[Mpi3-subsetting] Subsetting scope: a POV

[Supalov, Alexander]

Hi everybody,
 
In the run-up to our kick-off, here's my POV on the subsetting and its
possible scope/role in the MPI-3. Your comments and suggestions are most
welcome. 
 
We start this activity because:
 
1) Certain industrial customers complain about MPI complexity and
inadequacy
2) Complexity is going to grow in MPI-3
3) Growing complexity may have growing performance implications
 
As a result of the above, customers drift away from the MPI to
home-grown libraries, usually based on sockets. This effectively
eliminates fast networks from their scope, unless they can profit from
fast IP emulation layers. Moreover, this customer drift, if continued,
may make MPI irrelevant in some HPC areas and lead to creation of
alternative interfaces there.
 
The main purpose of the Forum, as well as the subsetting WG, is thus to
react to customer demand and make MPI faster and easier to use,
especially in those areas that are subjected to the increasing customer
drift (think, e.g., massive master-slave computations).
 
Basing on these premises, the subsetting, in my mind, should try to:
 
1) Make MPI standard modular. This may include:
    a) Splitting the standard functionality into coherent groups that
users will be able to select/deselect at init time
    b) Making implementation of some modules/functionality optional
(think dynamic process support) as they are anyway now
    c) Addressing not only functional groups but also certain aspects of
the standard that may not be needed in certain use cases (think
communicator management, message tagging, derived datatypes,
MPI_ANY_SOURCE support, non-blocking communication, etc.)
 
2) As part of the modularization, optionally identify the minimum
functional MPI subset. This may be:
    a) Those 6 calls (Init, Rank, Size, Send, Recv, Finalize), possibly
w/o communicator management and derived datatypes.
    b) A more flexible combination of modules actually needed by the
user
 
3) Connect subsetting with other MPI-3 activities (FT, ABI, collectives,
etc.)
 
As a result of modularization, we should strive to achieve
 
1) Simplification of the standard for the newcomers
2) Performance advantages for reasonable module combinations
3) Influence upon the overall shape of the MPI-3 standard
 
There are certainly quite a few concerns here:
 
1) We may end up complicating the standard and its implementation even
further
2) We may facilitate a split of the standard into several mutually
incompatible implementation "families"
3) We may cause some valid MPI-3 applications break if they use optional
modules not available in the implementation involved
4) We may get carried away by academic considerations and miss the
actual customer demands in the process
5) We may be engaging into a lost battle because the MPI standard is way
too rigid by design/purpose to be simplified
 
To guard against all this, we need to work closely with other WGs and
the Forum as a whole, define our goals as early as possible, and solicit
extensive Forum and customer feedback.
 
>From all this, by the time of the Forum meeting in March, we should have
at least a couple of slides reflecting our intentions and plans.
 
Best regards.
 
Alexander
 

--
Dr Alexander Supalov
Intel GmbH
Hermuelheimer Strasse 8a
50321 Bruehl, Germany
Phone:          +49 2232 209034
Mobile:          +49 173 511 8735
Fax:              +49 2232 209029
 
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