[Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)

[Richard Treumann]

While it is not the Forum's responsibility to mandate performance for a
particular implementation, it is certainly within the Forum's purview to
press for models that can be implemented with high performance and
deprecate models that are destined for poor performance on many or most
architectures.

Broad support for non-contiguous and user defined datatypes could easily be
a deal breaker for a high performance implementation.

This is another case where MPI_Init time assertions could be useful.  If an
application could declare in MPI_Init_asserted that it will only uses
predefined, contiguous datatypes in RMA operations then an implementation
would be able to exploit any optimizations that depend on contiguous
datatypes and raise an error it the application tried to use a
non-contiguous or user defined datatype.

The assertion would not relieve the implementation of the need to include
support for non-contiguous or user defined datatypes because any
application the did not assert it would use ONLY contiguous & predefined
types would have a right to expect the broader support.

If generalized readiness for RMA calls that just might use an arbitrary
datatype mean all RMA calls in the application pay a performance penalty
then the application author will need to decide whether he really needs
non-contiguous RMAs badly enough to pay the penalty.  I suspect many or
even most applications can do what they need to do without resorting to
non-contiguous RMA.

Recall that an assertion by an application does not require an
implementation to do anything different. If some MPI implementation can
provide high performance RMA for contiguous & user defined datatypes and
still have robust support for generalized datatypes, it can simply ignore
the assertion.)

          Dick

Dick Treumann  -  MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846         Fax (845) 433-8363


mpi3-rma-bounces at lists.mpi-forum.org wrote on 09/15/2009 09:18:42 PM:

> [image removed]
>
> Re: [Mpi3-rma] non-contiguous support in RMA & one-sided pack/unpack (?)
>
> Jeff Hammond
>
> to:
>
> MPI 3.0 Remote Memory Access working group
>
> 09/15/2009 09:19 PM
>
> Sent by:
>
> mpi3-rma-bounces at lists.mpi-forum.org
>
> Please respond to "MPI 3.0 Remote Memory Access working group"
>
> I must be blind for missing that.  Sorry.
>
> I understand that it is not MPI Forum's responsibility to ensure
> efficient implementations of the standard, but I am still concerned
> about the performance of even simple non-contiguous operations based
> upon what I see with ARMCI.  I guess I'll have to wait and see what
> the various groups/vendors produce.
>
> Thanks,
>
> jeff
>
>
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