4 ms·

Much ado about nothing, IMHO. AVX-512 isn't advertised by Intel as a 12th gen desktop CPU feature and doesn't provide any advantages for most desktop users any

by qayxc 5y ago

Much ado about nothing, IMHO. AVX-512 isn't advertised by Intel as a 12th gen desktop CPU feature and doesn't provide any advantages for most desktop users anyway. Sure, there's niche applications like some particular PS3 emulator, but I don't share the author's opinion w.r.t. to performance and efficiency in desktop use.

nullifidian 5y ago
It doesn't provide any advantages because there is no install base, so desktop software doesn't utilize it. It's just a wider SIMD instruction set, and I'm confused why people relegate it to HPC only category.
- CodesInChaos 5y ago
  AFAIK it downclocks the CPU, even if the AVX512 instructions are only a tiny fraction of instructions. So you only benefit from AVX512 if you use it enough. While 128 bit SIMD give you a speedup if you use it for a few functions that need it, without slowing down the rest of your code or even unrelated applications.
  - nullifidian 5y ago
    In heavily vectorized applications you still get a sizable speed up over AVX2, e.g. in tasks like video encoding. In mixed/real-time applications this issue could be solved by dedicating/pinning a thread with the AVX-512 payload to a core, automatically or manually. Not sure if such policies have been implemented in practice.
  - dragontamer 5y ago
    There's significant downclocking only on on Skylake-X (aka: 1st generation teething problems). On more recent cores, there's only ~100 MHz of downclocking, which is so small you can pretty much ignore it.
    - jeffbee 5y ago
      It's amazing how sticky the myth has become, though. On Ice Lake Xeon there is basically no penalty for using the AVX-512 unit.
      - dragontamer 5y ago
        It was true for several years, because Intel got "stuck" on Skylake due to issues in their manufacturing. If Intel was able to get Icelake out on time, it may have reduced the "stickiness" of the myth.
        jeffbee 5y ago
        True. But the power penalty from 512 was already quite reduced in Cannon Lake (2018) and completely gone in Tiger Lake (2020).
josephcsible 5y ago
I wouldn't say that Intel taking away a working, if niche, feature from their hardware retroactively after users bought it is "nothing".
- qayxc 5y ago
  It worked for one SKU at launch and by disabling up to 50% of the cores of the others. Calling that a working feature is a bit of a stretch already.
  - dr_zoidberg 5y ago
    That was before Ice Lake, where it had a very decent performance (just downclocking 10%) and then on Rocket Lake it didn't even downclock [0]. But the thing that stuck in our collective mind was that "it's bad because it downclocks". And seeing the RKL numbers, which are new to me (I had only read de Ice Lake numbers back in 2020) it wasn't even 14nm's fault, just an implementation issue which Intel eventually worked out. I don't know if it was super good (Linus Torvalds certainly was loud against it, and from his POV, his critic makes lots of sense), but I run some numeric code from time to time that _in theory_ could benefit from it. So it's a bit of shame that it won't be supported in future products? Let's see what AMD does about it on Zen 4 and if they manage to force Intel back to it, if only for competitions sake. [0] https://travisdowns.github.io/blog/2020/08/19/icl-avx512-freq.html https://travisdowns.github.io/blog/2020/08/19/icl-avx512-fre...
    - qayxc 5y ago
      > That was before Ice Lake My comment was about Alder Lake, where only P-cores even have AVX-512 units. The i9 has 8 P-cores and 8 E-cores and users need to disable the 8 E-cores to use AVX-512. That's 50% of the cores gone for very niche use cases. Considering that the PS3 emulator only got ~10% better performance on the highest end, it'd be better to simply get an 11th gen high-end part instead of a lower end 12th gen CPU for that use case anyway, so I really don't understand what the fuss is about.
      - 5y ago

by qayxc 5y ago

nullifidian 5y ago

It doesn't provide any advantages because there is no install base, so desktop software doesn't utilize it. It's just a wider SIMD instruction set, and I'm confused why people relegate it to HPC only category.

CodesInChaos 5y ago
AFAIK it downclocks the CPU, even if the AVX512 instructions are only a tiny fraction of instructions. So you only benefit from AVX512 if you use it enough. While 128 bit SIMD give you a speedup if you use it for a few functions that need it, without slowing down the rest of your code or even unrelated applications.
- nullifidian 5y ago
  In heavily vectorized applications you still get a sizable speed up over AVX2, e.g. in tasks like video encoding. In mixed/real-time applications this issue could be solved by dedicating/pinning a thread with the AVX-512 payload to a core, automatically or manually. Not sure if such policies have been implemented in practice.
- dragontamer 5y ago
  There's significant downclocking only on on Skylake-X (aka: 1st generation teething problems). On more recent cores, there's only ~100 MHz of downclocking, which is so small you can pretty much ignore it.
  - jeffbee 5y ago
    It's amazing how sticky the myth has become, though. On Ice Lake Xeon there is basically no penalty for using the AVX-512 unit.
    - dragontamer 5y ago
      It was true for several years, because Intel got "stuck" on Skylake due to issues in their manufacturing. If Intel was able to get Icelake out on time, it may have reduced the "stickiness" of the myth.
      - jeffbee 5y ago
        True. But the power penalty from 512 was already quite reduced in Cannon Lake (2018) and completely gone in Tiger Lake (2020).

josephcsible 5y ago

I wouldn't say that Intel taking away a working, if niche, feature from their hardware retroactively after users bought it is "nothing".

qayxc 5y ago
It worked for one SKU at launch and by disabling up to 50% of the cores of the others. Calling that a working feature is a bit of a stretch already.
- dr_zoidberg 5y ago
  That was before Ice Lake, where it had a very decent performance (just downclocking 10%) and then on Rocket Lake it didn't even downclock [0]. But the thing that stuck in our collective mind was that "it's bad because it downclocks". And seeing the RKL numbers, which are new to me (I had only read de Ice Lake numbers back in 2020) it wasn't even 14nm's fault, just an implementation issue which Intel eventually worked out. I don't know if it was super good (Linus Torvalds certainly was loud against it, and from his POV, his critic makes lots of sense), but I run some numeric code from time to time that _in theory_ could benefit from it. So it's a bit of shame that it won't be supported in future products? Let's see what AMD does about it on Zen 4 and if they manage to force Intel back to it, if only for competitions sake. [0] https://travisdowns.github.io/blog/2020/08/19/icl-avx512-freq.html https://travisdowns.github.io/blog/2020/08/19/icl-avx512-fre...
  - qayxc 5y ago
    > That was before Ice Lake My comment was about Alder Lake, where only P-cores even have AVX-512 units. The i9 has 8 P-cores and 8 E-cores and users need to disable the 8 E-cores to use AVX-512. That's 50% of the cores gone for very niche use cases. Considering that the PS3 emulator only got ~10% better performance on the highest end, it'd be better to simply get an 11th gen high-end part instead of a lower end 12th gen CPU for that use case anyway, so I really don't understand what the fuss is about.
    - 5y ago