[jboss-jira] [JBoss JIRA] Commented: (JBMESSAGING-180) Design and implement a fairer message priority algorithm

[Attila Szegedi (JIRA)]

    [ https://jira.jboss.org/jira/browse/JBMESSAGING-180?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12447956#action_12447956 ] 

Attila Szegedi commented on JBMESSAGING-180:
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I don't have issues with starvation - it is really a problem if it causes a priority inversion in the application, which, honestly, is an application level problem and shouldn't be a concern of the messaging middleware. If the app suffers from priority inversion problems, an anti-starvation solution will only "help" it insofar as it'll allow the application to still run, but more slowly, instead of deadlocking (which happens without anti-starvation). I might actually prefer a deadlock (I can detect and analyze those nicely as all the participants are standing still) to a hard to diagnose slowdown. 

That said, a simple yet effective anti-starvation algorithm would be if you'd choose a random integer between 0 and 9 every now and then (every second?), and then only deliver messages with that priority or higher (as long as there's actual contention, of course). This'll make sure priority 9 messages are always delivered, priority 5 messages are suppressed half the time, and priority 0 messages are suppressed 9/10th of the time. (A different distribution can be achieved by nonlinear mapping of a wider random number range to the priority range). The solution only needs a random number generator local to the server-side processing component, and thus should scale nicely.

Anyway, just my 2c.


> Design and implement a fairer message priority algorithm
> --------------------------------------------------------
>
>                 Key: JBMESSAGING-180
>                 URL: https://jira.jboss.org/jira/browse/JBMESSAGING-180
>             Project: JBoss Messaging
>          Issue Type: Task
>          Components: Messaging Core
>            Reporter: Tim Fox
>            Assignee: Tim Fox
>             Fix For: 2.0.0 Beta
>
>
> JBoss Messaging queues can contain messages of different priorities from 0 (the lowest priority) to 9 (the highest priority). When delivering messages to consumers JBoss Messaging will attempt to deliver higher priority messages before lower priority messages.
> The current implementation simply delivers any higher priority messages before any lower priority ones. This can result in starvation (i.e. they are never delivered) of lower priority messages if higher priority messages continue to arrive.
> The goal of this project is to design a fairer algorithm which still attempts to deliver higher priority messages first but does not completely starve lower priority messages. The algorithm also needs to be extremely efficient so as not to provide any significant performance overhead on the message delivery process.
> By definition a message priority algorithm cannot be completely "fair" but it should certainly be "fairer" than it is now! 

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