Author: Matthew Hull
Date: 10:52:53 09/03/03
Go up one level in this thread
On September 03, 2003 at 13:29:02, Robert Hyatt wrote: >On September 03, 2003 at 13:12:34, Gian-Carlo Pascutto wrote: > >>On September 03, 2003 at 12:05:43, Robert Hyatt wrote: >> >>>>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. >> >>I don't understand. >> >>Even the slowest access on a NUMA Opeteron is twice as fast as on a SMP >>Xeon. >> >>How can it be slower then? > >You keep changing the subject. I am not comparing apples to oranges. I am >comparing two machines that are _identical_ in every way except one has a pure >SMP memory interconnection while the other has a pure NUMA interconnection. > >No references to X86 vs Opteron. No references to Cray vs Sun. If you give >me two boxes that are identical except for SMP vs NUMA, the SMP box will >_always_ have a speed advantage. It might not be much for small numbers of >processors, but it _will_ be there. > >But if you want to compare opteron NUMA to something else, I'll take a Cray T932 >which is a pure crossbar SMP machine. Want to compare latencies there? The >Cray is _always_ 120 ns. No matter what part of memory from which processor >you access. > >However, that is just as unfair as opteron to X86. > >NUMA is worse, period, when compared to an equivalent non-NUMA machine, in all >respects _but_ pricing. That is where NUMA shines, and it is why NUMA was >developed in the first place. NUMA was _not_ a solution to a performance >problem. It was a solution to a _pricing_ problem. The Crossbar was a solution >to a performance problem. I think I understand exactly what you are saying (this discussion has been very imformative for me). Question: Touching on what GCP seemed to be getting at, if I were to go out and buy one of these quad Opteron systems and compile a current SMP version of Crafty on it, could it get a similar n-way speedup percentage like the Xeon quads, even though it's not SMP, but NUMA with some kind of fast latency (is that an oxymoron)? I understand the sentiment of "why settle for that when you can optimise for NUMA and go yet faster". Thanks, Matt > >> >>-- >>GCP
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