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https://issues.apache.org/jira/browse/JCS-171?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15828534#comment-15828534
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Wiktor N commented on JCS-171:
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In both cases you have 8 threads processing cache events. In thread pool mode
any thread may get event for any cache region (and file). So you can have a
situation, that you have two threads trying to process event for one cache.
Because of the locking in *DiskCache, you can have only one writing thread. So
the other waits.
In single queue, the processing thread process the events only for it's own
cache. So there is no lock contention and basically, only cache reads may block
it from writing the cache.
As for why you get OOM in threaded mode - the only guess I have, is that single
mode process events so fast, that the queue doesn't grow that much, thus you
don't get OOM's. It's interesting, what is the High Water Mark on the queue in
the single mode, and then - check - how many events would be dropped, with such
boundarySize of thread queue.
> Multiple CacheEventQueue.QProcessor spawned for the same cache region
> ---------------------------------------------------------------------
>
> Key: JCS-171
> URL: https://issues.apache.org/jira/browse/JCS-171
> Project: Commons JCS
> Issue Type: Bug
> Components: Composite Cache
> Affects Versions: jcs-2.0
> Reporter: Wiktor N
> Assignee: Thomas Vandahl
> Fix For: jcs-2.1
>
> Attachments: CacheEventQueue.patch, jcs-perf-test.zip
>
>
> I noticed that running on new version of JCS I get multiple
> CacheEventQueue.QProcessor thread. They spawn from time to time.
> I've checked recent changes and changes few things in r1774925 look
> suspicious:
> 1. In previous code we spawned a new thread in synchronized section. This got
> us a guarantee, that there will be no two threads trying to spawn a new
> thread in the same time. Maybe some locking is needed around thread creation?
> 2. QProcessor uses isAlive() method. But this is defined by Thread.isAlive()
> while it should probably check for CacheEventQueue.this.isAlive()
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