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From "Eddie Epstein (JIRA)" <uima-...@incubator.apache.org>
Subject [jira] Commented: (UIMA-1110) The default setting of prefetch=1 is causing problems
Date Fri, 09 Jan 2009 19:12:00 GMT

    [ https://issues.apache.org/jira/browse/UIMA-1110?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12662478#action_12662478
] 

Eddie Epstein commented on UIMA-1110:
-------------------------------------

This situation appears to be resolved.

1. the default prefetch value for a service input queue is 0, which results in accurate load
balancing. If there is a bandwidth problem with 0, which we have not seen yet, it can be overridden
in the deployment descriptor.

2. remote delegates now always have a [temp] reply queue on the remote broker, and the prefetch
value for these are always 1. Testing has shown a big improvement going from 0 to 1, but little
or no difference between 1 and 10. Values larger than 1 can be problematic when reply CASes
are very large.

3. the free CAS queue should always use a value of prefetch>0. We need to confirm this
is done.

> The default setting of prefetch=1 is causing problems
> -----------------------------------------------------
>
>                 Key: UIMA-1110
>                 URL: https://issues.apache.org/jira/browse/UIMA-1110
>             Project: UIMA
>          Issue Type: Bug
>          Components: Async Scaleout
>            Reporter: Eddie Epstein
>            Assignee: Eddie Epstein
>
> The new JMX monitor occasionally shows idle times of many seconds for a scaled out remote
delegate that is getting many CASes per second. Assuming the monitor is correct, the problem
is that AMQ is not evenly distributing the work to all the service instances. Other recent
experiences have demonstrated that uneven distribution is solved with prefetch=0. 
> While there is some possibility that prefetch > 0 may be useful in some situations
for remote delegates, for colocated delegates there is little doubt that a prefetch greater
than 0 cannot help performance and can only screw up load balancing.
> prefetch=0 should be the default everywhere.

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