Hi Bill, list Bill: This is very interesting indeed. Thanks for sharing!
Bill's graph seem to show that Shanghai and Barcelona scale (almost) linearly with the number of cores, whereas Nehalem stops scaling and flattens out at 4 cores. The Nehalem 8 cores and 4 cores curves are virtually indistinguishable, and for very large arrays 4 cores is ahead. Only for huge arrays (>16M) Nehalem gets ahead of Shanghai and Barcelona. Did I interpret the graph right? Wasn't this type of scaling problem that plagued the Clovertown and Harpertown? Any possibility that kernels, BIOS, etc, are not yet ready for Nehalem? Thanks, Gus Correa --------------------------------------------------------------------- Gustavo Correa Lamont-Doherty Earth Observatory - Columbia University Palisades, NY, 10964-8000 - USA --------------------------------------------------------------------- Bill Broadley wrote:
I've been working on a pthread memory benchmark that is loosely modeled on McCalpin's stream. It's been quite a challenge to remove all the noise/lost performance from the benchmark to get close to performance I expected. Some of the obstacles: * For the compilers that tend to be better at stream (open64 and pathscale), you lose the performance if you just replace double a[],b[],c[] with double *a,*b,*c. Patch[1] available. I don't have a work around for this, suggestions welcome. Is it really necessary for dynamic arrays to be substantially slower than static? * You have to be very careful with pointer alignment both with cache lines, and each other * cpu_affinity (by CPU id) * numa (by socket id) The results are relatively smooth graphs, here's an example, it's uselessly busy until you toggle off a few graphs (by clicking on the key): http://cse.ucdavis.edu/bill/pstream.svg The biggest puzzle I have now is what the previous generation intel quads, the current generation AMD quads, and numerous other CPUs show a big benefit in L1, while the nehalem shows no benefit. [1] http://cse.ucdavis.edu/bill/stream-malloc.patch _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
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