsearch 7.17.1 (Lucene 8.11) - lucene_default codec is
> used to bypass the terms dict compression
> name type time_min time_max
> time_p50 time_p90
> 7.17.0-27.1-SNAPSHOT.json.2 total 48 96 63
> 75
> 7.17.0-27.1-SNAPSHOT.json.2 SearchJoinRequest1 19 44 25
> 31
> 7.17.0-27.1-SNAPSHOT.json.2 SearchTaskBroadcastRequest2 23 42 29
> 37
> ❯ times_tasks Elasticsearch 8.0 (Lucene 9.0) - with terms dict compression
> name type time_min time_max
> time_p50 time_p90
> 8.0.0-28.0-SNAPSHOT.json total 260 327 287
> 313
> 8.0.0-28.0-SNAPSHOT.json SearchJoinRequest1 122 168 148
> 158
> 8.0.0-28.0-SNAPSHOT.json SearchTaskBroadcastRequest2 123 165 139
> 155
> {code}
> We can clearly see in the benchmark the impact of the terms dict compression
> in our workload. Profiling the execution indicates that the bottleneck is the
> {{{}LZ4.decompress{}}}. We have attached two screenshots of a flamegraph.
> The CPU time of the {{TermsDict.next}} method with Lucene 8.11 with no terms
> dict compression is around 2 seconds, while the CPU time of the same method
> in Lucene 9.0 is 12 seconds. This was measured on a small benchmark reading a
> fixed number of times a sorted set doc values field. Each document is created
> with a single keyword value that represents a UUID.
>
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