[Beowulf] Infiniband modular switches

[Patrick Geoffray]

Hi Don,

Don Holmgren wrote:
> latency difference here matters to many codes).  Perhaps of more 
> significance, though, is that you can use oversubscription to lower the 
> cost of your fabric.  Instead of connecting 12 ports of a leaf switch to 
> nodes and using the other 12 ports as uplinks, you might get away with 
> 18 nodes and 6 uplinks, or 20 nodes and 4 uplinks.  As core counts are 
> increasing, this is becoming more and more viable for some applications.

It's important to note that the "full-bisection" touted by vendors is on 
paper only. In reality, static routing provides full-bisection for a 
very small subset of patterns, the average effective bisection on a 
diameter-3 Clos is ~40% of link rate (adaptive routing improves that a 
lot, but breaks packet order on the wire which is a requirement for some 
network protocols).

In practice, "paper" full-bisection is near free when using a single 
enclosure, since all spine cables are on the backplane. For larger 
networks, where you have to pay for real cables to the spine level, then 
it may make sense to be oversubscribed if the effective bisection is 
already bad (static routing), or if your collective communication on 
large jobs are not bandwidth bounded. However, the later is often false 
on many-cores.

Patrick
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