[Beowulf] MPI_Alltoall

[Joachim Worringen]

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

-- 
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de