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https://issues.apache.org/jira/browse/LUCENE-8920?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16931367#comment-16931367
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Mike Sokolov commented on LUCENE-8920:
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This is cool. Regarding the strategy for which encoding to apply, I'll just
call out the current heuristics:
{{doFixedArray = }}{{(node depth (distance from root node) <= 3 and N >= 5) or
N >= 10}}
{{writeDirectly = doFixedArray && (max_label - min_label) < 4 * N}}
{{I think we can still consider that we would apply list-encoding for small N,
and consider open addressing as a variant within "doFixedArray," where we now
can choose among direct addressing (for least load factors L), open addressing
(for intermediate case), and binary search for highest L. Does that sound
right?}}
{{I wonder if we could work backwards from a single parameter L: maximum memory
cost (vs list encoding). The API would guarantee that no set of arcs is ever
encoded using more than L * the minimum possible, and then internally we choose
the best (ie fastest lookup) encoding that achieves that, possibly with some
tweak for higher-order arcs (ie near the root).}}
> 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: Mike 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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