On Feb 11, 2010, at 2:14 PM, Lisandro Dalcin wrote:
> But you did not answer my previous question... What's the rationale
> for requiring sendbuf!=recvbuf when count=0? I would argue you want a
> free ticket :-) to put restrictions on user code (without an actual
> rationale) in order to simplify your implementation.
I don't understand your assertion. The MPI spec clearly says that sendbuf must
!= recvbuf. If you want the sendbuf to be the same as the recvbuf, MPI
supports MPI_IN_PLACE for several operations.
I realize that's not what you're trying to do, but these are the semantics that
MPI has defined.
> > While zero-length arrays/sequence/containers do appears in real code, they
> > are not equal to NULL. If they are NULL, that means they do not contain any
> > useful data, and they don't need to be source or target of any kind of
> > [collective or point-to-point] communications.
And even stronger than this: remember that NULL *is* a valid pointer for MPI
when it is paired with an appropriate datatype. As I said in an earlier mail,
NULL is therefore not a special case buffer for sendbuf or recvbuf.
To be absolutely clear: none of OMPI's MPI API calls have checks of the form:
if (NULL == choice_buffer)
return error;
> Yes, I know. Moreover, I agree with you. NULL should be reserved for
> invalid pointers, not for zero-length array...
But it is not. MPI's datatype mechanism is so general that NULL is valid.
So yes, passing MPI_REDUCE(NULL, NULL, ...) violates the sendbuf!=recvbuf rule
(partially because there is only one datatype in MPI_REDUCE). If a language
may convert a buffer representation to NULL for you behind the scenes, then
it's up to the language binding to catch/correct that.
...at least by the wording in today's MPI spec. That being said, your python
example of buffers a and b unknowingly being transformed to NULL behind the
scenes seems like a good thing that MPI should support better. It's exactly
these kinds of issues that would be helpful to know / discuss / propose
improvements for MPI-3.
Could we convince you to come to a Forum meeting? :-)
--
Jeff Squyres
jsquy...@cisco.com
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