AVX-512 downclocking not as bad as thought?

By: Maynard Handley (name99.delete@this.name99.org), August 24, 2018 12:26 pm
Room: Moderated Discussions
Jeff S. (fakity.delete@this.fake.com) on August 24, 2018 12:24 pm wrote:
> Maynard Handley (name99.delete@this.name99.org) on August 24, 2018 12:03 pm wrote:
> > Is there a real (and realistic) fear that these instructions can generate so much heat so fast
> > that the existing thermal tracking is too slow? And something that could be fast enough (eg
> > as has been suggested, limiting instruction throughput at some point --- maybe issue, maybe
> > decode) is not realistic why? Because the entire thermal modeling system runs at say 100th
> > CPU frequency and it was too hard at the time to bolt on a new, better targeted, system?
>
> I have heard more concern around the input voltage sagging on sudden heavy draw more than heat accumulation,
> so maybe the thermal capacity of the die is larger in a sense than the electrical capacitance afforded by
> the on-die and on-package systems. It seems like the 512b FMA units can collectively suck down more than half
> a core's power budget by themselves, and it might not take very many cycles of suddenly doubled (or worse)
> power consumption to drain instantaneous power supply below the threshold of reliable operation.
>
> This is the kind of thing I suspect David would very succinctly summarize as "dI/dt
> concerns", but I don't have any real data or secret sources for you unfortunately.

Hmm, yeah, makes sense.

It seems that, in PRINCIPLE, you should be able to monitor the charge of a capacitor over time, and dynamically engage in clock-by-clock throttling (through things like halting fetch, decode, or issue); and that this would be more performant than just clubbing the frequency?
Of course it takes time to design a scheme like that, but this has been an issue for years...

It will be interesting to see if Apple has anything along those lines in the A12. Obviously the details are different, but they are clearly aware that they have their own problem with SoC power draw possibly exceeding what an (aged) battery can supply.
One solution to that is the Intel solution, just reducing frequency (and only improved over the A11 and earlier solution in that there might be more intermediate frequency steps available, and/or a more dynamic monitor of when current draw is going high).
But a more performant solution would be a variant of what I've described for AVX, something that perhaps monitors the big capacitor rather than the battery current, and throttles on a cycle by cycle basis.
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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
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        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
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    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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