AVX-512 possibly in its own clock domain?

By: Brett (ggtgp.delete@this.yahoo.com), August 25, 2018 12:58 pm
Room: Moderated Discussions
Travis Downs (travis.downs.delete@this.gmail.com) on August 25, 2018 9:18 am wrote:
> 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).

That means the high FPU units are the same design as the low FPU units, which further limits performance.
The high FPU units could have been designed for low clocks, using different transitors and different gate delays. But that means switching rates clocks fast.
I could see a design where you have an extra FPU just for non-vector math at 4+GHz, and a full set of vector units designed for low clocks.

There is also no law that the float/vector unit has to run at the same clock rate as the rest of the CPU.
You just have to buffer the cache interface and the incoming decoded float/vector instruction stream, and buffer the incoming instruction completion signals for write commits.

This would completely hide single slow vector instructions, you could detect this by having say every 20th instruction a vector op, the CPU should stay at 4+GHz.

> 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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      Gold 6130 resultsTravis2018/08/24 10:02 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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