[MPI3 Fortran] Proposed solution for the volatile buffer problem

N.M. Maclaren nmm1 at cam.ac.uk
Thu Jan 8 12:38:40 CST 2009

On Dec 30 2008, Bill Long wrote:
>I've thought about the various proposed solutions to the problem of 
>protecting a buffer used for a split send/receive - wait pair of calls.  
>In particular, the need to prevent motion of code involving the buffer 
>across the wait call. Most of them seem to have at least one defect.  
>Here is a possible solution (not entirely original) that I think works, 
>and involves minimal and localized changes in the Fortran spec:
>Introduce a new subprogram prefix named VOLATILE.
>A VOLATILE subroutine:
>1) must also be BIND(C)
>2) may modify any variable accessible in the program unit that calls the 
>The VOLATILE prefix is not required to match between an interface and 
>the actual definition of the subprogram.
>Restriction (1) gets around the objection that the concept really makes 
>no sense in the Fortran-Fortran case.  Restriction (2) will require that 
>variables be reloaded from memory after a call to a volatile subroutine, 
>and that code motion across the call is effectively blocked. 

Sorry, but that dog won't hunt.

Firstly, MPI non-blocking buffers have ASYNCHRONOUS semantics, and so can be
updated at any time between the isend/irecv and the wait.  In particular,
many programs put wrappers around the MPI calls (often to enable RDMA
transports as an option), and so the buffer variable may not be visible in
the procedure that calls the wait.

Secondly, there are big problems with all visible variables. Some variables 
are syntactically read-only, but that is the easy bit. What about DO-loop 
indices surrounding the wait, or arguments associated with expressions? 
That specification makes such usages undefined.

Thirdly, it is horribly unstructured and leads to catastrophic inefficiency
if extended to allow asynchronous update (as would be needed), because all
visible variables would be effectively VOLATILE.

Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

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