[
https://issues.apache.org/jira/browse/LUCENE-10449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17499629#comment-17499629
]
Robert Muir commented on LUCENE-10449:
--------------------------------------
Looks like you aren't using BinaryDocValues, your source code pointers point to
SortedDocValues/SortedSetDocValues term dictionary code.
If you want to retrieve a fast per-document binary value, you should use
BinaryDocValues. There's no compression and it just returns the per-document
byte-array. Sounds like this is what you should be using for the field.
The SortedDocValues/SortedSetDocValues sorts, deduplicates, dereferences and
assigns numeric ordinals to each document. Each document then has a fast
per-document numeric value, which can be used for sorting and faceting
computations. Separately there is a terms dictionary with additional
compression (prefix, LZ4, etc) which can be used to "resolve" the numeric
ordinals back to binary. This is slower, but e.g. only applies to top-N of
sort/facet so its ok.
Again, if you want to lookup per-document byte[], switch to the BinaryDocValues
field.
> Performance regression due to decompressBlock method introduced with
> compression on binary doc values
> ------------------------------------------------------------------------------------------------------
>
> 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-9211 introduced a compression mechanism for binary doc values, which
> was then removed at a later stage in LUCENE-9843 as it was impacting
> performance on some workload.
> However, LUCENE-9843 didn't revert the code as it was prior to that. Instead
> of reading the block directly from the {{IndexInput}} as in [1], the
> {{decompressBlock()}} call [2] is kept which is decompressing a non-compress
> block (from our understanding). The {{decompressBlock}} method deleguates to
> {{LZ4.decompress }}and it looks like this is adding a significant overhead
> (e.g., {{{}readByte{}}}).
> This has quite an impact on our workloads which heavily uses 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 binary 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 binary 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 binary 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) - no binary 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 that in Lucene 9.0, even after the removal of the binary
> doc values compression, the performance didn't improve. 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
> 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 binary doc values field. Each document is created
> with a single binary value that represents a UUID.
>
> [1]
> [https://github.com/apache/lucene-solr/blob/releases/lucene-solr/8.11.0/lucene/core/src/java/org/apache/lucene/codecs/lucene80/Lucene80DocValuesProducer.java#L1159]
> [2]
> [https://github.com/apache/lucene/commit/a7a02519f0a5652110a186f4909347ac3349092d#diff-ab443662a6310fda675a4bd6d01fabf3a38c4c825ec2acef8f9a34af79f0b252R1022]
>
--
This message was sent by Atlassian Jira
(v8.20.1#820001)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
