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Subject: Re: [OMPI users] openmpi credits for eager messages
From: Patrick Geoffray (patrick_at_[hidden])
Date: 2008-02-04 15:23:44


Brightwell, Ronald wrote:
> I'm looking at a network where the number of endpoints is large enough that
> everybody can't have a credit to start with, and the "offender" isn't any
> single process, but rather a combination of processes doing N-to-1 where N
> is sufficiently large. I can't just tell one process to slow down. I have
> to tell them all to slow down and do it quickly...

When you have N->1 patterns, then the hardware flow-control will
throttle the senders, or drop packets if there is no hardware
flow-control. If you don't have HOL blocking but the receiver does not
consume for any reasons (busy, sleeping, dead, whatever), then you can
still drop packets on the receiver (NIC, driver, thread) at a last
resort, this is what TCP does. The key is have exponential backoff (or a
reasonably large resend timeout) to no continue the hammering.

It costs nothing in the common case (unlike the credits approach), but
it does handle corner cases without affecting too much other nodes
(unlike hardware flow-control).

But you know all that. You are just being mean to your users because you
can :-) The sick part is that I think I envy you...

Patrick