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https://issues.apache.org/jira/browse/SOLR-16838?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17730247#comment-17730247
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Rahul Goswami commented on SOLR-16838:
--------------------------------------
The regression seems to be in the Lucene layer. Quoting the discussion on this
issue on the Lucene list:
" - 8.0 moved the terms index off-heap for non-PK fields with
MMapDirectory. [https://github.com/apache/lucene/issues/9681]
- Then in 8.6 the FST was moved off-heap all the time.
[https://github.com/apache/lucene/issues/10297]";
So now the terms index is off-heap, and due to Lucene's FST reading bytes
backwards readByte() call causes disk access for every single byte . The below
tickets have been opened by Adrien Grand on the issue for further discussion:
[https://github.com/apache/lucene/issues/12355] and
[https://github.com/apache/lucene/issues/12356].
> Atomic updates too slow in Solr 8 vs Solr 7
> -------------------------------------------
>
> Key: SOLR-16838
> URL: https://issues.apache.org/jira/browse/SOLR-16838
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SearchComponents - other
> Affects Versions: 8.11.1
> Reporter: Rahul Goswami
> Priority: Major
> Labels: RTG, RealTimeGet, atomicupdate
>
> Started experiencing slowness with updates in production after upgrading from
> Solr 7.7.2 to 8.11.1. Upon comparing the performance it turns out that
> indexing 20 million docs via atomic updates through the same client program
> (running 15 parallel threads indexing in batches of 1000) takes below time:
>
> Solr 7 : 78 mins
> Solr 8: 370 mins
>
> Environment details:
> - Java 11 on Windows server
> - Xms1536m Xmx3072m
> - Indexing client code running 15 parallel threads indexing in batches of 1000
> - using SimpleFSDirectoryFactory (since Mmap doesn't quite work well on
> Windows for our index sizes which commonly run north of 1 TB)
>
> Looking at the thread dump, the bottleneck seems to be RealTimeGet and I can
> see that Solr 7 takes a different code path than Solr 8. Note that the
> performance of regular updates (non-atomic) is still pretty good on Solr 8
> completing in < 1 hour for the same 20 million data set.
>
> Sharing the indexing code, solrconfig, schema and thread dumps in the link
> below:
> [https://drive.google.com/drive/folders/1q2DPNTYQEU6fi3NeXIKJhaoq3KPnms0h?usp=sharing]
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