[Beowulf] Connecting two 24-port IB edge switches to core switch:extra switch hop overhead

Nifty Tom Mitchell niftyompi at niftyegg.com
Tue Feb 10 12:55:03 PST 2009

With 36 hosts the best connectivity between the 36 hosts will be with a 
single 48 port switch and use many of the extra ports to link to the existing 

It is not insane to cost out and plan a fabric design with two, three or more
24 port switches including cables.   Three times 24  (or more) switch
designs can make it clear what value a big switch brings to your game.

What we do not know is the job mix and the demands that mix will place
on the fabric.  If the job mix is 99% 32 host jobs that are bandwidth
and latency limited then the big switch may quickly show it's value.
If the mix is lots of 23 or less host jobs then 24 port switch solutions
will behave nearly ideal.

Having looked at a lot of university clusters lots of 23 or less host
jobs seems like a common work load.  Thus a pair of 24 port switches
will be fine with the right job scheduling.

My gut is that two 24 port switches that: share five links, have
18 hosts per switch and with the last two links connected to your
existing fabric will operate quite well.

One important IB cluster design point is the cable link lengths at fast data
rates.  Smaller switches can be located to reduce host to switch and switch
to switch link lengths.    Also for fast link speeds watch: bend radius, cable quality
and other cable management issues, they matter.

On Tue, Feb 10, 2009 at 12:01:26AM -0500, Ivan Oleynik wrote:
>    It would be nice to have non-blocking communication within the entire
>    system but the critical part is the 36-node complex to be connected to
>    the main cluster.
>    On Mon, Feb 9, 2009 at 1:33 AM, Gilad Shainer <[1]Shainer at mellanox.com>
>    wrote:
>    Do you plan to have full not blocking communications between the next
>    systems and the core switch?
>      __________________________________________________________________
>    From: [2]beowulf-bounces at beowulf.org
>    [mailto:[3]beowulf-bounces at beowulf.org] On Behalf Of Ivan Oleynik
>    Sent: Sunday, February 08, 2009 8:20 PM
>    To: [4]beowulf at beowulf.org
>    Subject: [Beowulf] Connecting two 24-port IB edge switches to core
>    switch:extra switch hop overhead
>    I am purchasing 36-node cluster that will be integrated to already
>    existing system. I am exploring the possibility to use two 24 4X port
>    IB edge switches in core/leaf design that have maximum capability of
>    960Gb (DDR)/480Gb (SDR). They would be connected to the main Qlogic
>    Silverstorm switch.
>    I would appreciate receiving some info regarding the communication
>    overhead incurred by this setup. I am trying to minimize the cost of IB
>    communication hardware. It looks like buying single 48-port switch is
>    really an expensive option.
>    Thanks,
>    Ivan

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

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