MPI_ALLreduce函数

此函数和MPI_Reduce的意思相同，只是最后结果在所有的进程中。相当于先做MPI_Reduce，然后再做MPI_Scatter.

program main

implicit none

include "mpif.h"

integer myid, ierr

real(kind=8) val, max0

call MPI_INIT(ierr)

call MPI_COMM_RANK(MPI_COMM_WORLD, myid, ierr)

val = myid + 5.d0

print *, 'value:',val

call MPI_ALLreduce(val,max0,1,MPI_DOUBLE_PRECISION,MPI_MAX,

& MPI_COMM_WORLD,IERR)

if(myid.eq.0) print *, 'max:',max0

call MPI_FINALIZE(ierr)

stop

end program main

result:

[root@c0109 ~]# mpif90 hello.F -o hello

[root@c0109 ~]# mpiexec -n 10 ./hello

value: 5.00000000000000

max: 14.0000000000000

value: 6.00000000000000

value: 7.00000000000000

value: 11.0000000000000

value: 10.0000000000000

value: 8.00000000000000

value: 13.0000000000000

value: 14.0000000000000

value: 9.00000000000000

value: 12.0000000000000

[root@c0109 ~]#