----- Forwarded message from Ashley Pittman <[EMAIL PROTECTED]> -----
From: Ashley Pittman <[EMAIL PROTECTED]> Date: Wed, 20 Oct 2004 14:44:49 +0100 To: Mark Hahn <[EMAIL PROTECTED]> Cc: [EMAIL PROTECTED] Subject: Re: [Beowulf] bandwidth: who needs it? X-Mailer: Ximian Evolution 1.4.6 On Sat, 2004-10-16 at 22:36, Mark Hahn wrote: > do you have applications that are pushing the limits of MPI bandwidth? > for instance, code that actually comes close to using the 8-900 MB/s > that current high-end interconnect provides? > > we have a fairly wide variety of codes inside SHARCnet, but I haven't > found anyone who is even complaining about our last-generation fabric > (quadrics elan3, around 250 MB/s). is it just that we don't have the > right researchers? I've heard people mutter about earthquake researchers > being able to pin a 800 MB/s network, and claims that big FFT folk can > do so as well. by contrast, many people claim to notice improvements > in latency from old/mundane (6-7 us) to new/good (<2 us). > > I'd be interested in hearing about applications you know of which are > very sensitive to having large bandwidth (say, .8 GB/s today). It's not so much that you don't have the right researchers, it's the type of projects they are researching or at least the way they are attacking the problem. Latency is every bit as critical as bandwidth and in many cases more so. Latency at scale is also critical, multi-hop networks dictate the need to use nearest-neighbour algorithms and therefore have trouble scaling to large CPU counts. It's also harder for newcomers and non technical people to conceptualise latency and especially scalable latency. >From code optimisation that I've done in the past I've also found that bandwidth is easier to hide via pipelining than latency and therefore is less critical to wall clock time. Also don't forget that SMP boxes are getting wider, think in terms of Mb/s/CPU and todays 900Mb/s network bandwidth suddenly doesn't sound that much. The good news here however is that the large SMPs tend to have multiple PCI-X busses so can use multiple networks effectively. Ashley, _______________________________________________ Beowulf mailing list, [EMAIL PROTECTED] To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org";>leitl</a> ______________________________________________________________ ICBM: 48.07078, 11.61144 http://www.leitl.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE http://moleculardevices.org http://nanomachines.net ------- To unsubscribe, change your address, or temporarily deactivate your subscription, please go to http://v2.listbox.com/member/[EMAIL PROTECTED]
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