AVX-512 downclocking not as bad as thought?

By: Travis (travis.downs.delete@this.gmail.com), August 25, 2018 7:34 pm
Room: Moderated Discussions
anon (spam.delete.delete@this.this.spam.com) on August 25, 2018 5:06 am wrote:
> Travis (travis.downs.delete@this.gmail.com) on August 24, 2018 10:54 pm wrote:
> > David Hess (davidwhess.delete@this.gmail.com) on August 24, 2018 10:08 pm wrote:
> > > Ricardo B (ricardo.b.delete@this.xxxxx.xx) on August 24, 2018 5:22 am wrote:
> > > >
> > > > Naively, I would say that the best approach for dealing with power hungry instructions
> > > > would be to run at normal clock speed and restrict the instruction issue rate until
> > > > some criteria are met and then down clock the core and un-restrict the issue rate.
> > >
> > > Running at a slower clock would allow the extra complex instructions
> > > to take advantage of more gate delays per stage.
> >
> > Yes, but only if they only ran at those slower speeds, right? Here it seems you
> > can run the most complex instructions (full width FMAs) at a higher speed ("middle
> > tier"), but just not at a high rate (but they have the expected latency).
> >
> > So the ALU must be designed to accommodate the gate delay associated with that higher
> > frequency, unless it can somehow reconfigure itself when running at a lower freq?
> >
> >
>
> Some AVX-512 only instructions could rely on it.
> The ALU might also be asymmetric with the lowest 128b having the lowest number of gate delays
> while the higher 128b and highest 256b only need to meet more relaxed timing requirements.
> Since the high bits can be powergated they're on a separate supply
> anyway so in theory different voltages would be an option as well.

Sure - but this only works if the upper bits are never used at the highest frequency (i.e., the downclocking occurs before any instruction that requires the relaxed timing is executed).

As I mentioned above, this is not what I observe: the CPU is happy to execute with fairly high intensity (1 every 2 cycles) full-width FMAs (presumably the most complex instruction) at the higher frequency. So the unit has to be designed to run at the higher frequency as well.

This makes sense on these single FMA chips anyways: there is no dedicated 512-bit FP ALU really: it's the same unit as the 2x 256-bit ALUs. So in fact since you can run 2 256-bit FMAs at not only the middle tier (AVX2-turbo) but even the fastest frequency (non-AVX turbo) you can be sure the unit has to be designed for that frequency.

It could be different for the other FMA unit on chips that have them: that doesn't need to also support 256-bit FMAs, so perhaps it is never used until necessary and then only after downclocking has occurred.



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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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