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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2075) Share the Term -> TermInfo cache across threads
Date Sat, 21 Nov 2009 11:04:39 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2075?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780946#action_12780946
] 

Michael McCandless commented on LUCENE-2075:
--------------------------------------------

{quote}
bq. Also, the results for ConcurrentLRUCache are invalid (its hit rate is way too high) -
I think this is because its eviction process can take a longish amount of time, which temporarily
allows the map to hold way too many entries, and means it's using up alot more transient RAM
than it should.

Yep - there's no hard limit. It's not an issue in practice in Solr since doing the work to
generate a new entry to put in the cache is much more expensive than cache cleaning (i.e.
generation will never swamp cleaning).  Seems like a realistic benchmark would do some amount
of work on a cache miss? Or perhaps putting it in lucene and doing real benchmarks?
{quote}

I agree the test is synthetic, so the blowup we're seeing is a worse
case sitatuion, but are you really sure this can never be hit in
practice?

EG as CPUs gain more and more cores... it becomes more and more
possible with time that the 1 thread that's trying to do the cleaning
will be swamped by the great many threads generating.  Then if the CPU
is over-saturated (too many threads running), that 1 thread doing the
cleaning only gets slices of CPU time vs all the other threads that
may be generating...

It makes me nervous using a collection that, in the "perfect storm",
suddenly consumes way too much RAM.  It's a leaky abstraction.

That said, I agree the test is obviously very synthetic.  It's not
like a real Lucene installation will be pushing 2M QPS through Lucene
any time soon...

But still I'm more comfortable w/ the simplicity of the double-barrel
approach.  In my tests its performance is in the same ballpark as
ConcurrentLRUCache; it's much simpler; and the .clear() calls appear
in practice to very quickly free up the entries.

{quote}
bq. Another idea: I wonder whether a simple cache-line like cache would be sufficient. Ie,
we hash to a fixed slot and we evict whatever is there.

We need to balance the overhead of the cache with the hit ratio and the cost of a miss. for
the String intern cache, the cost of a miss is very low, hence lowering overhead but giving
up hit ratio is the right trade-off. For this term cache, the cost of a miss seems relatively
high, and warrants increasing overhead to increase the hit ratio.
{quote}

OK I agree.

Yet another option... would be to create some sort of "thread-private
Query scope", ie, a store that's created & cleared per-Query where
Lucene can store things.  When a Term's info is retrieved, it'd be
stored here, and then that "query-private" cache is consulted whenever
that Term is looked up again within that query.  This would be the
"perfect cache" in that a single query would never see its terms
evicted due to other queries burning through the cache...

Though, net/net I suspect the overhead of creating/pulling from this
new cache would just be an overall search slowdown in practice.


> Share the Term -> TermInfo cache across threads
> -----------------------------------------------
>
>                 Key: LUCENE-2075
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2075
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael McCandless
>            Priority: Minor
>             Fix For: 3.1
>
>         Attachments: ConcurrentLRUCache.java, LUCENE-2075.patch, LUCENE-2075.patch, LUCENE-2075.patch,
LUCENE-2075.patch, LUCENE-2075.patch, LUCENE-2075.patch
>
>
> Right now each thread creates its own (thread private) SimpleLRUCache,
> holding up to 1024 terms.
> This is rather wasteful, since if there are a high number of threads
> that come through Lucene, you're multiplying the RAM usage.  You're
> also cutting way back on likelihood of a cache hit (except the known
> multiple times we lookup a term within-query, which uses one thread).
> In NRT search we open new SegmentReaders (on tiny segments) often
> which each thread must then spend CPU/RAM creating & populating.
> Now that we are on 1.5 we can use java.util.concurrent.*, eg
> ConcurrentHashMap.  One simple approach could be a double-barrel LRU
> cache, using 2 maps (primary, secondary).  You check the cache by
> first checking primary; if that's a miss, you check secondary and if
> you get a hit you promote it to primary.  Once primary is full you
> clear secondary and swap them.
> Or... any other suggested approach?

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