[Beowulf] Re: Re: Home beowulf - NIC latencies
joachim at ccrl-nece.de
Tue Feb 15 03:20:48 EST 2005
Patrick Geoffray wrote:
> While we are at it, here is my wish list for the next MPI specs:
> a) only non-blocking calls. If there are no blocking calls, nobody will
> use them.
While this makes sense technically, nobody will probably offer an MPI
implementation without MPI_Send for the next 20 years for compatibility
reasons, so we can just forget about it.
> b) non-blocking calls for collectives too, there is no excuse. Yes, even
> an asynchronous barrier.
No problem here - barrier_enter() and barrier_leaver() are not new.
> c) ban of the ANY_SENDER wildcard: a world of optimization goes away
> with this convenience.
I think this could best be achieved with an assertion like those for
one-sided and I/O. There are situations where ANY_SENDER is needed, or
at least avoids large programming overheads.
> d) throw away the user defined datatypes, or at least restrict it to
> regular strides.
This is nonsense: user-defined datatypes do not cause any overhead if
you don't use them, there are ways to implemenent them very efficiently,
and you can't do without in many situations (like MPI-IO).
> e) get rid of one-sided communications: if someone is serious about it,
> it uses something like ARMCI or UPC or even low level vendor interfaces.
Instead, I propose to rework the MPI one-sided communications for a more
simple and flexible semantic. The current definition does not match
todays network capabilities, but was designed to allow a simple
implemenentation for slow/non-RDMA networks.
> Rob, you are politically connected, could you make it happen, please ?
One person alone can't do this. The best place to discuss such things is
the MPI users group meeting (EuroPVM/MPI, this year in Capri/Italy).
Also, adding mpi.h to the standard to define an ABI is a good thing.
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de
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