3 ms·

From what I've read on AVX-512, the big disadvantage is that the AVX-512 instructions are very CPU intensive so the max clock speeds is reduced if you heavily u

From what I've read on AVX-512, the big disadvantage is that the AVX-512 instructions are very CPU intensive so the max clock speeds is reduced if you heavily use AVX-512 instructions. Another disadvantage is that you have to recompile your code to use AVX512 but it seems general enough that compilers will use the instructions (to an extent) without specialized code [1]. [1] https://www.phoronix.com/scan.php?page=news_item&px=GCC-8-AVX-512-Benchmarks https://www.phoronix.com/scan.php?page=news_item&px=GCC-8-AV...

robocat 7y ago
The problem appears to be that whether you get a performance increase or a performance penalty depends on the duty cycle of your AVX512 instructions. What is especially deceiving is profiling a function in a loop for more than say 50ms, when the normal function execution takes say 0.5ms. Long running functions get the most gain, while short running functions cause the most pain. That is because downclocking AVX512 lasts 2ms (with a 0.5ms setup). Certain instruction mixes will cause a general slowdown (10% degradation measured by CloudFlare under actual usage) even though the test profiling might predicts a performance gain. Single AVX512 instructions when the CPU is running at full speed have a counterintuitive perverse performance penalty - apparently running 4x slower than when the CPU changes to the slower L1 or L2 clocks. Sustained AVX512 usage has predictable performance. “ Intel made more aggressive use of AVX-512 instructions in earlier versions of the icc compiler, but has since removed most use unless the user asks for it with a special command line option.” is a strong indication that you need to be very careful about where you use the instructions. Running an encoder for 1 second - likely candidate. Occasional 1ms functions or single AVX512 instructions on a web server - likely penalty.

robocat 7y ago

The problem appears to be that whether you get a performance increase or a performance penalty depends on the duty cycle of your AVX512 instructions. What is especially deceiving is profiling a function in a loop for more than say 50ms, when the normal function execution takes say 0.5ms. Long running functions get the most gain, while short running functions cause the most pain. That is because downclocking AVX512 lasts 2ms (with a 0.5ms setup). Certain instruction mixes will cause a general slowdown (10% degradation measured by CloudFlare under actual usage) even though the test profiling might predicts a performance gain. Single AVX512 instructions when the CPU is running at full speed have a counterintuitive perverse performance penalty - apparently running 4x slower than when the CPU changes to the slower L1 or L2 clocks. Sustained AVX512 usage has predictable performance. “ Intel made more aggressive use of AVX-512 instructions in earlier versions of the icc compiler, but has since removed most use unless the user asks for it with a special command line option.” is a strong indication that you need to be very careful about where you use the instructions. Running an encoder for 1 second - likely candidate. Occasional 1ms functions or single AVX512 instructions on a web server - likely penalty.