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https://issues.apache.org/jira/browse/LUCENE-7905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16084281#comment-16084281
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Michael McCandless commented on LUCENE-7905:
--------------------------------------------
bq. Its a bit tricky to see the diffs since the file got moved too, but
basically it just replaces MultiTermsEnum with a standard PQ?
That's it, except I also changed:
{noformat}
while (segmentOrds[segmentIndex] <= segmentOrd) {
ordDeltaBits[segmentIndex] |= delta;
ordDeltas[segmentIndex].add(delta);
segmentOrds[segmentIndex]++;
}
{noformat}
to:
{noformat}
assert segmentOrds[segmentIndex] <= segmentOrd;
do {
ordDeltas[segmentIndex].add(delta);
segmentOrds[segmentIndex]++;
} while (segmentOrds[segmentIndex] <= segmentOrd);
{noformat}
Which should make the branch easier to predict (since the loop will always run
the first time), but maybe the effect is negligible.
I think likely the cost we're saving from MTE is its {{TermMergeQueue.fillTop}}
method? It's doing a lot of work, sort of recursing into the PQ, with a if
inside a for inside a while, to find all subs on the current term, and then it
has to do {{pushTop}} after that. In general MTE is not allowed to .next() the
subs because it doesn't know if the caller will ask for postings on this term.
[~rcmuir] suggested we could maybe make pullTop()/pushTop() lazy which is a
neat idea...
> Optimizations for OrdinalMap
> ----------------------------
>
> Key: LUCENE-7905
> URL: https://issues.apache.org/jira/browse/LUCENE-7905
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Fix For: 7.1
>
> Attachments: LUCENE-7905.patch
>
>
> {{OrdinalMap}} is a useful class to quickly map per-segment ordinals to
> global space, but it's fairly costly to build, which must typically be done
> on every NRT refresh.
> I'm using it quite heavily in two different places, one for
> {{SortedSetDocValuesFacetCounts}}, and another custom usage, and I found some
> small optimizations to improve its construction time.
> I switched it to use a simple priority queue to merge the terms instead of
> the more general {{MultiTermsEnum}}, which does extra work since it must also
> provide postings, implement seekExact, etc.
> I also pulled {{OrdinalMap}} out into its own oal.index class.
> When testing construction time for my case the patch is ~16% faster (159.9s
> -> 134.2s) in one case with 91.4 M terms and ~9% faster (115.6s -> 105.7s) in
> another case with 26.6 M terms.
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