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https://issues.apache.org/jira/browse/LUCENE-10033?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17390827#comment-17390827
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Greg Miller commented on LUCENE-10033:
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{quote}I like this hybrid idea! Maybe another idea (which is hybrid too!) would
consist of doing part of the decoding for the entire block and part of the
decoding on a per-value basis. E.g. in the case of GCD compression, maybe we
could do the bit unpacking for the entire block but, but only apply the
multiplicative factor when fetching a single value.
{quote}
Oh yeah, that's a really interesting way to approach it as well! I wonder how
it would perform to always delay the application of GCD and/or the min delta
addition. It would be nice if no heuristics were needed to decide
whether-or-not to apply for the whole block up-front vs. delay, but I think
this code is auto-vectorizing when applying across the whole block? So maybe
some inefficiency in the "dense" cases if done on-demand. Hmm... cool idea!
{quote}I've tried to push some more optimizations by reusing ForUtil for low
numbers of bits per value and only subtracting the min value when it would
likely save space ...
{quote}
I'll see if I can pull your latest code down early next week and re-run our
internal benchmarks to see if there's much of a difference.
> Encode doc values in smaller blocks of values, like postings
> ------------------------------------------------------------
>
> Key: LUCENE-10033
> URL: https://issues.apache.org/jira/browse/LUCENE-10033
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Adrien Grand
> Priority: Minor
> Time Spent: 40m
> Remaining Estimate: 0h
>
> This is a follow-up to the discussion on this thread:
> https://lists.apache.org/thread.html/r7b757074d5f02874ce3a295b0007dff486bc10d08fb0b5e5a4ba72c5%40%3Cdev.lucene.apache.org%3E.
> Our current approach for doc values uses large blocks of 16k values where
> values can be decompressed independently, using DirectWriter/DirectReader.
> This is a bit inefficient in some cases, e.g. a single outlier can grow the
> number of bits per value for the entire block, we can't easily use run-length
> compression, etc. Plus, it encourages using a different sub-class for every
> compression technique, which puts pressure on the JVM.
> We'd like to move to an approach that would be more similar to postings with
> smaller blocks (e.g. 128 values) whose values get all decompressed at once
> (using SIMD instructions), with skip data within blocks in order to
> efficiently skip to arbitrary doc IDs (or maybe still use jump tables as
> today's doc values, and as discussed here for postings:
> https://lists.apache.org/thread.html/r7c3cb7ab143fd4ecbc05c04064d10ef9fb50c5b4d6479b0f35732677%40%3Cdev.lucene.apache.org%3E).
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