AVX-512 downclocking not as bad as thought?

By: Travis (travis.downs.delete@this.gmail.com), August 24, 2018 7:41 am
Room: Moderated Discussions
Jeff S. (fakity.delete@this.fake.com) on August 24, 2018 7:22 am wrote:
> The evidence Alex saw in his early investigations with y-cruncher suggested (IIRC) that warming up fuller widths
> of the FMA banks was what triggered downclocking, but that the effects had a non-trivial lag during which execution
> might have been handled through dual issue of half-width operations. Your new turbo tester may or may not be
> more accurate in measuring this, but at least keep his hypothesis in mind. He's still out in CA after Hot Chips
> visiting friends and family (and hopefully not actually working right now...), but you mind find some of the
> discussions he's had on this on Agner's blog or the Mersenne forum until he gets back.

I haven't tried to look at the transition behavior yet - these measurements are all basically "fully warmed up" because I do a lot of warmup iterations and then use the median results from many trials. I check both the actual CPU frequency and the "implied" frequency which is based on the number of operations executed and their known latency and in every case the actual and implied frequencies are consistent, so the CPU is running at full speed.

I have no doubt that there is a transition period where things are weird though: there is also such a period on Skylake client (and some Haswell chips) when any 256-bit operation is executed for the first time in a while, and during this period 256-bit operations execute at about 25% of their usual throughput.

There is a chance though that even in steady state, AVX-512 FMA operations are being executed via dual issue and reassembled and that this happens without any loss in performance, including no latency effect (after all, this is a 1 AVX2-FMA chip, so the total throughput is the same for AVX2 or AVX-512 FMA). That seems unlikely (and in any case, what is the distinction between parallel dual issue of 2 256-bit FMAs or single issue of 1 512-bit to the same hardware)?

Note that I always saw some downclocking with AVX-512: I never saw a case where AVX-512 ran at the full 3.2 GHz base frequency: it was always at the 2.8 GHz or 2.4 GHz tier. The oddity was that it was running at 2.8 in some cases where I'd expect it to have been at 2.4.

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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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