[Beowulf] fast interconnects, HT 3.0 ...

Jim Lux James.P.Lux at jpl.nasa.gov
Tue May 23 18:00:43 PDT 2006

At 09:35 AM 5/23/2006, Eugen Leitl wrote:
>On Tue, May 23, 2006 at 11:04:52AM -0500, Richard Walsh wrote:
>If I have some 10^3 nodes,

or if your nodes are spread out over some volume of space..

>and the context is not read-only
>I always have to wait to make sure nobody is trying to write to
>the same location. It's a worst case, but in a relativistic universe
>maintaining the illusion of coherence over many copies is an
>expensive one. Lots of signalling back and forth, until you
>know the state is settled for sure. This might work for 8, 16, maybe 32 
>in a close enough location -- but with 10^3 or 10^6 nodes it
>has to give.
> >    That is where the pGAS programming models become more efficient.  Remote
> >    memory references expressed in the syntax and compiled to
> > instructions for
> >    direct puts and gets without management or translation by a NIC.  It
>We're talking lunatic fringe interconnects

lunatic fringe today, regular order of business in 3-4 or 10 years, so we'd 
better figure out how to deal with "non-simultaneity" pretty soon.

>where the wire or the fibre
>is your FIFO, and the switch makes a routing decision after a few bits
>of the headers have streamed past -- which is reasonably close to c.
>With 10 GBit data rates and above that's a quick decision to take.
>At 10 GBit/s your serial bit is just ~3 cm or 100 ps short -- in vacuum.
>Shorter in glass, and much shorter in copper. So a very short message
>can arrive within a few ns, which is order of magnitude RAM access.
glass (silica) has a index of about 1.5, so 5ns/meter.  Coax with velocity 
factor 0.66 (solid PE dielectric) would be about the same.  Foam dielectric 
(i.e. cable TV coax) has a faster propagation speed (VF=0.70 to 
VF=0.80).  Twisted pairs depending on the twist rate and dielectrics 
involved could be anywhere from 0.5 (6ns/meter) to 0.9 (3.3 ns/meter)

> > would seem
> >    that HT 3.0 supports this model across chassis as long as the
> > programmer manages
> >    memory synchronization.
>You have to bite the bullet and manage synchronization by higher-order
>protocols. The physical world at the bottom is fundamentally message-passing.
>You might notice it very much if you're working on us scale, but
>in ns and below it you can't ignore it.
Exactly... when bit time (or message length) starts to get comparable to 
light time between endpoints, you've got to start thinking about it.


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