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From "Jake Mannix (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1997) Explore performance of multi-PQ vs single-PQ sorting API
Date Wed, 28 Oct 2009 17:45:59 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1997?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12771017#action_12771017
] 

Jake Mannix commented on LUCENE-1997:
-------------------------------------

But part of the point is that you don't have to get the values - you can have a fast in-memory
structure which just encodes their sort-order, right?  This is the whole point of using the
ordinal - you pre-sort all of the possible values to get the ordinals, and now arbitrarily
complex comparator reduces to int compare at sort time(*).  In the custom comparators we use,
for example, this allows for even sorting by multivalued fields in a custom way, via the simple
compare(int doca, int docb) way.

(*) This reminds me: Mike, you switched the compare for ord values from being "return ordA
- ordB" to being "return ordA > ordB ? 1 : (ordA == ordB ? 0 : -1)", on the basis of int
overflow at some point, right?  This is only true if we're really sorting by integers, which
could overflow - if they're ordinals, then these are both non-negative numbers, and their
difference will always be greater than -MAX_INT, so the branching can be avoided in this innermost
comparison in this case.

> Explore performance of multi-PQ vs single-PQ sorting API
> --------------------------------------------------------
>
>                 Key: LUCENE-1997
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1997
>             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
>
>
> 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/sortBench.py).
> 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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