[Beowulf] torus versus (fat) tree topologies

Patrick Geoffray patrick at myri.com
Thu Nov 18 23:39:23 EST 2004


Ashley Pittman wrote:
> On Fri, 2004-11-12 at 17:51 -0500, Patrick Geoffray wrote:
> 
>>Going trough a crossbar cost about 100-150ns these days.
> 
> 
> This isn't necessarily true, we reckon about 25ns on elan4.  This
> doesn't give you perfect latency scaling across very large networks but
> it's very close.

It's ~50ns on the Myrinet Xbar32 but you need to add the one-time cost 
of the SerDes for the fiber (~100 ns). You can do without if you use 
bulky noisy degrade-with-time copper cables :-) ).

Of course, the crossbar overheads add up and the number of crossbars to 
cross depends on the number of ports per crossbar. Last time I looked, 
the crossbars on the Quadrics Elite switch had 8 ports each, but it may 
be more by now.

Patrick
-- 

Patrick Geoffray
Myricom, Inc.
http://www.myri.com
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