3 ms·

Thanks for the link, I can only find "when the processor detect AVX instruction additional voltage is applied, the processor can run hotter which can require th

by acqq 10y ago

Thanks for the link, I can only find "when the processor detect AVX instruction additional voltage is applied, the processor can run hotter which can require the frequency to be reduced" but I don't see anywhere mentioned that the base frequency is 200 MHz. If you mean 200 MHz lower than TDP marked frequency, but processing twice as much data, it doesn't sound so bad, it's still 1.7 times more power efficient than the shorter instructions spending twice as much time at the marked TDP frequency. And I'd be surprised that AES is magically not needing serious processing too. Otherwise it would be already implemented to be much faster than it is now.

honkhonkpants 10y ago
It really depends on your instruction mix. If only one in twenty instruction uses AVX, the rest of your instructions are running slower due to the lower clock and they aren't getting double the throughput. On top of that it could be some other thread using AVX, clocking down the entire core and harming the given thread that isn't using AVX. Intel has done a lot of things to try to balance this. One of those things is they don't even bother turning half the vector unit on unless you use it a lot. If you seldom issue an op with 512-bit operands, the CPU will actually dispatch them as multiple 256-bit operations, in which case you won't incur the drop in clock, but you also don't get the supposed benefit of double throughput. Furthermore the performance may be much worse if the CPU decides to turn up the remaining vector bits, because the clock drops dramatically while those units are charging up. So you can see that for someone trying to wring out every last bit of performance on a recent Intel CPU using all the advertised vector capabilities, optimization can become quite complicated.

3 ms·

Thanks for the link, I can only find "when the processor detect AVX instruction additional voltage is applied, the processor can run hotter which can require th

by acqq 10y ago

honkhonkpants 10y ago
It really depends on your instruction mix. If only one in twenty instruction uses AVX, the rest of your instructions are running slower due to the lower clock and they aren't getting double the throughput. On top of that it could be some other thread using AVX, clocking down the entire core and harming the given thread that isn't using AVX. Intel has done a lot of things to try to balance this. One of those things is they don't even bother turning half the vector unit on unless you use it a lot. If you seldom issue an op with 512-bit operands, the CPU will actually dispatch them as multiple 256-bit operations, in which case you won't incur the drop in clock, but you also don't get the supposed benefit of double throughput. Furthermore the performance may be much worse if the CPU decides to turn up the remaining vector bits, because the clock drops dramatically while those units are charging up. So you can see that for someone trying to wring out every last bit of performance on a recent Intel CPU using all the advertised vector capabilities, optimization can become quite complicated.