Hi!
I have a weird problem with executing a singleton OpenMPI program, where
an additional thread causes full load, while the master thread performs
the actual calculations.
In contrast, executing "mpirun -np 1 [executable]" performs the same
calculation at the same speed but the additional thread is idling.
In my understanding, both calculations should behave in the same way
(i.e., one working thread) for a program which is simply moving some
data around (mainly some MPI_BCAST and MPI_GATHER commands).
I could observe this behaviour in OpenMPI 1.10.1 with ifort 16.0.1 and
gfortran 5.3.0. I could create a minimal working example, which is
appended to this mail.
Am I missing something?
Best regards,
Stefan
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MWE: Compile this with "mpifort main.f90". When executing with
"./a.out", there is thread wasting cycles, while the master thread waits
for input. When executing with "mpirun -np 1 ./a.out" this thread is idling.
program main
use mpi_f08
implicit none
integer :: ierror,rank
call MPI_Init(ierror)
call MPI_Comm_Rank(MPI_Comm_World,rank,ierror)
! let master thread wait on [RETURN]-key
if (rank == 0) then
read(*,*)
end if
write(*,*) rank
call mpi_barrier(mpi_comm_world, ierror)
end program
