View source for CoE Cluster november 2011/MPI

=Programmering med MPI=
http://www.lam-mpi.org/tutorials/one-step/ezstart.php
==Geany IDE==
Der findes et IDE man kan tilrette til cluster kompilering og eksekvering kaldet geany.
Installation:
<pre>
aptitude install geany
</pre>
For at kompilere det med mpicc og eksekvere med mpiexec skal man lige rette lidt til under "Build > Set Includes and Arguments".
{|
|[[Image:Geany IDE.jpg|800px|left|thumb|Geany IDE]]
|}
<pre>mpicc -Wall "%f" -o "%e"

mpiexec -n 20 -hostfile ~/mpd.hosts "%e"
</pre>
Og til C++ bruger man mpic++ -Wall "%f" -o "%e"<br/><br/>
Programmet man skriver skal selvfølgelig ligge i /var/mirror for de andre noder også har adgang til det.<br/><br/>
Og den kan man selvfølgelig X'e over med
<pre>
env DISPLAY=172.16.4.105:1 geany
</pre>

==Hello World eksempel==
HelloWorld program
<source lang=c>
#include "mpi.h"
#include <stdio.h>

int main (int argc, char *argv[] )
{
        int rank, size, namelen;
        char processor_name[MPI_MAX_PROCESSOR_NAME];


        MPI_Init( &argc, &argv );

        MPI_Get_processor_name( processor_name, &namelen );
        MPI_Comm_rank( MPI_COMM_WORLD, &rank );
        MPI_Comm_size( MPI_COMM_WORLD, &size );
        printf( "Hello World from process %d, of %d on %s\n", rank, size, processor_name );
        MPI_Finalize();
        return 0;
}
</source>
Compile det med
<pre> mpicxx -o helloWorld helloWorld.c</pre>
og kør det med
<pre>mpiexec -n 10 /var/mirror/helloWorld</pre>
Resultatet skulle gerne blive
<pre>
Hello World from process 2, of 10 on C05
Hello World from process 6, of 10 on C05
Hello World from process 3, of 10 on C06
Hello World from process 1, of 10 on C04
Hello World from process 5, of 10 on C04
Hello World from process 7, of 10 on C06
Hello World from process 9, of 10 on C04
Hello World from process 0, of 10 on C01
Hello World from process 4, of 10 on C01
Hello World from process 8, of 10 on C01
</pre>
==Hello World med MPI==
Det forrige eksempel brugte ikke rigtig MPI til noget, ud over at få rank og size.<br/>
Hvis vi skal lave det om til at rank 0 er den der printer til skærmen og alle de andre sender via MPI til den, ville det se sådan ud:
<source lang=c>
#include "mpi.h"
#include <stdio.h>
#include <string.h>

int main (int argc, char *argv[] )
{
        int rank, size, namelen, i;
        char processor_name[MPI_MAX_PROCESSOR_NAME];
        char greeting[MPI_MAX_PROCESSOR_NAME + 100];
        MPI_Status status;

        MPI_Init( &argc, &argv );

        MPI_Get_processor_name( processor_name, &namelen );
        MPI_Comm_rank( MPI_COMM_WORLD, &rank );
        MPI_Comm_size( MPI_COMM_WORLD, &size );
        sprintf( greeting,  "Hello World from process %d, of %d on %s\n", rank, size, processor_name );

        if ( rank == 0 ) {
                printf( "%s", greeting );
                for (i = 1; i < size; i++ ) {
                        MPI_Recv( greeting, sizeof( greeting ), MPI_CHAR, i, 1, MPI_COMM_WORLD, &status );
                        printf( "%s", greeting );
                }
        }
        else {
                MPI_Send( greeting, strlen( greeting ) + 1, MPI_CHAR, 0, 1, MPI_COMM_WORLD );
        }
        MPI_Finalize();
        return 0;
}
</source>
Hvilket returnenrer:
<pre>0: Hello World from process 0, of 10 on C01
0: Hello World from process 1, of 10 on C04
0: Hello World from process 2, of 10 on C05
0: Hello World from process 3, of 10 on C06
0: Hello World from process 4, of 10 on C01
0: Hello World from process 5, of 10 on C04
0: Hello World from process 6, of 10 on C05
0: Hello World from process 7, of 10 on C06
0: Hello World from process 8, of 10 on C01
0: Hello World from process 9, of 10 on C04
</pre>