[Mpi3-rma] RMA window attributes for locking

[Jeff Hammond]

I will try to participate when not on other conference calls :|

One addition to lock types is MPI_WIN_ASSOCIATIVE_LOCK, which would
obviate the need for locks on the window.  Such a window would only
permit access via Get/Copy OR Acc (and no non-MPI access), since these
operations are invariant with respect to re-ordering.  My motivation
for this feature is that read-only and accumulate-only are ubiquitous
usage patterns in application codes (over 90% of all GA/ARMCI calls in
codes I work with act within these epochs) and there are a variety of
optimizations that can occur if this window attribute is explicitly
specified.

For read-only and accumulate-only operations, message ordering within
the lifetime of the window (with this attribute active) is irrelevant
and there is almost no need for internal consistency checking.
Get/Copy calls can be completely overlapped in the implementation and
Acc calls can be overlapped as long as the accumulate function is
atomic.

Alternatively, if Acc is bad for non-cache-coherent then
MPI_WIN_READONLY_LOCK provides half of what I want and allows for even
more optimization in a heterogeneous context.  Local memory attached
to a read-only window could be migrated to memory banks closer to the
NIC to prevent bus contention I observe on NUMA nodes.  For a
Cell/GPU-like node where the window was on the attached processor (I
assume this will soon be possible), then such an optimization would be
very valuable.

Hopefully this idea isn't too stupid.  If I need to provide more
justification, please let me know.

Jeff

On Sun, Mar 7, 2010 at 6:42 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
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-- 
Jeff Hammond
Argonne Leadership Computing Facility
jhammond at mcs.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond