[MPICH] Problem with MPI_Gather

[Jonathan Patterson]

	Hello!
	I've recently put mpich2-1.0.6p1 on a machine running Scientific Linux 5.0.
	One of my users is having a problem with MPI_Gather - it's working fine for 
<7 processes, but anything more and the MPI_Gather function seems to mess up 
the ordering of the data coming back from the non-root processes. The 
resulting memory does not therefore contain what it should - the gathering of 
data has got jumbled up. The error happens regardless of wether all processes 
are on the same machine, or each are on a seperate machine.
	The test programs included with mpich2 for allgather do work, but they use 
MPI_Type_vector to make a different datatype for the call to MPI_Gather. From 
the documentation, though, this doesn't seem needed.
	I've put together a program to reproduce the problem, which works fine with 
the original mpich (ie. not mpich2).
	My question is basically this - should the below program work? Am I using the 
right method to gather the data? Because it doesn't work - for anything >7 
processes, it fails to gather the data properly. Though if you reduce 
DATASIZE enough, it works.
	Any help gratefully appreciated!

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

#define DATASIZE 500
#define MAX_PROCESSES 20

// ----------------------------------------------------------------------- 
MAIN
int main(int argc,char *argv[])
{
   int myid,np;

   MPI_Init (&argc,&argv);
   MPI_Comm_rank(MPI_COMM_WORLD,&myid);
   MPI_Comm_size(MPI_COMM_WORLD,&np);

   printf("I am %i of %i\n",myid+1,np);
   fflush(stdout);

   MPI_Barrier(MPI_COMM_WORLD);

   // Test MPI_Gather
   int fakeData[DATASIZE];
   int recvData[DATASIZE*MAX_PROCESSES];
   int c;

   // Initialise fake data
   for(c=0;c<DATASIZE;c++)  { fakeData[c]=c+(myid*DATASIZE); };
   for(c=0;c<DATASIZE*MAX_PROCESSES;c++) { recvData[c]=-1; };

   printf("%i: Sending %i -> %i\n",myid,fakeData[0],fakeData[DATASIZE-1]);
   
MPI_Gather(fakeData,DATASIZE,MPI_INT,recvData,DATASIZE,MPI_INT,0,MPI_COMM_WORLD);

   MPI_Barrier(MPI_COMM_WORLD);

   if (myid==0)
   {   printf("Checking...\n");
       for(c=0;c<DATASIZE*np;c++)
       {        if (recvData[c]!=c) { printf("%i=%i  ",c,recvData[c]); 
fflush(stdout); };
       };
       printf("Check done.\n"); fflush(stdout);
   };

   fflush(stdout);
   MPI_Finalize();
}