[Beowulf] Home beowulf - NIC latencies
joachim at ccrl-nece.de
Thu Feb 10 01:28:27 PST 2005
Josip Loncaric wrote:
> Are there any projects that would expand the ability of MPI application
> programmers to provide performance hints to the MPI library? For
> example, hints indicating that certain messages are latency sensitive
> whereas others need optimal bandwidth and low CPU overhead?
MPI offers a lot of different send modes already. If you use a ready
send, the MPI library can assume that you are interested in low-latency
delivery; if you use a non-blocking send, it should be o.k. for the
library to assume that you are interested in overlapping computation and
communication and so on. On the receiving side, a hybrid
polling-blocking approach for receiving can be applied.
I do not think that there is serious demand for more explicit "steering"
of the MPI library. User's make much to little use of the existing ways
(that I described above). But, if you really want to do such stuff, you
could use (implementation-specific) attributes which you assign to
different communicators, one for "low-latency" delivery and one for
"low-cpu", or whatever. But this has more effect on the sending side
than on the receiving side. I wouldn't invest work into this unless you
have very good reasons. Esp. as this would be non-portable, few users
would ever take notice.
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de
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