Patrick,

Looking at the log files you sent I'm seeing JGroups doing a lot of 
retransmission of messages. Basically for each ~60K of content JGroups sends a 
message; if the receiver detects some messages are missing it asks for 
retransmission of the missing message and won't deliver messages to the 
application until the missing message is received. If you get a lot of that it 
will certainly slow things down.

I'm going to assume you are using the standard JGroups configurations that ship 
with the AS and that the problem isn't there.

Much more likely is it's an OS-level issue; OS isn't providing large enough 
buffers for UDP datagrams. Result is datagrams are lost at the network layer, 
so JGroups never sees the messages and needs to have them retransmitted. The 
default UDP buffer size on many OSs is very small.

http://www.29west.com/docs/THPM/udp-buffer-sizing.html has a nice section on 
how to configure UDP buffer sizes on various OS's.  The stock JBoss AS config 
can utilize up to a 25MB in UDP buffer, so if you have memory resources to 
allow that much, go for it.  If not, provide as much as you can but I'd say at 
least 2MB.

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