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https://issues.apache.org/jira/browse/LUCENE-9827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17299477#comment-17299477
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Adrien Grand commented on LUCENE-9827:
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In the worst-case scenario where someone does 1-document flushes with small
documents, I believe that this would replace half of the semi-optimized merges
with optimized merges, so this would certainly help indexing speed.
{{getNumDirtyDocs}} is bounded by the max number of docs per chunk. So assuming
1kB documents (before compression), each chunk has 480 documents with
BEST_COMPRESSION (480kB block size). So segments that have 1 dirty chunk cannot
have more than 479 dirty documents and {{candidate.getNumDirtyDocs() * 100 >
candidate.getNumDocs()}} always evaluates false if the input segment has more
than 479*100=47,900 documents. So this change may only affect small segments
and should have a negligible impact on the space efficiency of large indices.
+1 to merge this change
> Small segments are slower to merge due to stored fields since 8.7
> -----------------------------------------------------------------
>
> Key: LUCENE-9827
> URL: https://issues.apache.org/jira/browse/LUCENE-9827
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Adrien Grand
> Priority: Minor
> Attachments: total-merge-time-by-num-docs-on-small-segments.png
>
>
> [~dm] and [~dimitrisli] looked into an interesting case where indexing slowed
> down after upgrading to 8.7. After digging we identified that this was due to
> the merging of stored fields, which had become slower on average.
> This is due to changes to stored fields, which now have top-level blocks that
> are then split into sub-blocks and compressed using shared dictionaries (one
> dictionary per top-level block). As the top-level blocks are larger than they
> were before, segments are more likely to be considered "dirty" by the merging
> logic. Dirty segments are segments were 1% of the data or more consists of
> incomplete blocks. For large segments, the size of blocks doesn't really
> affect the dirtiness of segments: if you flush a segment that has 100 blocks
> or more, it will never be considered dirty as only the last block may be
> incomplete. But for small segments it does: for instance if your segment is
> only 10 blocks, it is very likely considered dirty given that the last block
> is always incomplete. And the fact that we increased the top-level block size
> means that segments that used to be considered clean might now be considered
> dirty.
> And indeed benchmarks reported that while large stored fields merges became
> slightly faster after upgrading to 8.7, the smaller merges actually became
> slower. See attached chart, which gives the total merge time as a function of
> the number of documents in the segment.
> I don't know how we can address this, this is a natural consequence of the
> larger block size, which is needed to achieve better compression ratios. But
> I wanted to open an issue about it in case someone has a bright idea how we
> could make things better.
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