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I can check gstack, but just because I see the listener threads on the recv call does not mean that call is not returning. One other thing I thought was to do something like:

netstat -lupe | grep <PROCESS_PID>

to check the inodes of the sockets listening via udp. Then possibly try watching (watch command) what /proc/net/udp says about the receive and transmit queues of those inodes. I'm not sure if this is a good way to diagnose this, however. Trying to listen to the same multicast groups to check for data coming in is also not really a guarantee that the process in question is receiving it.

What are some other ways or things I could probe or try?

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Look at netstat -gn output to see joins.

tcpdump can help actually see activity... e.g: tcpdump -i eth1 -s0 -net

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