On 11/9/2012 17:17, Vincent Diepeveen wrote: > The latency estimate taking 5 hops seems a tad optimistic to me > except when i read the English wrong and they mean 1.7 microseconds a > hop making it for a 5 hop 5 * 1.7 = 8.5 microseconds in total.
What they state is that nearest neighbour is 1.2 usec. Of those 1.2 usec, only a fraction is actual fabric latency, which is why they can do 5 fabric hops in 1.7 (meaning you have rougly 100ns per fabric hop which is very reasonable with 12.5/14 Gbps links). The rest of the 1.2/1.7 usec is getting in and out of the chip (to the CPUs). Cheers, -- Steffen Persvold, Chief Architect NumaChip Numascale AS - www.numascale.com Tel: +47 92 49 25 54 Skype: spersvold PS: Not working for Cray or have ever touched one so I can't confirm the numbers, but I know a thing or two about network chips and what they quote is not unreasonable at all :) > > "Not every node is only one hop away, of course. On a fully > configured system, you are five hopes away maximum from any socket, > so there is some latency. But the delta is pretty small with > Dragonfly, with a minimum of about 1.2 microseconds for a short hop, > an average of 1.5 microseconds on average, and a maximum of 1.7 > microseconds for the five-hop jump, according to Bolding." > > On Nov 8, 2012, at 7:13 PM, Hearns, John wrote: > >> Well worth a read: >> >> >> >> http://www.theregister.co.uk/2012/11/08/ >> cray_cascade_xc30_supercomputer/ >> >> >> >> John Hearns | CFD Hardware Specialist | McLaren Racing Limited >> McLaren Technology Centre, Chertsey Road, Woking, Surrey GU21 4YH, UK >> >> >> T: +44 (0) 1483 262000 >> >> D: +44 (0) 1483 262352 >> >> F: +44 (0) 1483 261928 >> E: [email protected] >> >> W: www.mclaren.com >> >> >> >> The contents of this email are confidential and for the exclusive >> use of the intended recipient. If you receive this email in error >> you should not copy it, retransmit it, use it or disclose its >> contents but should return it to the sender immediately and delete >> your copy. >> >> _______________________________________________ >> Beowulf mailing list, [email protected] sponsored by Penguin >> Computing >> To change your subscription (digest mode or unsubscribe) visit >> http://www.beowulf.org/mailman/listinfo/beowulf > > _______________________________________________ > Beowulf mailing list, [email protected] sponsored by Penguin Computing > To change your subscription (digest mode or unsubscribe) visit > http://www.beowulf.org/mailman/listinfo/beowulf > _______________________________________________ Beowulf mailing list, [email protected] sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
