[Mpi3-ft] Piggybacking API

Terry Dontje Terry.Dontje at Sun.COM
Tue Apr 22 05:54:45 CDT 2008

Martin Schulz wrote:
> At 06:36 AM 4/21/2008, Terry Dontje wrote:
>> So I reread the piggybacking document on wiki.  I am not thrilled with
>> the amount of new APIs this would be adding to the standard but can also
>> see the point of the paper.  I am curious how the new API is expected to
>> be used?  The proposal say's this API is needed for user-level fault
>> tolerance solutions.  So do we expect a user to change all application
>> calls to the MPI library to use the PB calls?  I wonder if a more
>> general solution that doesn't require a direct change to the API would work.
> Layers like the one for fault tolerance will have to intercept
> all MPI communication calls anyway (otherwise it will not be
> possible to capture the state of the MPI layer) and hence the
> most likely mechanism to implement them is the PMPI layer. At
> this point it will be very easy and fully transparent to also
> replace communication calls with the piggyback counterparts.
> The application would not see the difference (if we get the API
> right :) ).
>> I wonder if there might be a way one could register piggybacking with a
>> communicator and somehow have the actual piggybacking occur as a
>> callback from an implementations messaging layer?
> Registering piggyback data beforehand (whether per communicator
> or globally) has the problem that in a fully multithreaded use
> of MPI piggyback data can no longer be associated with a single
> MPI call/message (unless we make the piggyback data thread local,
> but this would require a concept of threads in MPI and I don't
> think we should go there).
Fair enough, however with the numerous variations of send calls it is
already confusing as to which ones to use.  IMO adding PB api's
for each of the send apis is just asking for more user confusion. 

> Martin
>> Just a thought,
>> --td
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> _______________________________________________________________________
> Martin Schulz, schulzm at llnl.gov, http://people.llnl.gov/schulz6
> CASC @ Lawrence Livermore National Laboratory, Livermore, USA  
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