[mpich-discuss] MPI_REDUCE and complex numbers

[Brian Dushaw]

I recently ran across what may be a bug in MPI_REDUCE using the 
latest version of mpich2-1.4., (although I can't be sure...)

I have fortran 90 code that calculates a complex array of size
about 400X6000.  Each row of the array is calculated independently,
so I apportion subsets of rows across a small cluster for calculating.
On each node, the rows not calculated are just filled with zeros.

when all done, I called:

call MPI_REDUCE(psif,psiff,itemp,MPI_DOUBLE_COMPLEX,MPI_SUM,0,MPI_COMM_WORLD,ierr)

which just adds up all the arrays from each node, giving me the final
filled up array complex psiff on the master node (myid=0). itemp is size
ncol*nrow of psiff (or about 400*6000).

This worked fine for smaller sized arrays, but for the larger size
of 400X6000 (which doesn't seem all that large to me), I would just get
almost all zeros in the psiff array.

I wondered if the array size was so large as to overflow a maximum buffer
size for MPI_REDUCE.

In any case, I rewrote the above to send back real and imaginary parts
separately, calling instead:

call MPI_REDUCE(rpsif,rpsiff,itemp,MPI_DOUBLE_PRECISION,MPI_SUM,0,MPI_COMM_WORLD,ierr)
call MPI_REDUCE(ipsif,ipsiff,itemp,MPI_DOUBLE_PRECISION,MPI_SUM,0,MPI_COMM_WORLD,ierr)

And this seems to work fine.

Does anybody have any comments or wise words to enlighten this situation?
The code takes quite a while to run, so it is slow to debug.  I suppose I 
could write a small test case to check more simply whether this is a bug
or not.  Does MPI_REDUCE have a maximum buffer size that one should worry about?

Thanks -