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https://issues.apache.org/jira/browse/LUCENE-8920?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16948770#comment-16948770
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Michael Sokolov commented on LUCENE-8920:
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> Can we simply change the constant fst.Builder.DIRECT_ARC_LOAD_FACTOR to 1.5f
> (making it float)?
That seems fine as an interim measure. It's hard to know what the impact would
be on the ES benchmarks though.
I saw performance improvements primarily in Suggesters which are heavy users of
FSTs; numbers are posted in https://issues.apache.org/jira/browse/LUCENE-8781
> Reduce size of FSTs due to use of direct-addressing encoding
> -------------------------------------------------------------
>
> Key: LUCENE-8920
> URL: https://issues.apache.org/jira/browse/LUCENE-8920
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael Sokolov
> Priority: Blocker
> Fix For: 8.3
>
> Time Spent: 1h 10m
> Remaining Estimate: 0h
>
> Some data can lead to worst-case ~4x RAM usage due to this optimization.
> Several ideas were suggested to combat this on the mailing list:
> bq. I think we can improve thesituation here by tracking, per-FST instance,
> the size increase we're seeing while building (or perhaps do a preliminary
> pass before building) in order to decide whether to apply the encoding.
> bq. we could also make the encoding a bit more efficient. For instance I
> noticed that arc metadata is pretty large in some cases (in the 10-20 bytes)
> which make gaps very costly. Associating each label with a dense id and
> having an intermediate lookup, ie. lookup label -> id and then id->arc offset
> instead of doing label->arc directly could save a lot of space in some cases?
> Also it seems that we are repeating the label in the arc metadata when
> array-with-gaps is used, even though it shouldn't be necessary since the
> label is implicit from the address?
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