[mpiwg-rma] Memory model and source code portability
rabenseifner at hlrs.de
Tue Sep 30 11:23:18 CDT 2014
The follwing discussion is (partially) independent from RMA.
The example is about MPI_Irecv and MPI_Wait.
> Comment (by gropp):
> I fully agree with Dave. As I've said before, MPI should only talk about
> MPI. What the compiler might to with non-MPI is not in scope for us.
> Yes, this makes it difficult to provide examples that are guaranteed to
> work; that's part of the point of Boehm's article. But that doesn't
> change the fact that the MPI Forum must not specify the behavior of C (or
> Fortran) codes that happen to be accessing shared memory.
> Ticket URL:
For me it looks like that you give up with the important goal
that the MPI standard should provide source code portability.
I mean source code portability for MPI applications written in
one of the supported language bindings.
For me, the MPI standard has to specify all that is needed that
a source code that is consistent with the MPI standard is
portable from one MPI library with companion compiler
to another MPI library with its companion compiler.
With MPI-3.0 and the new Fortran interface,
we tried to achieve this goal. Maybe that further
investigation is needed.
People always thought that there is no such problem with C.
MPI-3.0 Section 17.1.20 "Comparison with C" tries to say this.
As I learned in this discussion, such expectation about
brave C is completely wrong, as seen in
There is no source code portability for the MPI C Interface
if C = C99 or older:
Such a small code
may be compiled by C99 (not C11) into
MPI_Wait(&rq,...) with internally: buf=received_value
And with an MPI library making progress between MPI_Irecv and MPI_Wait,
the outcome may be
with internally by the MPI lib progress engine:
buf=reg0; /* i.e. the received value is
overwritten by the old value!!! */
Or in other words, according to
- MPI-3.0 Section 17.1.20, and
already MPI-1.1 for C required the C11/C11++ memory model,
which does not allow such optimizations.
This has direct influence on the wording we use in MPI-3.0 Section 2.6.
Saying "ISO C is enough" is outdated, i.e., closing the eyes
in front of the knowledge that exists.
I would like that the MPI Forum gets this right, in C and Fortran.
Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)
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