Kidger's comments on Quadric's design and performance

[Richard Fryer]

On Fri, 19 Apr 2002 14:06:00 +0100
Daniel Kidger <Daniel.Kidger at quadrics.com> wrote:

> after all as well as having the fastest line-speed, the Quadrics
> interconnect sends all data as virtual addresses (the NIC has its
> own MMU and TLB). That way any process can read and write
> the memory of any other node without any CPU overhead.

I appreciate getting a bit of technical detail on Quadrics interfaces.  Is
there a web location that might provide more information - comparative
benchmarks or protocol information or ???

This message also reminded me to ask if a long-held opinion is valid - and
that opinion is "that a cache coherent interconnect would offer performance
enhancement when applications are at the 'more tightly coupled' end of the
spectrum."  I know that present PCI based interfaces can't do that without
invoking software overhead and latencies.  Anyone have data - or an argument
for invalidating this opinion?

I did recently read that the AMD 'HyperTransport' interfaces ARE capable of
cache coherent transactions.  This would appear to allow protocols (such as
SCI) that support cache coherence to operate in that mode.  But I wonder if
it matters to the MPI world.  Seems to me that it would be a factor in
improving scalability (providing that other interconnect issues such as
bandwidth bottlenecks) don't prevent it.  My recollection is that the SCI
simulations I saw required very little added traffic to maintain coherency.

Also a brief note about the Dolphin product line, since the issue of link
saturation has come up:  - they DO also sell switches - or at least offer
them.  And if you check the SCI specification, you'll see that there are
some elaborate discussions of fabric architectures that the protocol
supports and switches enable.  What I DO NOT know is if the SCALI software
supports switch-based operation, and also don't know what the impact is on
the system cost per node.  My 'inexperienced' assessment of the appeal in
the Dolphin family is that you can start without the switch and later add it
if the performance benefit warrents.  That's what I'd say if I were selling
them anyway - and didn't know otherwise.  :-)

Richard Fryer
rfryer at beomax.com