[mpich-discuss] MPI_REDUCE and complex numbers

[Darius Buntinas]

There isn't a max buffer size (aside from the running out of memory, but you'd get an error for that).  Can you send us a short sample program that demonstrates this?  How many  processes are you using?

Thanks,
-d

On Aug 18, 2011, at 11:04 PM, Brian Dushaw wrote:

> 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 -
> 
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