Author: Robert Hyatt
Date: 09:05:43 09/03/03
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On September 03, 2003 at 00:15:32, Jeremiah Penery wrote: >On September 02, 2003 at 22:46:28, Robert Hyatt wrote: > >>On September 02, 2003 at 18:26:44, Jeremiah Penery wrote: >> >>>All I said is that being SMP doesn't magically make your latency better. It >>>depends on how the machine is built. There's no reason a big NUMA machine >>>couldn't be built where the average memory access wouldn't be as fast or faster >>>than a similar-sized SMP machine. There are several reasons why such a machine >>>isn't built, but not a real technical reason. >> >> >>The reason is $. NUMA scales well with respect to price per additional CPU. >>Crossbars to not. It is possible to build a NUMA box that switches just as >>fast as a Crossbar. There's no reason to do so, because you end up with a >>crossbar, and its associated high scaling cost. > >Exactly. :) > >As I've said, there's nothing that magically makes SMP *inherently* faster than >NUMA. Nothing more. That's not what I said. SMP _is_ inherently faster. Because all memory has the same access latency. You _do_ have to share _something_ in a parallel algorithm. ANd whatever that is will be slower than doing the same thing on a SMP box. Even if it is just one word, the SMP box will access that one word faster all around and the program will run faster. Perhaps not a lot faster for 1 word of shared data. But faster nonetheless.
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