[jira] [Commented] (LUCENE-8681) Prorated early termination

["Mike Sokolov (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-8681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16818266#comment-16818266 ] 

Mike Sokolov commented on LUCENE-8681:
--------------------------------------

I updated the PR with a new patch that changes the API for creating collectors that can early terminate to use an enum to see what it would look like. Is this what you had in mind [~rcmuir]? For example {{TopFieldCollector.create(int numHits, int countingThreshold)}} becomes  {{TopFieldCollector.create(int numHits, TerminationStrategy terminationStrategy)}}, and so on. This masks the complexity a bit, and adds an explanatory label at the coset of a small loss of flexibility (you can no longer specify exactly how many hits should be counted exactly; you just get the choice of counting the number of results, or counting up to 1000. The change is mostly impacting tests, and some internal calls in IndexSearcher. It certainly simplifies the calling API for the pro-rating so it no longer must explain what the thresholding parameter is all about, so I think that's an improvement.

> Prorated early termination
> --------------------------
>
>                 Key: LUCENE-8681
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8681
>             Project: Lucene - Core
>          Issue Type: Improvement
>          Components: core/search
>            Reporter: Mike Sokolov
>            Priority: Major
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> In this issue we'll exploit the distribution of top K documents among segments to extract performance gains when using early termination. The basic idea is we do not need to collect K documents from every segment and then merge. Rather we can collect a number of documents that is proportional to the segment's size plus an error bound derived from the combinatorics seen as a (multinomial) probability distribution.
> https://github.com/apache/lucene-solr/pull/564 has the proposed change.
> [~rcmuir] pointed out on the mailing list that this patch confounds two settings: (1) whether to collect all hits, ensuring correct hit counts, and (2) whether to guarantee that the top K hits are precisely the top K.
> The current patch treats this as the same thing. It takes the position that if the user says it's OK to have approximate counts, then it's also OK to introduce some small chance of ranking error; occasionally some of the top K we return may draw from the top K + epsilon.
> Instead we could provide some additional knobs to the user. Currently the public API is {{TopFieldCOllector.create(Sort, int, FieldDoc, int threshold)}}. The threshold parameter controls when to apply early termination; it allows the collector to terminate once the given number of documents have been collected.
> Instead of using the same threshold to control leaf-level early termination, we could provide an additional leaf-level parameter. For example, this could be a scale factor on the error bound, eg a number of standard deviations to apply. The patch uses 3, but a much more conservative bound would be 4 or even 5. With these values, some speedup would still result, but with a much lower level of ranking errors. A value of MAX_INT would ensure no leaf-level termination would ever occur.
> We could also hide the precise numerical bound and offer users a three-way enum (EXACT, APPROXIMATE_COUNT, APPROXIMATE_RANK) that controls whether to apply this optimization, using some predetermined error bound.
> I posted the patch without any user-level tuning since I think the user has already indicated a preference for speed over precision by specifying a finite (global) threshold, but if we want to provide finer control, these two options seem to make the most sense to me. Providing access to the number of standard deviation to allow from the expected distribution gives the user the finest control, but it could be hard to explain its proper use.



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)

---------------------------------------------------------------------
To unsubscribe, e-mail: dev-unsubscribe@lucene.apache.org
For additional commands, e-mail: dev-help@lucene.apache.org