[jboss-jira] [JBoss JIRA] (JGRP-1835) DONT_LOOPBACK flag causes credit exhaustion in MFC

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1835?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12966749#comment-12966749 ] 

Bela Ban commented on JGRP-1835:
--------------------------------

Solution #3 is not good either: when sending a message, after returning from sending the message, credits are decremented and sent to self if needed. This doesn't wait for the message to be received and delivered, but replenishes credits immediately, so the essentially bypasses flow control, too -> OOME.

> DONT_LOOPBACK flag causes credit exhaustion in MFC
> --------------------------------------------------
>
>                 Key: JGRP-1835
>                 URL: https://issues.jboss.org/browse/JGRP-1835
>             Project: JGroups
>          Issue Type: Bug
>    Affects Versions: 3.5
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 3.5
>
>
> When a message M is multicast by sender P and has the {{DONT_LOOPBACK}} transient flag set, it will not get looped back up the stack.
> When M passes MFC on the way down, it decrements the sent credits for P. When M is received by P, the received credits are decrements and possibly credits are sent back to P.
> However, M is *never received by P* because {{DONT_LOOPBACK}} causes it to be dropped.
> This results in credits for P getting exhausted if we send many messages. Credit requests sent by P (every 5 s by default) do replenish P's credits, but if P sends many messages this is not enough, and slows things down.
> SOLUTION:
> # Don't use {{DONT_LOOPBACK}} in upper protocols or application space. JGroups itself uses it for discovery (lower protocols)
> # Don't maintain credits for self; ie. P doesn't decrement or replenish credits for itself, only for others
> #* Make sure P doesn't starve processing of messages from all other members (who use flow control) by circumventing flow control...
> # When sending a message tagged as {{DONT_LOOPBACK}}, decrement the *received* credits immediately and send new credits if needed



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