如何指定mpirun的线程数？

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hello.f90的列表：

mpirun -n 4 myprog is only giving me 1 thread when I would like 4.  How do I specify 4 threads. mpiexec.hydra -n 4 correctly gives me 4 threads.

mpirun示例：

use mpi
implicit none
integer,parameter :: i4= selected_real_kind(4)
integer (i4) :: mype
integer (i4) :: nprocs
integer (i4) :: ierr
call mpi_init(ierr)
call mpi_comm_size(mpi_comm_world,nprocs,ierr)
call mpi_comm_rank(mpi_comm_world,mype,ierr)
write(6,*) 'Hello World',mype
call mpi_finalize(ierr)
stop
end

mpiexec示例：

 mpirun -n 4 a.out
 Hello World           0
 Hello World           0
 Hello World           0
 Hello World           0

lscpu的输出：

 mpiexec.hydra -n 4 a.out
 Hello World           3
 Hello World           1
 Hello World           2
 Hello World           0