AVX-512 downclocking not as bad as thought?

By: Travis Downs (travis.downs.delete@this.gmail.com), August 25, 2018 8:18 am
Room: Moderated Discussions
Royi (Royi.delete@this.yahoo.com) on August 25, 2018 4:36 am wrote:
> Travis (travis.downs.delete@this.gmail.com) on August 24, 2018 1:01 am wrote:
> >
> > If anyone wants to run my test on a more interesting system than the W-2104, you can find it here:
> >
> > https://github.com/travisdowns/avx-turbo
> >
> > By default it will run on 1 core up to the number of cores you have so you can see
> > the progression as active cores increase. In particular, you can use it to verify if
> > you're getting the published relationship between code type and active core count.
>
> There is another nice analysis by Daniel Lemire (He used your code):
>
> https://lemire.me/blog/2018/08/24/trying-harder-to-make-avx-512-look-bad-my-quantified-and-reproducible-results/
>
> https://lemire.me/blog/2018/08/25/avx-512-throttling-heavy-instructions-are-maybe-not-so-dangerous/
>
>

Indeed, I started looking into this because of claims there that AVX-512 downclocking maybe isn't much of a problem as people think. I had a fairly clear model in my head of how this worked, so I figured the cost should be obvious based on the frequency matrix.

I ended up finding that it was not so simple and that my model was wrong in ways that cut both directions : in favor of AVX-512, and also plain AVX/AVX2 it is harder to get into the "heavy instruction" license than I thought: unlike the light instructions it takes much more than a single instruction to switch licenses (the topic of this thread).

On the other hand, I also found that AVX-512 has an insidious effect on lots of code that has nothing to do with AVX-512 at all, including scalar FP, SSE integer and FP, and AVX/AVX2 code: the CPU can be running in a mode where all such instructions are extended to 512-bits internally, forcing you to the middle license, slowing your CPU and no doubt burning a ton of power.

At least on the system I tested (Ubuntu 16.04, kernel 4.4), processes seem to be run in this mode through no fault of their own.
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TopicPosted ByDate
AVX-512 downclocking not as bad as thought?Travis2018/08/24 12:01 AM
  AVX-512 downclocking not as bad as thought?Ricardo B2018/08/24 04:22 AM
    AVX-512 downclocking not as bad as thought?David Hess2018/08/24 09:08 PM
      AVX-512 downclocking not as bad as thought?Travis2018/08/24 09:54 PM
        AVX-512 downclocking not as bad as thought?anon2018/08/25 04:06 AM
          AVX-512 downclocking not as bad as thought?Travis2018/08/25 07:34 PM
        AVX-512 downclocking not as bad as thought?David Hess2018/08/25 09:44 AM
          AVX-512 downclocking not as bad as thought?Travis2018/08/25 07:35 PM
  AVX-512 downclocking not as bad as thought?Jeff S.2018/08/24 06:22 AM
    AVX-512 downclocking not as bad as thought?Travis2018/08/24 07:41 AM
      AVX-512 downclocking not as bad as thought?Jeff S.2018/08/24 09:24 AM
  AVX-512 downclocking not as bad as thought?Maynard Handley2018/08/24 11:03 AM
    AVX-512 downclocking not as bad as thought?Jeff S.2018/08/24 11:24 AM
      AVX-512 downclocking not as bad as thought?Maynard Handley2018/08/24 12:26 PM
        AVX-512 downclocking not as bad as thought?Jeff S.2018/08/24 12:39 PM
        AVX-512 downclocking not as bad as thought?megabytephreak2018/08/24 12:43 PM
          AVX-512 downclocking not as bad as thought?Maynard Handley2018/08/24 03:59 PM
          AVX-512 downclocking not as bad as thought?someone2018/09/12 12:20 AM
            AVX-512 downclocking not as bad as thought?anonymou52018/09/12 06:39 AM
        AVX-512 downclocking not as bad as thought?Travis2018/08/24 02:48 PM
    AVX-512 downclocking not as bad as thought?David Hess2018/08/24 09:26 PM
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  AVX-512 downclocking not as bad as thought?Adrian2018/08/24 08:30 PM
    AVX-512 downclocking not as bad as thought?Adrian2018/08/24 08:36 PM
      AVX-512 downclocking not as bad as thought?Adrian2018/08/24 08:39 PM
        AVX-512 downclocking not as bad as thought?Travis2018/08/24 10:06 PM
          AVX-512 downclocking not as bad as thought?Adrian2018/08/24 10:28 PM
  AVX-512 downclocking not as bad as thought?Royi2018/08/25 03:36 AM
    AVX-512 downclocking not as bad as thought?Travis Downs2018/08/25 08:18 AM
      AVX-512 possibly in its own clock domain?Brett2018/08/25 12:58 PM
        AVX-512 possibly in its own clock domain?Travis2018/08/25 07:40 PM
          AVX-512 possibly in its own clock domain?Brett2018/08/26 08:48 AM
            AVX-512 possibly in its own clock domain?Maynard Handley2018/08/26 04:28 PM
              AVX-512 possibly in its own clock domain?none2018/08/26 08:40 PM
                AVX-512 possibly in its own clock domain?Maynard Handley2018/08/27 09:37 AM
                  AVX-512 possibly in its own clock domain?none2018/08/27 10:28 AM
        AVX-512 possibly in its own clock domain?David Hess2018/08/26 09:20 AM
          AVX-512 possibly in its own clock domain?Maynard Handley2018/08/26 04:35 PM
            AVX-512 possibly in its own clock domain?Gabriele Svelto2018/08/27 12:46 PM
            AVX-512 possibly in its own clock domain?David Hess2018/08/27 06:03 PM
  AVX-512 downclocking not as bad as thought?Travis2018/09/07 07:32 PM
    AVX-512 downclocking not as bad as thought?anonymou52018/09/07 08:03 PM
      AVX-512 downclocking not as bad as thought?Travis2018/09/09 07:16 PM
    AVX-512 downclocking not as bad as thought?Tim McCaffrey2018/09/08 07:55 AM
      AVX-512 downclocking not as bad as thought?Travis Downs2018/09/08 02:21 PM
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