2 ms·

There is more recent work. This blog post by Travis Downs is the most detailed analysis of transition behavior I've seen: https://travisdowns.github.io/blog/202

There is more recent work. This blog post by Travis Downs is the most detailed analysis of transition behavior I've seen: https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html For general guidelines on when to use AVX-512, this (older post) remains best guide I've seen: 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...

google234123 6y ago
So, are programs that are compiled with those instructions faster or slower? In my experience they have been faster.
- klodolph 6y ago
  Short answer: Yes, faster. Long answer: It depends, and you may be measuring the wrong thing. Among other things, it depends on the workload and the exact processor. You can find plenty of cases where AVX512 makes things faster. You can also find cases where the entire system slows down because it is running sections of AVX512 code here and there—apparently, for certain Intel processors, the processor needs to turn on top 256 bits of the register files and interconnects, and to get full speed for AVX512 it will alter the processor’s voltage and clock speed. This reduces the speed of other instructions and even other cores on the same die (which may be surprising). While the specifics may be new, the generalities seem familiar—it has long been true that a well-intentioned improvement to a small section of your code base can improve performance locally while degrading overall system performance. The code that you’re working on occupies a smaller and smaller slice of your performance metrics, and meanwhile, the whole system is slowing down. There are so many reasons that this can happen, dynamic frequency scaling with AVX512 is just one more reason.
  - jfkebwjsbx 6y ago
    > apparently, for certain Intel processors Not certain Intel processors: all of them. The CPU will reduce its clocks, reducing its overall performance. Using AVX256 or AVX512 is easily a net negative on performance, depending on the software, input data and other processes running in the system. Unless you are certain you have enough data to offset the downclock, don't use them.

google234123 6y ago

So, are programs that are compiled with those instructions faster or slower? In my experience they have been faster.

klodolph 6y ago
Short answer: Yes, faster. Long answer: It depends, and you may be measuring the wrong thing. Among other things, it depends on the workload and the exact processor. You can find plenty of cases where AVX512 makes things faster. You can also find cases where the entire system slows down because it is running sections of AVX512 code here and there—apparently, for certain Intel processors, the processor needs to turn on top 256 bits of the register files and interconnects, and to get full speed for AVX512 it will alter the processor’s voltage and clock speed. This reduces the speed of other instructions and even other cores on the same die (which may be surprising). While the specifics may be new, the generalities seem familiar—it has long been true that a well-intentioned improvement to a small section of your code base can improve performance locally while degrading overall system performance. The code that you’re working on occupies a smaller and smaller slice of your performance metrics, and meanwhile, the whole system is slowing down. There are so many reasons that this can happen, dynamic frequency scaling with AVX512 is just one more reason.
- jfkebwjsbx 6y ago
  > apparently, for certain Intel processors Not certain Intel processors: all of them. The CPU will reduce its clocks, reducing its overall performance. Using AVX256 or AVX512 is easily a net negative on performance, depending on the software, input data and other processes running in the system. Unless you are certain you have enough data to offset the downclock, don't use them.