AVX-512 downclocking not as bad as thought?

By: Travis (travis.downs.delete@this.gmail.com), September 9, 2018 7:16 pm
Room: Moderated Discussions
anonymou5 (no.delete@this.spam.com) on September 7, 2018 9:03 pm wrote:

> maybe be more explicit about the cost of the frequency transition i.e. its dead time?
>
> and maybe mention power gating too i.e. extra delays upon use after periods of non-use

Yes, this is mostly not mentioned because I haven't yet tested SKX for these properties: the article is based on "steady state" tests that measure how everything runs once all the transitions are done.

I'll probably get around to testing the specific transition behavior, although I expect it to be fairly similar to past processors: roughly an 8-10us transition when the frequency needs to change (which is pretty much any time the license needs to change as there are few-to-no columns in the frequency matrix where the speed is the same for difference licenses), and also some periods where wide instructions issue at a lower rate but other instructions issue at the regular frequency.

This last part has been a feature apparently since Haswell: I guess it gives the core time to see if the wide instructions are really going to be common enough to bother switching licenses, or if it should just power through a few scatted wide instructions in "slow mode" rather than slowing down the whole CPU. It has been suggested that in this code the CPU is using a narrower ALU twice to execute the wide instructions (e.g., passing 256-bit AVX operations twice through a 128-bit ALU and combining the result), although of course there are other possibilities, such as simple dispatch throttling or using only one out of two ALUs, but at the full width.
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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
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          AVX-512 downclocking not as bad as thought?Travis2018/08/25 07:35 PM
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    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
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        AVX-512 downclocking not as bad as thought?Jeff S.2018/08/24 12:39 PM
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            AVX-512 downclocking not as bad as thought?anonymou52018/09/12 06:39 AM
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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?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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