Re:AMD Threadripper 3970X under heavy AVX2 load: Defective b...
by paulmd on Friday February 21, 2020 @01:59PM
(#22385724)
There is no explicit "warmup" instruction but you can perform a dummy AVX instruction wherever you want to start warming it up.
> I observed an interesting phenomenon when executing 256-bit vector instructions on the Skylake. There is a warm-up period of approximately 14 µs before it can execute 256-bit vector instructions at full speed. Apparently, the upper 128-bit half of the execution units and data buses is turned off in order to save power when it is not used. As soon as the processor sees a 256-bit instruction it starts to power up the upper half. It can still execute 256-bit instructions during the warm-up period, but it does so by using the lower 128-bit units twice for every 256-bit vector. The result is that the throughput for 256-bit vectors is 4-5 times slower during this warm-up period. If you know in advance that you will need to use 256-bit instructions soon, then you can start the warm-up process by placing a dummy 256-bit instruction at a strategic place in the code. My measurements showed that the upper half of the units is shut down again after 675 µs of inactivity.
Agner also notes that previous generations would just flatly stall all instructions (including non-vector) once they saw 256-bit instructions, which is probably why there is no "warmup" instruction. If there is an explicit stall of all instructions then there are very few reasons to incur that until the last possible second.
> I observed an interesting phenomenon when executing 256-bit vector instructions on the Skylake. There is a warm-up period of approximately 14 µs before it can execute 256-bit vector instructions at full speed. Apparently, the upper 128-bit half of the execution units and data buses is turned off in order to save power when it is not used. As soon as the processor sees a 256-bit instruction it starts to power up the upper half. It can still execute 256-bit instructions during the warm-up period, but it does so by using the lower 128-bit units twice for every 256-bit vector. The result is that the throughput for 256-bit vectors is 4-5 times slower during this warm-up period. If you know in advance that you will need to use 256-bit instructions soon, then you can start the warm-up process by placing a dummy 256-bit instruction at a strategic place in the code. My measurements showed that the upper half of the units is shut down again after 675 µs of inactivity.
https://www.agner.org/optimize/blog/read.php?i=628
Agner also notes that previous generations would just flatly stall all instructions (including non-vector) once they saw 256-bit instructions, which is probably why there is no "warmup" instruction. If there is an explicit stall of all instructions then there are very few reasons to incur that until the last possible second.