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https://issues.apache.org/jira/browse/OPENJPA-637?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12606078#action_12606078
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Jeremy Bauer commented on OPENJPA-637:
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Several attempts were made to tune the max size of the data cache with base
1.2.0 code. Performance started to degrade at a certain point by further
increasing the max cache size . The benchmark showed better results with a max
size of 5000 than it did for 10000+. (Default (1000) and 15000 were also
tested, 5000 appeared to be optimal for this workload.) The database tables
used in the benchmark fluctuate around 35000 rows, which roughly equate to
entities.
Out of curiousity, instead of using CacheMap as the cache store in
ConcurrentDataCache, java.util.concurrent.ConcurrentHashMap was used directly
in its place (soft ref, write locking, and pinning support were removed for
simplification). This also eliminated the max size and null handling aspects of
the cache. Benchmark performance with this configuration was very similar to
measurements taken with the data cache disabled. Database (on a separate
server) utilization was down considerably, which was good & expected, but the
benchmark was not showing a performance improvement. In contrast, 1.2.0 using
the 1.0.x code showed a ~20% improvement when the data cache (max cache size
5000) was enabled.
> Significant performance degradation when data cache is enabled
> --------------------------------------------------------------
>
> Key: OPENJPA-637
> URL: https://issues.apache.org/jira/browse/OPENJPA-637
> Project: OpenJPA
> Issue Type: Bug
> Components: datacache, lib
> Affects Versions: 1.2.0
> Reporter: Jeremy Bauer
> Attachments: OPENJPA-637.patch
>
>
> Performance testing is showing a severe data cache performance degradation
> when moving from 1.0.x OpenJPA code to 1.2.0 level code. Profiling showed
> the problem to be in the new random eviction scheme which runs when the cache
> reaches its maximum number of entries. This code was changed significantly
> when OpenJPA moved to Java 5 java.util.concurrent.ConcurrentHashMap and away
> from the OpenJPA implementation of ConcurrentHashMap. A macro-benchmark
> showed a 20% performance degradation from base 1.2.0 code when the cache
> reaches its maximum size; prompting eviction in order to add new cache
> entries.
> I've found that the new random eviction code appears to be improved in the
> very recent 666903 commit, but data cache performance remains considerably
> slower than the 1.0.x implementation. Profiles with the 666903 changes show
> test threads to be waiting on the reentrant write lock in the CacheMap
> wrapper (which now wrappers a max size capable, null handling, subclass of
> java.util.concurrent.ConcurrentHashMap). Investigation is underway to
> determine whether the write lock is necessary (ie. can
> java.util.conncurrentConcurrentHashMap manage the cache without the need for
> external locking) and/or if changes could be made which would result in a
> significant reduction in contention for the lock. Any thoughts/ideas on that
> would be extremely helpful.
> Performance tests run with the 1.2.0 code base, using the OpenJPA version of
> ConcurrentHashMap (instead of the Java 5
> java.util.concurrent.ConcurrentHashMap-based implementation) have shown that
> performance of the data cache is significantly better when the legacy OpenJPA
> implementation is used. Based on the results, it appears that OpenJPA should
> be using the the legacy ConcurrentHashMap instead of the Java 5-based
> implementation -- or the new Java 5-based implementation needs to be improved
> considerably in order to perform as well as 1.0.x.
> I am opening this as a 1.2.0 issue, although it very likely affects 1.1.x as
> well. Testing has not been performed on 1.1.x to confirm the problem exists
> on that release.
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