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

Nifty Tom Mitchell niftyompi at niftyegg.com
Thu Feb 12 16:45:01 EST 2009


If the bulk of your computation fits on 4-8 nodes then
a pair of 24 port switches will provide good service for you.

I doubt that there is a need for special queues to establish IB switch
specific host assignments.  Yet, it might pay to establish multiple queues
early on for other reasons.   

Latency should not be an issue in the 2 each 24 port switch .vs. 48 port case.
The larger switches are built out of the same ASICS that small switches
are. The internal links are almost equivalent to having an external
link... 


On Wed, Feb 11, 2009 at 10:44:23AM -0500, Ivan Oleynik wrote:
> 
>    Tom,
>    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 env\ironment.
>    Ivan
> 
>    On Tue, Feb 10, 2009 at 3:55 PM, Nifty Tom Mitchell
>    <[1]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][2]Shainer at mellanox.com>
> 
>    >    wrote:
>    >
>    >    Do you plan to have full not blocking communications between the
>    next
>    >    systems and the core switch?
>    >
>    __________________________________________________________________
>    >
> 
>      >    From: [2][3]beowulf-bounces at beowulf.org
>      >    [mailto:[3][4]beowulf-bounces at beowulf.org] On Behalf Of Ivan
>      Oleynik
> 
>    >    Sent: Sunday, February 08, 2009 8:20 PM
> 
>      >    To: [4][5]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?
> 
> References
> 
>    1. mailto:niftyompi at niftyegg.com
>    2. mailto:Shainer at mellanox.com
>    3. mailto:beowulf-bounces at beowulf.org
>    4. mailto:beowulf-bounces at beowulf.org
>    5. mailto:beowulf at beowulf.org

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

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