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https://issues.apache.org/jira/browse/LUCENE-7258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15290023#comment-15290023
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Adrien Grand commented on LUCENE-7258:
--------------------------------------
[~dsmiley] This is because of the following block of code. If the largest
buffer is large enough to hold all docs then we reuse it. I tried to generalize
your suggestion to reuse the buffer if there is a single buffer.
{code}
+ int[] docs = largestBuffer.array;
+ if (docs.length < totalLength + 1) {
+ docs = Arrays.copyOf(docs, totalLength + 1);
+ }
{code}
> Tune DocIdSetBuilder allocation rate
> ------------------------------------
>
> Key: LUCENE-7258
> URL: https://issues.apache.org/jira/browse/LUCENE-7258
> Project: Lucene - Core
> Issue Type: Improvement
> Components: modules/spatial
> Reporter: Jeff Wartes
> Attachments:
> LUCENE-7258-Tune-memory-allocation-rate-for-Intersec.patch,
> LUCENE-7258-Tune-memory-allocation-rate-for-Intersec.patch,
> LUCENE-7258-expanding.patch, LUCENE-7258.patch, allocation_plot.jpg
>
>
> LUCENE-7211 converted IntersectsPrefixTreeQuery to use DocIdSetBuilder, but
> didn't actually reduce garbage generation for my Solr index.
> Since something like 40% of my garbage (by space) is now attributed to
> DocIdSetBuilder.growBuffer, I charted a few different allocation strategies
> to see if I could tune things more.
> See here: http://i.imgur.com/7sXLAYv.jpg
> The jump-then-flatline at the right would be where DocIdSetBuilder gives up
> and allocates a FixedBitSet for a 100M-doc index. (The 1M-doc index
> curve/cutoff looked similar)
> Perhaps unsurprisingly, the 1/8th growth factor in ArrayUtil.oversize is
> terrible from an allocation standpoint if you're doing a lot of expansions,
> and is especially terrible when used to build a short-lived data structure
> like this one.
> By the time it goes with the FBS, it's allocated around twice as much memory
> for the buffer as it would have needed for just the FBS.
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