[MPICH] about MPI_Reduce

[Rob Ross]

Thomas is correct that only the data described by the datatype, not the 
junk in the "holes", is transferred.

Rob

Thomas Moschny wrote:
> llwaeva at 21cn.com wrote:
> 
>>Thank you so much. The problem is solved. However, according to the
>>problem, does it mean that no matter what kind of datatype I specified
>>in MPI_Reduce, the whole buffer will be sent and received?
> 
> 
> No, I don't think so. When sending and receiving buffers using datatypes
> with holes in them, the bytes within the wholes should not be read or
> sent over the network and must not be written to.
> This holds for any communication operation in MPI.
> 
> - Thomas
>