[Mpi3-rma] MPI3 RMA Design Goals

Jeff Hammond jeff.science at gmail.com
Mon Aug 31 18:01:08 CDT 2009

Regarding: "We are still investigating with the input from the user
community to get an idea on if this is necessary or a smaller sub-set
of Reduce Operations are sufficient
to be used as OP TYPE values (e.g. RMA ACC SUM, RMA ACC PROD, RMA ACC
LXOR, etc.)."

What is supported by ARMCI - Put, Get, Acc with multiplicative factor
ala AXPY or min/max-like operations - is sufficient for everything I
can imagine doing with MPI RMA, but perhaps that is because my use of
ARMCI has limited my imagination.

Regarding "target_mem":

It appears that there is a special (read: restricted?) interface for
the remote memory segment but nothing locally?  Will it be possible to
implement no-copy one-sided on all platforms if the local access is
done not on memory encapsulated by such a special structure but a
plain pointer ("origin_addr")?  The DCMF_Put call
<http://dcmf.anl-external.org/docs/api/dcmf/group__PUT.html>, for
example, requires the use of DCMF_Memregion on both the source and the
receiver, so it would appear that either the local registration is
implicit in the "xfer" call or the operation cannot be no-copy.  If
local registration is implicit, and this process is non-trivial, then
the latency on "xfer" will be substantial since registered malloc can
take a while on some platforms.  It will probably be easier on
implementers to use a more restrictive form of the "xfer" operation
which uses "target_mem"-like restricted memory regions for both the
local and remote processes.  It may also be of practical value when
RMA operations are used in a multi-threaded context since the
programmer could impose restrictions upon the local memory region so
that it was accessible by only one thread (e.g. during Acc RMA)or
accessible by all threads (read-only i.e. Put RMA) and unsafe usage
would throw an error rather.

I am not a computer scientist and apologize for any stupidity
contained in this email.



Jeff Hammond
Argonne Leadership Computing Facility
jhammond at mcs.anl.gov / (630) 252-5381

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