Ok let's take the simple example here, I might have use wrong terms and
I apologize for it
While the rank 0 process is sleeping the other ones are in bcast waiting
for data
program test
use mpi
implicit none
integer :: mpi_wsize, mpi_rank, mpi_err
integer :: data
call mpi_init(mpi_err)
call mpi_comm_size(MPI_COMM_WORLD, mpi_wsize, mpi_err)
call mpi_comm_rank(MPI_COMM_WORLD, mpi_rank, mpi_err)
if(mpi_rank.eq.0) then
call sleep(100)
data = 10
end if
call mpi_bcast(data, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, mpi_err)
print *, "Done in #", mpi_rank, " => data=", data
end program test
George Bosilca wrote:
On Sep 3, 2008, at 6:11 PM, Vincent Rotival wrote:
Eugene,
No what I'd like is that when doing something like
call mpi_bcast(data, 1, MPI_INTEGER, 0, .....)
the program continues AFTER the Bcast is completed (so no control
returned to user), but while threads with rank > 0 are waiting in
Bcast they are not taking CPU resources
Threads with rank > 0 ? Now, this scares me !!! If all your threads
are going in the bcast, then I guess the application is not correct
from the MPI standard perspective (i.e. on each communicator there is
only one collective at every moment). In MPI, each process (and not
each thread) has a rank, and each process exists in each communicator
only once. In other words, as each collective is bounded to a specific
communicator, on each of your processes, only one thread should go in
the MPI_Bcast, if you want only ONE collective.
george.
I hope it is more clear, I apologize for not being clear in the first
place
Vincent
Eugene Loh wrote:
Vincent Rotival wrote:
The solution I retained was for the main thread to isend data
separately to each other threads that are using Irecv + loop on
mpi_test to test the finish of the Irecv. It mught be dirty but
works much better than using Bcast
Thanks for the clarification.
But this strikes me more as a question about the MPI standard than
about the Open MPI implementation. That is, what you really want is
for the MPI API to support a non-blocking form of collectives. You
want control to return to the user program before the
barrier/bcast/etc. operation has completed. That's an API change.
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