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From "Jake Mannix (JIRA)" <>
Subject [jira] Commented: (LUCENE-1997) Explore performance of multi-PQ vs single-PQ sorting API
Date Tue, 03 Nov 2009 18:51:32 GMT


Jake Mannix commented on LUCENE-1997:

bq. Since each approach has distinct advantages, why not offer both ("simple" and "expert")
comparator extensions APIs?

+1 from me on this one, as long as the simpler one is around.  I'll bet we'll find that we
regret keeping the "expert" one by 3.2 or so though, but I'll take any compromise which gets
the simpler API in there.

bq. Don't forget that this is multiplied by however many queries are currently in flight.

Sure, so if you're running with 100 queries per second on a single shard (pretty fast!), with
100 segments, and you want to do sorting by value on the top 1000 values (how far down the
long tail of extreme cases are we at now?  Do librarians hit their search servers with 100
QPS and have indices poorly built with hundreds of segments and can't take downtime to *ever*
optimize?), we're now talking about 40MB.  

*Forty megabytes*.  On a beefy machine which is supposed to be handling 100QPS across an index
big enough to need 100 segments.  How much heap would such a machine already be allocating?
 4GB?  6?  More? 

We're talking about less than 1% of the heap is being used by the multiPQ approach in comparison
to singlePQ.

> Explore performance of multi-PQ vs single-PQ sorting API
> --------------------------------------------------------
>                 Key: LUCENE-1997
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.9
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>         Attachments: LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch,
LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch
> Spinoff from recent "lucene 2.9 sorting algorithm" thread on java-dev,
> where a simpler (non-segment-based) comparator API is proposed that
> gathers results into multiple PQs (one per segment) and then merges
> them in the end.
> I started from John's multi-PQ code and worked it into
> contrib/benchmark so that we could run perf tests.  Then I generified
> the Python script I use for running search benchmarks (in
> contrib/benchmark/
> The script first creates indexes with 1M docs (based on
> SortableSingleDocSource, and based on wikipedia, if available).  Then
> it runs various combinations:
>   * Index with 20 balanced segments vs index with the "normal" log
>     segment size
>   * Queries with different numbers of hits (only for wikipedia index)
>   * Different top N
>   * Different sorts (by title, for wikipedia, and by random string,
>     random int, and country for the random index)
> For each test, 7 search rounds are run and the best QPS is kept.  The
> script runs singlePQ then multiPQ, and records the resulting best QPS
> for each and produces table (in Jira format) as output.

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