2 ms·

I don't know specifically, e.g. if there are any such pauses on Zen. Also, Zen doesn't yet support AVX-512 so a big possible source of variation is moot. I don

I don't know specifically, e.g. if there are any such pauses on Zen. Also, Zen doesn't yet support AVX-512 so a big possible source of variation is moot. I don't know if any AMD chip has ever had different turbo speeds for any ISA. It should be noted that even without that, any chip can still run slower with heavier instructions because they hit some other limit: thermal, TDP, current, etc. AMD has used an interesting "adaptive clocking" scheme since steamroller, and apparently this is still in effect in Zen: https://www.realworldtech.com/steamroller-clocking/ https://www.realworldtech.com/steamroller-clocking/ This handles the same type of voltage droop worst case that Intel apparently handles by dispatch throttling. It would be interesting to test it, since the clock elongation should be visible when you measure instruction timing relative to a clock not affected by the adaptation.

celrod 7y ago
On the desktop chips (x299) it's easy to adjust all the clock speeds in the bios. If the workloads I'm most interested in are all avx512-heavy (why I bought x299 instead of threadripper), do you think there'd be a reason to set the clock speeds to be equal, regardless of ISA? That is, if I currently have 4.6/4.3/4.1 GHz no-avx/avx(2)/avx512, when might it be worth setting all three of these to 4.1 GHz? I suspect "never" is the answer? I have the impression that Zen's clocking algorithm is much smarter than Intel's heuristic approach.
- BeeOnRope 7y ago
  I don't think it makes sense, because the maximum penalty due to the tranitions is fairly low (~30 us out of 650 us, and only under pretty much a malicious load that transitions at exactly the right points), and mostly you want the higher frequencies when you can get them: they quickly overwhelm the small transition periods. Also, someone indicated to me in private correspondence that even when the frequencies are manually set so no transition takes place, the throttling periods may still take place (which makes sense since the required voltage may still be higher).

celrod 7y ago

On the desktop chips (x299) it's easy to adjust all the clock speeds in the bios. If the workloads I'm most interested in are all avx512-heavy (why I bought x299 instead of threadripper), do you think there'd be a reason to set the clock speeds to be equal, regardless of ISA? That is, if I currently have 4.6/4.3/4.1 GHz no-avx/avx(2)/avx512, when might it be worth setting all three of these to 4.1 GHz? I suspect "never" is the answer? I have the impression that Zen's clocking algorithm is much smarter than Intel's heuristic approach.

BeeOnRope 7y ago
I don't think it makes sense, because the maximum penalty due to the tranitions is fairly low (~30 us out of 650 us, and only under pretty much a malicious load that transitions at exactly the right points), and mostly you want the higher frequencies when you can get them: they quickly overwhelm the small transition periods. Also, someone indicated to me in private correspondence that even when the frequencies are manually set so no transition takes place, the throttling periods may still take place (which makes sense since the required voltage may still be higher).