[Beowulf] fast interconnects, HT 3.0 ...

Jim Lux James.P.Lux at jpl.nasa.gov
Tue May 23 17:42:30 PDT 2006

At 06:49 AM 5/23/2006, Richard Walsh wrote:
>Eugen Leitl wrote:
>>In regards to keeping the wires short, does this IBM trick of keeping all
>>wires equal-length work well on 3d lattices, and above? This would seem to
>>be a must for those coming (hopefully) Hypertransport motherboards with 
>    Speaking of Hyper Transport 3.0 and its AC chassis-to-chassis 
> capabilities and 10 to 20
>    Gbps performance maximums one-way (non-coherent, off chassis I 
> believe), what do the
>    people that know say about scalability.  Are we looking at coherency 
> within the board complex
>    and basic reference ability off board or something else?

WHere coherency means precisely what?  I don't see lockstep execution, 
because when you're talking tens or hundreds of picoseconds per bit, 
"simultaneous" is hard to define.  Two boxes a foot apart are going to be 
many bit times different.  Something's got to take up the timing slack, and 
while the classic "everything synchronous to a master clock" has a 
simplicity of design, it has real speed limits (the light time across the 
physical extent of the computational unit being but one).

>    Sounds like the Cray X1E pGAS memory model.  Is there a role for 
> switches?  And then there is the
>    its intersection with the  pGAS language extensions (UPC and CAF) ... 
> raising the prospect of
>    much better performance in a commodity regime, with possible 
> implications for MPI use.

Get to nanosecond latencies for messages, and corresponding fine grained 
computation, and you're looking at algorithm design that is latency 
tolerant, or, at least, latency aware.  As long as your "computational 
granules" are microseconds, propagation delay can probably be subsumed into 
a basic "computation interval", some part of which is the time required to 
get the data from one place to another.

At finer times, you're looking at things like systolic arrays and pipelines.

>    Anyone have a crystal ball or insights on this?
>    rbw


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