C
/*
* This program demonstrates the use of MPI_Alltoall when
* transposing a square matrix.
* For simplicity, the number of processes is 4 and the dimension
* of the matrix is fixed to NROW
*/
#include <stdio.h>
#include <mpi.h>
#define NROW 131072
#define NP 1024
#define NBLK NROW/NP
#define ONEML 99999999999
void
trans (double *a, int n)
/* transpose square matrix a, dimension nxn
* Consider this as a black box for the MPI course
*/
{
int i, j;
int ij, ji, l;
double tmp;
ij = 0;
l = -1;
for (i = 0; i < n; i++)
{
l += n + 1;
ji = l;
ij += i + 1;
for (j = i+1; j < n; j++)
{
tmp = a[ij];
a[ij] = a[ji];
a[ji] = tmp;
ij++;
ji += n;
}
}
}
int main (int argc, char *argv[])
{
double a[NROW][NBLK];
double b[NROW][NBLK];
int i, j, nprocs, rank,provided,itemp;
double r0;
MPI_Init (&argc, &argv);
MPI_Comm_size (MPI_COMM_WORLD, &nprocs);
MPI_Comm_rank (MPI_COMM_WORLD, &rank);
if(rank == 0)
{
printf("Transposing a %d x %d matrix, divided among %d processors\n",NROW,NROW,NP);
}
if (nprocs != NP)
{
if (rank == 0)
printf ("Error, number of processes must be %d\n",NP);
MPI_Finalize ();
return 1;
}
r0 = MPI_Wtime();
for (i = 0; i < NROW; i++)
for (j = 0; j < NBLK; j++)
a[i][j] = ONEML * i + j + NBLK * rank; /* give every element a unique value */
r0 = MPI_Wtime()-r0;
if(rank == 0)
{
printf("Building matrix time (sec) %f\n",r0);
}
/* do the MPI part of the transpose */
/* Tricky here is the number of items to send and receive.
* Not NROWxNBLK as one may guess, but the amount to send to one process
* and the amount to receive from any process */
r0 = MPI_Wtime();
/* MPI_Alltoall does not a transpose of the data received, we have to
* do this ourself: */
MPI_Alltoall (&a[0][0], /* address of data to send */
NBLK * NBLK, /* number of items to send to one process */
MPI_DOUBLE, /* type of data */
&b[0][0], /* address for receiving the data */
/* NOTE: send data and receive data may NOT overlap */
NBLK * NBLK, /* number of items to receive
from any process */
MPI_DOUBLE, /* type of receive data */
MPI_COMM_WORLD);
r0 = MPI_Wtime()-r0;
if(rank == 0)
{
printf("MPI_Alltoall time (sec) %f\n",r0);
}
/* transpose NP square matrices, order NBLKxNBLK: */
r0 = MPI_Wtime();
for (i = 0; i < NP; i++)
trans (&b[i * NBLK][0], NBLK);
r0 = MPI_Wtime()-r0;
if(rank == 0)
{
printf("Transpose block matrices time (sec) %f\n",r0);
}
/* now check the result */
for (i = 0; i < NROW; i++)
for (j = 0; j < NBLK; j++)
{
if (b[i][j] != ONEML * (j + NBLK * rank) + i )
{
printf ("process %d found b[%d][%d] = %f, but %f was expected\n",
rank, i, j, b[i][j], (double) (ONEML * (j + NBLK * rank) + i));
MPI_Abort (MPI_COMM_WORLD,1);
return 1;
}
}
if (rank == 0)
printf ("Transpose seems ok\n");
MPI_Finalize ();
return 0;
}
FORTRAN
subroutine trans (a, n)
!* transpose square matrix a, dimension nxn
!* Consider this as a black box for the MPI course
!*/
integer i, j, ij, ji, l,n
double precision tmp
double precision a(*)
ij = 0
l = -1
do i = 0, n-1
l = l+ n + 1
ji = l
ij = ij+i + 1
do j=i+1,n-1
tmp = a(ij+1)
a(ij+1) = a(ji+1)
a(ji+1) = tmp
ij = ij+1
ji = ji+ n
enddo
enddo
return
end subroutine trans
!*
!* This program demonstrates the use of MPI_Alltoall when
!* transposing a square matrix.
!* For simplicity, the number of processes is 4 and the dimension
!* of the matrix is fixed to NROW
!*/
program main
implicit none
include 'mpif.h'
integer :: nprocs
integer :: proc_me,proc_up,proc_down
integer ierr
integer,parameter::NROW=131072
integer,parameter::NP=1024
integer,parameter::NBLK=NROW/NP
double precision,parameter::ONEML=99999999999.0
double precision,allocatable:: a(:,:)
double precision,allocatable:: b(:,:)
integer i, j, rank,provided,itemp
double precision r0
call MPI_INIT(ierr)
call MPI_COMM_SIZE(MPI_COMM_WORLD,nprocs,ierr)
call MPI_COMM_RANK(MPI_COMM_WORLD,proc_me,ierr)
allocate( a(NBLK,NROW),b(NBLK,NROW))
if(proc_me.eq.0) write(*,*) 'Transposing a',NROW,'x',NROW,' matrix, divided among',NP,'processors'
if (nprocs.ne.NP) then
if (proc_me.eq.0) then
write(*,*) 'Error, number of processes must be',NP
end if
call MPI_Finalize (ierr)
stop
endif
r0 = MPI_Wtime()
do j=1,NROW
do i=1,NBLK
a(i,j) = ONEML * j + i + NBLK *proc_me
enddo
enddo
r0 = MPI_Wtime()-r0
if(proc_me.eq.0) write(*,*) 'Building matrix time (sec)',r0
!* do the MPI part of the transpose */
!/* Tricky here is the number of items to send and receive.
! * Not NROWxNBLK as one may guess, but the amount to send to one process
! * and the amount to receive from any process */
r0 = MPI_Wtime()
!/* MPI_Alltoall does not a transpose of the data received, we have to
! * do this ourself: */
call MPI_Alltoall (a(1,1), NBLK * NBLK, MPI_DOUBLE_PRECISION, b(1,1), NBLK * NBLK, MPI_DOUBLE_PRECISION, MPI_COMM_WORLD,ierr)
r0 = MPI_Wtime()-r0
if(proc_me.eq.0) write(*,*) 'MPI_Alltoall time (sec)',r0
!* transpose NP square matrices, order NBLKxNBLK: */
r0 = MPI_Wtime()
do i=1,NP
call trans(b(1,(i-1) * NBLK+1),NBLK)
enddo
r0 = MPI_Wtime()-r0;
if(proc_me.eq.0) write(*,*) 'Transpose block matrices time (sec)',r0
!* now check the result */
do j=1,NROW
do i=1,NBLK
if (b(i,j).ne. ONEML * (i + NBLK * proc_me) + j ) then
write(*,*) 'process',proc_me,'b',b(i,j),'expected', ONEML * (i + NBLK * proc_me) + j
call MPI_Abort (MPI_COMM_WORLD,1, ierr)
endif
enddo
enddo
if (proc_me.eq.0) write(*,*) 'Transpose seems ok!'
call MPI_Finalize (ierr)
end program main
Q\A exercise 15 answer
