[Beowulf] Mutiple IB networks in one cluster

atchley tds.net atchley at tds.net
Wed Feb 5 09:33:42 PST 2014

Hi Bill,

I am not clear as to what you are saying.

I understand non-oversubscribed, fat-tree networks to be described as
non-blocking for "some" bi-sections of the nodes, but not for all
bi-sections. It is possible to block in a "non-blocking" network except on
the local crossbar.

At least that is what Chuck Seitz drove into our heads whenever we said
that a Clos was non-blocking. :-)


On Tue, Feb 4, 2014 at 4:06 PM, Bill Broadley <bill at cse.ucdavis.edu> wrote:

> On 02/01/2014 08:17 AM, atchley tds.net wrote:
> > The cross-bar switch only guarantees non-blocking if the two ports are on
> > the same line card (i.e. using the same crossbar). Once you start
> > traversing multiple crossbars, you are sharing links and can experience
> > congestion.
> Full backplane means no shared links, even if multiple switches are
> involved.
> Shared links are used when you oversubscribe the uplinks, like with a half
> or
> quarter backplane configuration.
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