4 ms·

Reminder that nothing is a panacea: I've heard from game engine authors and cryptographers that on Intel chips _over-using_ SIMD can actually heat up the chips

Reminder that nothing is a panacea: I've heard from game engine authors and cryptographers that on Intel chips _over-using_ SIMD can actually heat up the chips too much such that it'll cause the system to then adjust the clockrate lower to cool down and you can degrade performance beyond not using SIMD at all. Before hearing that, I had never considered thermal properties of particular instructions.

saagarjha 7y ago
That’s probably AVX-512.
corysama 7y ago
It's not a problem for SSE and AVX1. But, with AVX2/AVX-512, the deal is that you should not just dip your toe with an occasional call to a small SIMD task using such heavy-hitting features. Either do enough SIMD work to overcome the down-clock, or use a lower-end SIMD functionality for smaller tasks. And, even within AVX2/512 there are huge sets of added functionality that are really "AVX1-enhanced" without going wider. Those are fine to use to without worrying about downclocking.
- calaphos 7y ago
  The AVX ALUs also go into power saving when not used and take a couple of cycles to switch back on, delaying the first AVX instruction. There is afaik even a paper on a side channel attack that uses this.
robocat 7y ago
“Intel cores can run in one of three modes: license 0 (L0) is the fastest (and is associated with the turbo frequencies written on the box), license 1 (L1) is slower and license 2 (L2) is the slowest. To get into license 2, you need sustained use of heavy 512-bit instructions, where sustained means approximately one such instruction every cycle. Similarly, if you are using 256-bit heavy instructions in a sustained manner, you will move to L1. The processor does not immediately move to a higher license when encountering heavy instructions: it will first execute these instructions with reduced performance (say 4x slower) and only when there are many of them will the processor change its frequency. Otherwise, any other 512-bit instructions will move the core to L1: the processor stops and changes its frequency as soon as an instruction is encountered.” Great info from here: https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-use-these-new-instructions/ https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-us... Be careful to benchmark real loads as there are perverse interactions e.g. “Downclocking, when it happens, is per core and for a short time after you have used particular instructions (e.g., ~2ms).“ so a function using AVX512 can affect the speed of unrelated code (similar to thermal throttling).

saagarjha 7y ago

That’s probably AVX-512.

corysama 7y ago

It's not a problem for SSE and AVX1. But, with AVX2/AVX-512, the deal is that you should not just dip your toe with an occasional call to a small SIMD task using such heavy-hitting features. Either do enough SIMD work to overcome the down-clock, or use a lower-end SIMD functionality for smaller tasks. And, even within AVX2/512 there are huge sets of added functionality that are really "AVX1-enhanced" without going wider. Those are fine to use to without worrying about downclocking.

calaphos 7y ago
The AVX ALUs also go into power saving when not used and take a couple of cycles to switch back on, delaying the first AVX instruction. There is afaik even a paper on a side channel attack that uses this.

robocat 7y ago

“Intel cores can run in one of three modes: license 0 (L0) is the fastest (and is associated with the turbo frequencies written on the box), license 1 (L1) is slower and license 2 (L2) is the slowest. To get into license 2, you need sustained use of heavy 512-bit instructions, where sustained means approximately one such instruction every cycle. Similarly, if you are using 256-bit heavy instructions in a sustained manner, you will move to L1. The processor does not immediately move to a higher license when encountering heavy instructions: it will first execute these instructions with reduced performance (say 4x slower) and only when there are many of them will the processor change its frequency. Otherwise, any other 512-bit instructions will move the core to L1: the processor stops and changes its frequency as soon as an instruction is encountered.” Great info from here: https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-use-these-new-instructions/ https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-us... Be careful to benchmark real loads as there are perverse interactions e.g. “Downclocking, when it happens, is per core and for a short time after you have used particular instructions (e.g., ~2ms).“ so a function using AVX512 can affect the speed of unrelated code (similar to thermal throttling).