[mpich-discuss] MPI_Reduce syntax

[Dave Goodell]

Hi Martin,

Yes, that is valid MPI code.  A key observation is that slightly  
earlier in the subsection, in the description of the "recvbuf"  
parameter, it says "significant only at root".

Perhaps that second sentence could be reworded to be a bit less  
confusing in a future version of the MPI Standard.  The sentence is  
really trying to say that the send and recv buffers need to be the  
same size with elements of the same type at the root, and that the  
send buffers need to be the same size with elements of the same type  
on all processes, including the root.  I'll take a note to myself to  
discuss it with others at the next meeting of the MPI Forum in January.

-Dave

On Dec 7, 2009, at 10:41 PM, Martin Siegert wrote:

> Hi,
>
> the MPI standard
> http://www.mpi-forum.org/docs/mpi22-report/node103.htm#Node103
> says the following about MPI_Reduce:
>
> "The routine is called by all group members using the same arguments  
> for
> count, datatype, op, root and  comm. Thus, all processes provide input
> buffers and output buffers of the same length, with elements of the
> same type."
>
> Thus, the following appears to be illegal:
>
> buf = (double *)malloc(l*sizeof(double);
> random(buf, l); /* fill buf with something */
> if (myid == 0) {
>   MPI_Reduce(MPI_IN_PLACE, buf, l, MPI_DOUBLE, MPI_SUM, 0,  
> MPI_COMM_WORLD);
> } else {
>   MPI_Reduce(buf, NULL, l, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
> }
>
> because it violates the "same length" requirement of input and output
> buffers. But why do I need to provide a valid output buffer for  
> processes
> other than the receiving one?
>
> Will the code using NULL as the receive buffer for those processes
> that do not receive anything work with mpich2?
>
> Cheers,
> Martin
>
> -- 
> Martin Siegert
> Head, Research Computing
> WestGrid Site Lead
> IT Services                                phone: 778 782-4691
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> Burnaby, British Columbia                  email: siegert at sfu.ca
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