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https://issues.apache.org/jira/browse/LUCENE-2075?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Michael McCandless updated LUCENE-2075:
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Attachment: LUCENE-2075.patch
First cut at a benchmark. First, download
http://concurrentlinkedhashmap.googlecode.com/files/clhm-production.jar
and put into your lib subdir, then run "ant -lib
lib/clhm-production.jar compile-core", then run it something like
this:
{code}
java -server -Xmx1g -Xms1g -cp build/classes/java:lib/clhm-production.jar
org.apache.lucene.util.cache.LRUBench 4 5.0 0.0 1024 1024
{code}
The args are:
* numThreads
* runSec
* sharePct -- what %tg of the terms should be shared b/w the threads
* cacheSize
* termCountPerThread -- how many terms each thread will cycle through
The benchmark first sets up arrays of strings, per thread, based
termsCountPerThread & sharePct. Then each thread steps through the
array, and for each entry, tries to get the string, and if it's not
present, puts it. It records the hit & miss count, and prints summary
stats in the end, doing 3 rounds.
To mimic Lucene, each entry is tested twice in a row, ie, the 2nd time
we test the entry, it should be a hit. Ie we expect a hit rate of 50%
if sharePct is 0.
Here's my output from the above command line, using Java 1.6.0_14 (64
bit) on OpenSolaris:
{code}
numThreads=4 runSec=5.0 sharePct=0.0 cacheSize=1024 termCountPerThread=1024
LRU cache size is 1024; each thread steps through 1024 strings; 0 of which are
common
round 0
sync(LinkedHashMap): Mops/sec=2.472 hitRate=50.734
DoubleBarreLRU: Mops/sec=20.502 hitRate=50
ConcurrentLRU: Mops/sec=17.936 hitRate=84.409
ConcurrentLinkedHashMap: Mops/sec=1.248 hitRate=50.033
round 1
sync(LinkedHashMap): Mops/sec=2.766 hitRate=50.031
DoubleBarreLRU: Mops/sec=17.66 hitRate=50
ConcurrentLRU: Mops/sec=17.82 hitRate=83.726
ConcurrentLinkedHashMap: Mops/sec=1.266 hitRate=50.331
round 2
sync(LinkedHashMap): Mops/sec=2.714 hitRate=50.168
DoubleBarreLRU: Mops/sec=17.912 hitRate=50
ConcurrentLRU: Mops/sec=17.866 hitRate=84.156
ConcurrentLinkedHashMap: Mops/sec=1.26 hitRate=50.254
{code}
NOTE: I'm not sure about the correctness of DoubleBarrelLRU -- I just
quickly wrote it.
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.
In theory DoubleBarrelLRU should be vulnerable to the same issue, but
in practice it seems to affect it much less (I guess because
CHM.clear() must be very fast).
I'm not sure how to fix the benchmark to workaround that... maybe we
bring back the cleaning thread (from Solr's version), and give it a
high priority?
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.
> 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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