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

Ivan Oleynik iioleynik at gmail.com
Wed Feb 11 07:44:23 PST 2009


Thanks for your reply. As I explained in my original email 48-port IB switch
would be ideal because the jobs on these 36 nodes will mostly be run locally
within the 36-node complex. However, 48-port IB switch is too expensive,
that is why I am considering alternative cost-effective solutions. I think
we major pattern of the load will be a bunch of 32-64 cpu jobs at maximum,
i.e each of them can fit into 4-8 nodes. These jobs are MPI the
applications, therefore, they require the best bandwidth-latency


On Tue, Feb 10, 2009 at 3:55 PM, Nifty Tom Mitchell
<niftyompi at niftyegg.com>wrote:

> 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
> fabric.
> 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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