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https://issues.apache.org/jira/browse/OAK-4882?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15605371#comment-15605371
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Tomek Rękawek commented on OAK-4882:
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[~mmarth], the problem here is as follows:
a) in case of a high load, there is a lot of persistent cache updates,
b) the H2 MVStore used for the persistence, sometimes need to maintain its
files,
c) the maintenance takes place in the thread invoking {{put()}} method. The
maintenance process blocks all the other threads from putting anything into the
cache (so they have to wait until maintenance is done) and basically makes the
impression that the whole instance hangs.
That's why the OAK-2761 introduced mechanism to {{put()}} entries to the
MVStore asynchronously, using some queue. The question is what to do if the
queue is full. Should we:
1) block the caller - however, this will lead to (c) case again, as after some
time of high load the queue will be full again,
2) remove the oldest or drop the most recent {{put()}} entries - this option
has disadvantage described in your post
3) ?
> Bottleneck in the asynchronous persistent cache
> -----------------------------------------------
>
> Key: OAK-4882
> URL: https://issues.apache.org/jira/browse/OAK-4882
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: cache, documentmk
> Affects Versions: 1.5.10, 1.4.8
> Reporter: Tomek Rękawek
> Assignee: Tomek Rękawek
> Fix For: 1.6
>
>
> The class responsible for accepting new cache operations which will be
> handled asynchronously is
> [CacheActionDispatcher|https://github.com/apache/jackrabbit-oak/blob/trunk/oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/document/persistentCache/async/CacheActionDispatcher.java].
> In case of a high load, when the queue is full (=1024 entries), the
> [add()|https://github.com/apache/jackrabbit-oak/blob/trunk/oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/document/persistentCache/async/CacheActionDispatcher.java#L86]
> method removes the oldest 256 entries. However, we can't afford losing the
> updates (as it may result in having stale entries in the cache), so all the
> removed entries are compacted into one big invalidate action.
> The compaction action
> ([CacheActionDispatcher#cleanTheQueue|https://github.com/apache/jackrabbit-oak/blob/trunk/oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/document/persistentCache/async/CacheActionDispatcher.java#L97])
> still holds the lock taken in add() method, so threads which tries to add
> something to the queue have to wait until cleanTheQueue() ends.
> Maybe we can optimise the CacheActionDispatcher#add->cleanTheQueue part, so
> it won't hold the lock for the whole time.
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