Hi there,
When I was debugging somebody's MPI code, I found the
running code using LAM has a deadlock with MPICH. So I
wrote a tiny toy program to test the idea.
The code is attached at the end.
The idea is that to see whether MPI_Send is blocking
or not. The code actually only uses 1 process to
demonstrate the idea.
In LAM under linux, it runs fine for any SIZE.
But for MPICH under linux and MPIPro under windows,
for SIZE larger than some value(say 10), the code
runs into a deadlock.
I checked the mannual of all the manpage of MPI_Send,
they all say that MPI_Send "May" block, which I
interpret as it blocks under some condition and not
otherwise.
My questions are:
1. Any detailed explanation for all that's happened?
2. Is LAM smarter than the other two to know that the
code is dealing with a single node so that it won't
block?
3. In principle, shall I always regard MPI_Send as a
blocking function and should avoid something like:
loop rank for all
{
MPI_Send( to rank)
MPI_Recv( from rank)
}
Because that's the situation we encountered in the
real situation inside a loop when the rank index has
a chance to be the same as its own rank.
4. Finally, the fact that LAM is differnt from the
others makes me nervous. Shall I awayls try to compile
the code against different implementations to make
sure it runs fine with all of them in order to make
sure it is really portable?
Thanks a lot
/////////////// localLock.c //////////////
#include<stdio.h>
#include<mpi.h>
#define SIZE 9999999
int main(int argc, char * argv[])
{
int err,rank,numProcs;
double* data;
double megaSize;
MPI_Status status;
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD, &numProcs);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
data=(double*)malloc(SIZE*sizeof(double));
megaSize=SIZE*sizeof(double)/1024.0/1024.0;
printf("Data size=%f MBytes\n",(int)megaSize);
MPI_Send(data,SIZE,MPI_DOUBLE,0,1,MPI_COMM_WORLD);
MPI_Recv(data,SIZE,MPI_DOUBLE,0,1,MPI_COMM_WORLD,&status);
printf("Done!\n");
free(data);
MPI_Finalize();
}
/////////////////// End of code
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