joachim at ccrl-nece.de
Tue Apr 12 06:34:04 PDT 2005
Rita Zrour wrote:
> Hello I have a question,
> when i do many MPI_Alltoall in my program always the
> first MPI_Alltoall take too much time to be done.
You should be more precise:
- which MPI implementation, which interconnect?
- a code and timing example would be helpful
> I don't know where the first communication is always
> expensive. Is that a problem of memory???????
One possible reason: some MPI implementations do not set up the
"connections" between the processes until communication is actually
taking place. When the first communication call is all_to_all, then this
setup procedure takes some more time because each process communicates
with every other one.
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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