Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!ptimtc!nntp-server.caltech.edu!jkubicky
From: jkubicky@nntp-server.caltech.edu (Joseph J. Kubicky)
Newsgroups: comp.arch
Subject: Touchstone Delta followup
Message-ID: <1991Jun3.230539.7162@nntp-server.caltech.edu>
Date: 3 Jun 91 23:05:39 GMT
Organization: California Institute of Technology, Pasadena
Lines: 26


I posted an article about the Touchstone Delta the other day and I have
received several letters requesting additional information and pointing
out some potential problems.

I talked to one of the people in the router group here and he gave me some
better information about the inter-node latency.  Basically, the 75-125ns
figure is wrong.  The node-to-node jump latency is around 50ns.  So, for the
entire mesh, the maximum transport latency is around 2us.  However,
it turns out that the OS that each node runs accounts for considerably more
delay than this.  I get the impression that, as the software is
now written, we're talking at least 10us or so to actually get a message
somewhere (although, as I also pointed out, each node can multi-task, so
this delay at least need not contribute to wasted processing time).  However,
the theoretical lower limit (that is, that imposed by the harware), is
something like 2us (this also ignores delays in message delivery due to
messages already being delivered, but the OS automatically breaks
big messages up into little ones so this is minimized).

More information is availble in the 6th Annual MIT Conference on Advanced
Research in VLSI proceedings (maybe best to key on Advanced Research in VLSI).
Look for and article by C. Seitz called something like "Let's Route Packets
Instead of Wires".

Jay Kubicky
jkubicky@cobalt.cco.caltech.edu