[mpiwg-rma] MPI_Lock... for MPI shared memory

[Rolf Rabenseifner]

Hello,

my colleague Joseph detected that an MPI implementation is allowed 
to not allow passive target communication/synchronization (i.e.,
all forms of MPI_Lock and Unlock) on MPI shared memory windows.

Was it intented or is it only a historical review-bug that 
MPI_WIN_ALLOCATE_SHARED was never added to the list of 
MPI_ALLOC_MEM (since MPI-2.0), and MPI_WIN_ALLOCATE & 
MPI_WIN_ATTACH (since MPI-3.0) in the following text.

 
MPI-3.1 Sect. 11.5.3 on page 448 lines 1-12 say

"Implementors may restrict the use of RMA communication 
that is synchronized by lock calls to windows in 
memory allocated by MPI_ALLOC_MEM (Section 8.2),
MPI_WIN_ALLOCATE (Section 11.2.2), or attached with 
MPI_WIN_ATTACH (Section 11.2.4). 
Locks can be used portably only in such memory.

Rationale. The implementation of passive target 
communication when Memory is not shared may require 
an asynchronous software agent. Such an agent can be
implemented more easily, and can achieve better 
performance, if restricted to specially allocated memory. 
It can be avoided altogether if shared memory is used. 
It seems natural to impose restrictions that allows 
one to use shared memory for third party communication 
in shared memory machines.
(End of rationale.)"


The rationale not really fits to the current state
that MPI_WIN_ALLOCATE_SHARED is missing in the list. 
It would fit if MPI_WIN_ALLOCATE_SHARED would be added
to the list.

Best regards
Rolf

-- 
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
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