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Flow control delays traced to overloaded router
While working on the DMC version of the system, I noticed an issue that seems to be common to the most current release as well: some form of flow control stutter. To trigger it, I set up a group with 6-12 members all on one machine, and then send multicasts
at high rates. This is enough to overload the UDP IPMC layer of the kernel and trigger loss. In that situation, where multicasts get dropped now and then, the system seems to drop a message and freezes up very briefly, then retransmits and recovers.
So this is actually what you might expect, but the freeze-up lasts longer than it should. [Ken: deleted a speculation about what caused this that proved to be incorrect!]
So you get a stutter longer than it should be, but not super long. Maybe 150 or 250ms.
Since I can see what is going wrong, I should have this fixed fairly soon; I'll post a patch release when I have it. Meanwhile the system is definitely working perfectly well, which is probably why nobody reported this. But if you have an application that generates
steady loads of multicasts this way, you'll probably see delays too.