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https://issues.apache.org/jira/browse/AMQ-6239?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15235057#comment-15235057
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Tim Bain commented on AMQ-6239:
-------------------------------
This algorithm isn't thread-safe in the face of concurrent removals, so if we
expect to use it when not holding an exclusive lock, we need a change. The
find-the-next-iterator logic needs to exist in next() as well (and so should be
refactored out into a helper method that I've called getIteratorThatHasNext()
in the code below), to handle the case where the current iterator has a
remaining item when hasNext() is called but no longer does when next() is
called.
The logic of next() should be:
while (true) {
Iterator<MessageReference> iterator = getIteratorThatHasNext();
if (iterator == null) {
return null;
}
MessageReference messageReference = iterator.next;
if (messageReference != null) {
return messageReference;
}
}
This algorithm will keep getting the next iterator that should have a next
item, and if it turns out not to then we'll keep getting the next one, until we
either fine one with a non-null next() value or we run out of iterators to try.
> Performance issue in PrioritizedPendingListIterator
> ---------------------------------------------------
>
> Key: AMQ-6239
> URL: https://issues.apache.org/jira/browse/AMQ-6239
> Project: ActiveMQ
> Issue Type: Improvement
> Components: Broker
> Affects Versions: 5.12.2
> Reporter: Martin Lichtin
> Attachments: AMQ-6239-yourkit-1.jpg, PrioritizedPendingList.java.patch
>
>
> Sending and consuming 5000 messages to/from a queue, one can see heavy CPU
> use on the broker side (v 5.12.2).
> Yourkit shows
> PrioritizedPendingList$PrioritizedPendingListIterator.<init>
> as a hot spot method. It calls ArrayList.add(Object) around 12 mio times.
> Situation is that FilePendingMessageCursor.isEmpty() iterates over in-memory
> messages and therefore (as it is a prioritized queue) uses
> PrioritizedPendingListIterator which uses OrderedPendingList.getAsList()
> which overall turns out to be an expensive method as it converts the
> self-managed linked list to a Java ArrayList and then this list is filled
> into another ArrayList managed by PrioritizedPendingListIterator.
> PrioritizedPendingListIterator could be improved to walk the priority lists
> via OrderedPendingList iterators, as these are implemented efficiently.
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