[Beowulf] Сlos network vs fat tree

Nifty niftyompi Mitch niftyompi at niftyegg.com
Thu Nov 13 18:26:17 EST 2008


On Thu, Nov 13, 2008 at 08:54:35PM +0300, Mikhail Kuzminsky wrote:
> Sorry, is it correct to say that fat tree topology is equal to  
> *NON-BLOCKING* Clos network w/addition of "uplinks" ? I.e. any  
> non-blocking Clos network w/corresponding addition of uplinks gives fat 
> tree ?
>
> I read somewhere that exact evidence of "non-blocking" was performed for 
> Clos networks with >= 3 levels. But most popular Infiniband fat trees has 
> only 2 levels.
>
> (Yes, I know that "non-blocking" for Clos network isn't "absolute" :-))

Since Infiniband routing is static I suspect that the topology may match
but the behavior will not.

 http://en.wikipedia.org/wiki/Clos_network#Strict-sense_nonblocking_Clos_networks_.28m_.E2.89.A5_2n_-_1.29_-_the_original_1953_Clos_result

See the bit:  "If m ≥ n, the Clos network is rearrangeably nonblocking,
meaning that an unused input on an ingress switch can always be connected
to an unused output on an egress switch, but for this to take place,
existing calls may have to be rearranged by assigning them to different
centre stage switches in the Clos network [2]. To prove this, it is..."

The key word is "rearrangeably nonblocking".   

If 30 seconds of homework is sufficient the key to Clos topology research
is that it is focused on teleco switching where a call is 'routed' when
it is made and torn down on disconnect.    This is not the same problem
space as a packet switched network at a couple of levels.


-- 
	T o m  M i t c h e l l 
	Found me a new hat, now what?


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