MPI
From MohidWiki
Is an acronym standing for Message Passing Interface. MPI are programing directives which allow a program to launch child processes in different nodes (other computers) through the network and communicate between them. This allows parallel processing. To Build a MOHID project for MPI refer to Compiling Mohid with MPI.
MPICH
MPICH are the libraries containing the MPI directives.
Linux installation
- To build the MPICH libraries in linux, get them and:
>./configure FC=ifort --enable-f90 >make >make testing >make install
- To compile Mohid in linux in MPI refer to this wiki.
- To compile any program in linux simply type
>mpif90 -i-static foo.f90
- To use an MPI program on a Linux machine simply type these lines:
mpd & mpiexec -n 4 ./MohidWater mpdallexit
The three lines do the following: i) install the mpi daemon, ii) run MohidWater in 4 processes, iii)once the program is finished, kill all daemons.
Samples
cpi
This little C program calculates pi.
#include "mpi.h"
#include <stdio.h>
#include <math.h>
double f(double);
double f(double a)
{
return (4.0 / (1.0 + a*a));
}
int main(int argc,char *argv[])
{
int n, myid, numprocs, i;
double PI25DT = 3.141592653589793238462643;
double mypi, pi, h, sum, x;
double startwtime = 0.0, endwtime;
int namelen;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
MPI_Get_processor_name(processor_name,&namelen);
fprintf(stdout,"Process %d of %d is on %s\n",
myid, numprocs, processor_name);
fflush(stdout);
n = 10000; /* default # of rectangles */
if (myid == 0)
startwtime = MPI_Wtime();
MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
h = 1.0 / (double) n;
sum = 0.0;
/* A slightly better approach starts from large i and works back */
for (i = myid + 1; i <= n; i += numprocs)
{
x = h * ((double)i - 0.5);
sum += f(x);
}
mypi = h * sum;
MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
if (myid == 0) {
endwtime = MPI_Wtime();
printf("pi is approximately %.16f, Error is %.16f\n",
pi, fabs(pi - PI25DT));
printf("wall clock time = %f\n", endwtime-startwtime);
fflush(stdout);
}
MPI_Finalize();
return 0;
}
f90pi
The same little program in fortran 90:
!**********************************************************************
! pi3f90.f - compute pi by integrating f(x) = 4/(1 + x**2)
!
! (C) 2001 by Argonne National Laboratory.
! See COPYRIGHT in top-level directory.
!
! Each node:
! 1) receives the number of rectangles used in the approximation.
! 2) calculates the areas of it's rectangles.
! 3) Synchronizes for a global summation.
! Node 0 prints the result.
!
! Variables:
!
! pi the calculated result
! n number of points of integration.
! x midpoint of each rectangle's interval
! f function to integrate
! sum,pi area of rectangles
! tmp temporary scratch space for global summation
! i do loop index
!****************************************************************************
program main
use mpi
double precision PI25DT
parameter (PI25DT = 3.141592653589793238462643d0)
double precision mypi, pi, h, sum, x, f, a
integer n, myid, numprocs, i, rc
! function to integrate
f(a) = 4.d0 / (1.d0 + a*a)
call MPI_INIT( ierr )
call MPI_COMM_RANK( MPI_COMM_WORLD, myid, ierr )
call MPI_COMM_SIZE( MPI_COMM_WORLD, numprocs, ierr )
print *, 'Process ', myid, ' of ', numprocs, ' is alive'
sizetype = 1
sumtype = 2
do
if ( myid .eq. 0 ) then
write(6,98)
98 format('Enter the number of intervals: (0 quits)')
read(5,99) n
99 format(i10)
endif
call MPI_BCAST(n,1,MPI_INTEGER,0,MPI_COMM_WORLD,ierr)
! check for quit signal
if ( n .le. 0 ) exit
! calculate the interval size
h = 1.0d0/n
sum = 0.0d0
do i = myid+1, n, numprocs
x = h * (dble(i) - 0.5d0)
sum = sum + f(x)
enddo
mypi = h * sum
! collect all the partial sums
call MPI_REDUCE(mypi,pi,1,MPI_DOUBLE_PRECISION,MPI_SUM,0, &
MPI_COMM_WORLD,ierr)
! node 0 prints the answer.
if (myid .eq. 0) then
write(6, 97) pi, abs(pi - PI25DT)
97 format(' pi is approximately: ', F18.16, &
' Error is: ', F18.16)
endif
enddo
call MPI_FINALIZE(rc)
stop
end
