Bogus 100x speedups

Article: Computational Efficiency in Modern Processors
By: AM (myname4rwt.delete@this.jee.male), November 10, 2009 10:47 am
Room: Moderated Discussions
David Kanter (dkanter@realworldtech.com) on 11/10/09 wrote:
---------------------------
>Here's a few great examples on the N-body problem:
>
>http://www.cise.ufl.edu/~jgao/p_final.pdf
>
>They are claiming a very impressive 100X speed up. Look at the hardware they are
>comparing against though. I think they are also using the O(N^2) algorithm.
>
>http://markjstock.org/research/AIAA-2008-608-552.pdf
>
>This is a better paper where they actually are using an intelligent algorithm (better
>than O(N^2)) on the GPU. They see ~220GFLOP/s for N-body on a 8800 GTX...which
>is supposedly 17X better than a 2-core 2.4GHz Opteron.

Actually, on p.5 and 6 they report 140x speedup (218 GFLOP/s vs 1.6 GFLOP/s), 8800 GTX achieving about 63% of its peak capacity. I'm short of time to read it in detail at the moment, just a little puzzled about it.

>Just a quick reference check here:
>220 GFLOPs/17 = 13 GFLOP/s for the Opteron
>13 GFLOPs/2 cores = 6.5 GFLOP/s per core
>6.5 GFLOPs/2.4GHz = 2.7 FLOP/cycle per core
>
>Here's the catch. An Opteron can execute 8 FLOP/s cycle, so the SW isn't really tuned that well.
>
>More problematic, this is on small problem size (0.5M particles). When the problem
>size scales up, the algorithm starts to backfire and the speedup drops to 9.6X.
>
>
>Here's some real work on this area that touches on these issues directly:
>
>http://www.lanl.gov/conferences/lacss/2009/presentations/vuduc.pdf
>
>They only consider FMM which is O(N), instead of O(N^2) or O(N log N). That's
>quite reasonable since you should be using the most efficient algorithm. Although
>I suppose you could consider a CPU using the O(N) vs. a GPU using whatever their favorite algorithm is.
>
>See page 56: 2 socket Nehalem ~ 2 GPUs.
>
>And note the sensitivity of Nehalem to tuning - 25X speed up from tuning.
>
>I think that pretty clearly shows how specious these comparisons can be. The algorithm
>matters, tuning matters, hardware matters and most importantly - don't believe what
>you read until you can see the fine print.
>
>David
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TopicPosted ByDate
Article: Computational Efficiency in Modern Processors by DKMoTheG2009/11/08 07:02 AM
  Article: Computational Efficiency in Modern Processors by DKnone2009/11/08 07:15 AM
  Silverthorne and OoO vs. InOrdMoTheG2009/11/08 07:22 AM
    Silverthorne and OoO vs. InOrdDavid Kanter2009/11/08 04:11 PM
      Magical 100x speedupsAM2009/11/09 09:03 AM
        Magical 100x speedupsDavid Kanter2009/11/09 12:41 PM
          Magical 100x speedupsnone2009/11/09 01:36 PM
            Magical speedupsDavid Kanter2009/11/09 03:24 PM
              Magical speedupsnone2009/11/09 03:40 PM
              Hardware SpecsMS2009/11/09 05:49 PM
                44x faster than a single cpu coreVincent Diepeveen2009/11/10 08:17 AM
              Magical speedupsVincent Diepeveen2009/11/10 08:02 AM
          Xeon 130x speedup vs XeonEric Bron2009/11/10 08:20 AM
          Magical 100x speedupsAM2009/11/10 10:42 AM
            Magical 100x speedupsLinus Torvalds2009/11/10 01:19 PM
              Mega speedupsAM2009/11/11 06:21 AM
        Bogus 100x speedupsDavid Kanter2009/11/10 01:26 AM
          No speedups for CPUs for the general programming populaceMoTheG2009/11/10 05:26 AM
          Bogus 100x speedups?2009/11/10 05:45 AM
          Bogus 100x speedupshobold2009/11/10 07:31 AM
          Bogus 100x speedupsVincent Diepeveen2009/11/10 08:26 AM
          Bogus 100x speedupssylt2009/11/10 10:00 AM
          Bogus 100x speedupsAM2009/11/10 10:47 AM
      GPU vs. CPUMoTheG2009/11/09 11:30 AM
        GPU vs. CPUa reader2009/11/09 07:58 PM
          ease of programmingMoTheG2009/11/09 11:45 PM
            yes for GPU programming you need non-public infoVincent Diepeveen2009/11/10 08:36 AM
              yes for GPU programming you need non-public infoPotatoswatter2009/11/11 08:06 AM
                yes for GPU programming you need non-public infoVincent Diepeveen2009/11/11 11:23 AM
                  yes for GPU programming you need non-public infoPotatoswatter2009/11/11 01:26 PM
                  Real businesses use GPGPU.Jouni Osmala2009/11/11 11:00 PM
        GPU vs. CPU?2009/11/10 06:01 AM
          2. try but most is said, just clarifyingMoTheG2009/11/10 10:24 AM
            2. try but most is said, just clarifying?2009/11/11 01:11 AM
              you missread meMoTheG2009/11/12 12:33 AM
                you missread me?2009/11/12 01:18 AM
            2. try but most is said, just clarifyingPotatoswatter2009/11/11 08:22 AM
              2. try but most is said, just clarifying?2009/11/12 01:22 AM
                loose, not so orderlyMoTheG2009/11/12 12:47 PM
                  loose, not so orderlyPotatoswatter2009/11/12 06:50 PM
                2. try but most is said, just clarifyingrwessel2009/11/12 01:01 PM
                  2. try but most is said, just clarifyingGabriele Svelto2009/11/13 12:39 AM
                    2. try but most is said, just clarifying?2009/11/13 01:14 AM
                      2. try but most is said, just clarifyingGabriele Svelto2009/11/13 01:30 AM
                      2. try but most is said, just clarifyingrwessel2009/11/13 01:24 PM
                  2. try but most is said, just clarifyingMichael S2009/11/14 01:08 PM
                    2. try but most is said, just clarifyingGabriele Svelto2009/11/14 11:38 PM
                      2. try but most is said, just clarifyingAndi Kleen2009/11/15 01:19 AM
                      2. try but most is said, just clarifyingMichael S2009/11/15 01:58 AM
                        2. try but most is said, just clarifyingEric Bron2009/11/15 02:25 AM
                          /MP optionEric Bron2009/11/15 02:33 AM
                            /MP optionPaul2009/11/15 09:42 AM
                              /MP optionEric Bron2009/11/15 01:22 PM
                        2. try but most is said, just clarifying?2009/11/15 03:13 AM
                          2. try but most is said, just clarifyingMichael S2009/11/15 05:14 AM
                  2. try but most is said, just clarifyingEugene Nalimov2009/11/14 09:24 PM
    Atom pointAM2009/11/09 09:00 AM
      Atom TDPDavid Kanter2009/11/09 12:48 PM
        Atom TDPhobold2009/11/10 07:41 AM
        Atom TDPAM2009/11/10 10:49 AM
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