AVX-512 downclocking not as bad as thought?

By: Travis (travis.downs.delete@this.gmail.com), August 24, 2018 2:48 pm
Room: Moderated Discussions
Maynard Handley (name99.delete@this.name99.org) on August 24, 2018 1:26 pm wrote:

> 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...

Ignoring the complexity of implementation, clock-by-clock throttling isn't necessarily strictly superior to frequency reduction: you get very fine-grained control, which is good, but you aren't moving to another point on the VF curve, which is bad. So at for some steady state you might expect clock-by-clock to perform worse since it's running at a higher voltage and frequency (N^2 or N^3 effect on power) but then just gating down the issued instructions (linear effect), while moving to a different frequency allows a lower voltage and lower power.

Where clock-by-clock would shine is code that transitions quickly between various intensities of heavy instructions. Clock-by-clock also avoids the problem of lengthy state transitions: moving to a different frequency takes ~20,000 clocks on Skylake and that's much better than it used to be. If you do that too often you were better off just running at the lower speed all the time: this is the cause of the so called "turbo attenuation" behavior which decides the chip is moving between frequencies too much and tries to just keep it at a lower speed.

Of course, I guess you could combine the two.
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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
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      AVX-512 downclocking not as bad as thought?Travis2018/08/24 09:54 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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