[Mpi3-rma] Info accumulate_ops

[Torsten Hoefler]

On Tue, Oct 16, 2012 at 01:35:08PM +0000, Hubert Ritzdorf wrote:
>    Hi,
> 
>    I think that there is a problem in the definition of info argument
>    "accumulate_ops" of MPI_Win_create functions (Page 406, Line 4-9
>    of MPI 3.0 document).
> 
>    (*) A value is missing if not all concurrent accumulate calls perform the
>         same operation.
>         Possible values might be: "not_same_op" or "false"
This would be illegal (as described in the semantics section). The NO_OP
was an exception to allow querying a flag with our "fake atomic reads"
while performing atomic updates CAS etc.. So really only the two
mentioned combinations are legal.

>    (*) The actual default value of "same_op_no_op" breaks the backward
>          compatibility and causes problems in portability of applications,
>    since a MPI
>          implementation is not required to evaluate (recognize) info keys.
>          Therefore, "not_same_op" must be the default value of info argument
>          "accumulate_ops".
Default value means that this is the default semantic if the key is not
set (or not evaluated). So a valid user program in MPI-2.2 was only
allowed to use the same OP for concurrent accumulates. In MPI-3.0 it may
use the same op and also NO_OP in addition. Thus, this richer semantic
cannot break backward compatibility. Setting the info key enables the
user to enable the more limited MPI-2.2 semantics which may unlock a set
of richer optimizations possible in the library.

Best,
  Torsten

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