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https://issues.apache.org/activemq/browse/AMQ-2952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=62927#action_62927
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Rob Janney commented on AMQ-2952:
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We ran into this issue with one of our 5.4.1 AMQ instances in production. We
found that once all consumers are full, the remaining messages are all assigned
to a single consumer. When we reproduced this issue in test & stepped through
the Queue class in AMQ, we found that this happens because the group logic is
done before the full detection.
For example, we had 15 consumers with a prefetch size of 1 and a backlog of
1000 messages. Each consumer had at least 1 message assigned to it & some had
a few more than that (possibly finished processing the first message before all
consumers were full). Once all 15 consumers were full, the remaining 980 or so
messages were assigned to the same consumer :( This lead to a significant
performance problem where we had all the work waiting on a single thread and
the other 14 threads idle doing nothing.
We were able to work around this problem by setting the pre-fetch to 100 for
our 15 consumers. This allowed the messages to be dispatched in a
round-robin fashion to the first 1500 messages in the queue.
The fix above to only do group operations on non-full consumers should also
resolve the effect we observed.
> Message groups with small prefetch
> ----------------------------------
>
> Key: AMQ-2952
> URL: https://issues.apache.org/activemq/browse/AMQ-2952
> Project: ActiveMQ
> Issue Type: Bug
> Components: Broker
> Affects Versions: 5.4.1
> Reporter: Dejan Bosanac
> Assignee: Dejan Bosanac
> Fix For: 5.5.0
>
>
> Closing and starting message groups with a lot of consumers and a small
> prefetch could result in messages being dispatched to wrong consumers. The
> problem is that message group logic is done before we detecting weather the
> consumer is full or not.
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