[Beowulf] Infiniband modular switches

Gilad Shainer Shainer at mellanox.com
Wed Jun 25 00:38:25 EDT 2008

Mark Hahn wrote:
> > Static routing is the best approach if your pattern is 
> known. In other
> sure, but how often is the pattern actually known?  I mean in general:
> aren't most clusters used for multiple, shifting purposes?

There will be many arguments in each side. There are enough cases where
the patterns are know, or that the majority of the applications use the
same pattern, and there are cases that they are not. As I see it the key
is flexibility - choose what works best for you, and if you want to tune
it - that is possible as well

> > There are some vendors that uses only the 24 port switches to build 
> > very large scale clusters - 3000 nodes and above, without any 
> > oversubscription, and they find it more cost effective. Using single
> so the switch fabric would be a 'leaf' layer with 12 up and 
> 12 down, and a top layer with 24 down, right?  so 3000 nodes 
> means 250 leaves and 125 tops, 9000 total ports so 4500 cables.

The number of switch layers, or tiers, depends on the cluster size and
the oversubscription. For full non blocking,  2 layer of 24 switches get
you up to 288 ports switch, and with 36 ports up to 648 ports. With 3
layers you can build 3456 ports or more than 11K ports (24 or 36 ports
building blocks). Assume that you use 2 layers, you need the same number
of cables as number of nodes.  

> > enclosures is easier, but the cables are not expensive and 
> you can use 
> > the smaller components.
> in federated networks, I think cables wind up being 15-20% of 
> the network price.  for instance, if we take the simplest 
> possible approach, and equip this 3000-node cluster with a 
> non-blocking federated fabric (assuming just sdr) from 
> colfax's current price list:
> subtot	unit	n	what
> 375000	125	3000	ib nic
> 117000	39	3000	1m host ib cables
> 148500	99	1500	8m leaf-top ib cables
> 900000	2400	375	24pt unman switch
> 1540500 total (cable 17%)
> I'm still confused about IB pricing, since the street price 
> of nics, cables and switches are dramatically more expensive 
> than colfax.
> (to the paranoid, colfax would appear to be a mellanox shell 
> company...)

This is totally wrong. Colfax is one of many distributers and is in
business much before Mellanox. Mellanox do not need to have shell
companies. If you check with othersm you might see same pricing. 

> for completeness, here's the same bom with "normal" public prices:
> subtot	unit	n	what
> 2100000	700	3000	ib nic
> 330000	110	3000	1m host ib cables
> 330000	220	1500	8m leaf-top ib cables
> 1500000	4000	375	24pt unman switch
> 4260000 total (cable 15%)

I can not comment on any pricing from my customers and partners. 

> interestingly, if nodes were about 3700 apiece (about what 
> you'd expect for intel dual-socket quad-core 2G/core), the 
> interconnect winds up being 28% of the cost.

To the smart user, if your application require high speed interconnect,
than it worth the money (assuming your math is correct). Buying 300
servers and being able to utilize only half, not to mention the electric
bill - suddenly the interconnect seems very cheap.



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