[Beowulf] Re: Re: Home beowulf - NIC latencies

Joachim Worringen 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

-- 
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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