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Posted to dev@lucene.apache.org by Richard Marr <ri...@gmail.com> on 2009/07/30 12:28:59 UTC
Re: [jira] Commented: (LUCENE-1690) Morelikethis queries are very
slow compared to other search types
Yeah, having this stuff stored centrally behind the IndexReader seems
like a better idea than having it in client classes. My shallow
knowledge of the code isn't helping me explain why it's not performing
though.
Out of interest, how come it's a per-thread cache? I don't understand
all the issues involved but that surprised me.
2009/7/30 Michael McCandless (JIRA) <ji...@apache.org>:
>
> [ https://issues.apache.org/jira/browse/LUCENE-1690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12737059#action_12737059 ]
>
> Michael McCandless commented on LUCENE-1690:
> --------------------------------------------
>
> OK now I feel silly -- this cache is in fact very similar to the caching that Lucene already does, internally! Sorry I didn't catch this overlap sooner.
>
> In oal.index.TermInfosReader.java there's an LRU cache, default size 1024, that holds recently retrieved terms and their TermInfo. It uses oal.util.cache.SimpleLRUCache.
>
> There are some important differences from this new cache in MLT. EG, it holds the entire TermInfo, not just the docFreq. Plus, it's a central cache for any & all term lookups that go through the SegmentReader. Also, it's stored in thread-private storage, so each thread has its own cache.
>
> But, now I'm confused: how come you are not already seeing the benefits of this cache? You ought to see MLT queries going faster. This core cache was first added in 2.4.x; it looks like you were testing against 2.4.1 (from the "Affects Version" on this issue).
>
>> Morelikethis queries are very slow compared to other search types
>> -----------------------------------------------------------------
>>
>> Key: LUCENE-1690
>> URL: https://issues.apache.org/jira/browse/LUCENE-1690
>> Project: Lucene - Java
>> Issue Type: Improvement
>> Components: contrib/*
>> Affects Versions: 2.4.1
>> Reporter: Richard Marr
>> Priority: Minor
>> Attachments: LruCache.patch, LUCENE-1690.patch, LUCENE-1690.patch
>>
>> Original Estimate: 2h
>> Remaining Estimate: 2h
>>
>> The MoreLikeThis object performs term frequency lookups for every query. From my testing that's what seems to take up the majority of time for MoreLikeThis searches.
>> For some (I'd venture many) applications it's not necessary for term statistics to be looked up every time. A fairly naive opt-in caching mechanism tied to the life of the MoreLikeThis object would allow applications to cache term statistics for the duration that suits them.
>> I've got this working in my test code. I'll put together a patch file when I get a minute. From my testing this can improve performance by a factor of around 10.
>
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Richard Marr
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Re: [jira] Commented: (LUCENE-1690) Morelikethis queries are very
slow compared to other search types
Posted by Richard Marr <ri...@gmail.com>.
2009/7/30 Michael McCandless <lu...@mikemccandless.com>:
> Good question...
Good answer. Thanks.
I guess the next step then is to understand why the TermInfo cache
isn't getting the performance to where it could be. It'll take me a
while to get to the point where I can answer that question. If
anyone's in a hurry it'd probably be worth someone looking at it.
Rich
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Re: [jira] Commented: (LUCENE-1690) Morelikethis queries are very
slow compared to other search types
Posted by Michael Busch <bu...@gmail.com>.
On 7/30/09 4:10 AM, Michael McCandless wrote:
> Plus, the original motivation for this (LUCENE-1195) was because
> queries in general look up the same term at least 2 times during their
> execution (weight (idf computation), get postings), and so I think we
> wanted to ensure that a single thread doing its query would not see
> its terms evicted (due to many other threads coming through) by the
> 2nd time it needed to use them. But if we made the central cache
> "large enough", perhaps growing if it detects many threads, then this
> (other threads evicted my entries before I finished my query)
> shouldn't be a problem in practice.
>
> Mike
>
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>
Yes this was part of the motivation. Especially wildcard or range
queries could wipe out the entire cache before another thread does its
second term lookup.
If we had a lock-less cache then I agree simply making it larger would
probably be better than having separate caches per thread.
Also we should probably optimize the most common cases... if in rare
situations certain queries wipe out the cache it might not be such a big
deal.
Michael
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Re: [jira] Commented: (LUCENE-1690) Morelikethis queries are very
slow compared to other search types
Posted by Michael McCandless <lu...@mikemccandless.com>.
On Thu, Jul 30, 2009 at 6:28 AM, Richard Marr<ri...@gmail.com> wrote:
> Yeah, having this stuff stored centrally behind the IndexReader seems
> like a better idea than having it in client classes. My shallow
> knowledge of the code isn't helping me explain why it's not performing
> though.
>
> Out of interest, how come it's a per-thread cache? I don't understand
> all the issues involved but that surprised me.
Good question... making it thread private seems rather wasteful since
at heart this information (Term -> TermInfo) is constant across
threads and so we're wasting RAM.
Also, it's a non-trivial amount of RAM that we're tying up once the
cache is full: 1024 times maybe ~120 bytes per TermInfo on a 64bit jre
= ~120 KB, and it's somewhat devilish/unexpected ("principle of least
surprise") for Lucene to "do this" to any threads that come through
it.
I think one reason was to avoid having to synchronize on the lookups,
though with magic similar to LUCENE-1607 we could presumably make it
lockless.
Plus, the original motivation for this (LUCENE-1195) was because
queries in general look up the same term at least 2 times during their
execution (weight (idf computation), get postings), and so I think we
wanted to ensure that a single thread doing its query would not see
its terms evicted (due to many other threads coming through) by the
2nd time it needed to use them. But if we made the central cache
"large enough", perhaps growing if it detects many threads, then this
(other threads evicted my entries before I finished my query)
shouldn't be a problem in practice.
Mike
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