> 	this is one of the coolest things I've seen... it's a very modular
> architecture; so you just build exactly the computer you need, and when you
> want to expand it, you just add more pieces to it. instead of a fixed, rigid
> backplane, they use NUMAlink (formerly called CrayLink) cables to go from
> brick to brick. makes installation very flexible. 

Yup.  It's really slick.  If SGI every discontinued their Origin series,
I'd cry.  They're really enjoyable machines to admin.

> as I understand it; the C-brick has a NUMALink connector, and a XTown
> ('crosstown') connector on it. the NUMALink goes to the router brick; and
> the XTown goes to the I/O brick. If you add a graphics brick; I understand
> that it connects to the I-brick, and from there to the C-brick. 
> but how do you get decent bandwidth from the CPUs to the graphics unit,
> using such a long chain (with possibly 6 feet of cable between each brick)?
> Obviously the graphics unit has some pretty impressive onboard GPUs, and a
> sizeable cache of its own; but it seems strange to go through an
> intermediate unit on the way to main memory. 
> 
> or maybe I'm just misreading the product literature.

Nope, you've read it correctly.  The only to connect the G-brick is via the
I-brick.  The G-brick needs to be hooked to the I-brick because the I-brick
controls the systems console, and once the G-brick is attached, it becomes
the console.  The G-brick's only connection to the system is via that link.

Yeah, it's a bit of a kludge, but it seems to me that it was a sort of
'after the fact' setup.  I personally don't think it works all that well.
I'd rather have a desk-side Onyx3000 than an Origin with a G-brick (Note:
The G-brick is just an Onyx3000 without any 'cpu modules').  Of course, if
you need more than 8-16 cpus, then an Origin is the only option, but it's
still a PITA to manage, in my experience.

Gabe
-- 
Gabe Turner                                             gabe at msi.umn.edu
SGI Origin Systems Administrator,
University of Minnesota Supercomputing Institute
 for Digital Simulation and Advanced Computation         www.msi.umn.edu