On Feb 21, 2008, at 10:19 AM, Gleb Natapov wrote:
To further complicate things, this race condition is never seen
_if_ the
application uses the same QP to advertise (send a credit allowing the
peer to SEND) the RECV buffer availability. So if the app posts a
SEND
after the RECV is posted and that SEND allows the peer access to the
RECV buffer, then everything is ok. This is due to the fact that the
FW/HW will process the SEND only after processing the RECV. If the
app
uses a different QP to post the SEND advertising the RECV, then the
race
condition exists allowing the peer to SEND into that RECV buffer
before
the HW makes it ready.
OpenMPI can be configured to send credit updates over different QP.
I'll
try to stress it next week to see what happens.
FWIW: this is exactly where the question arose: Steve's working on the
iwarp port of OMPI, and since we send the flow control messages for
all QP's between a pair of processes over a single QP, this apparent
race condition can occur.
--
Jeff Squyres
Cisco Systems
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