[LAU] CPUs for low latency

Just looking through some of the results on: https://www.osadl.org/Hardware-overview.qa-farm-hardware.0.html Some of the higher speed many core CPUs have the worst latency: https://www.osadl.org/Latency-plot-of-system-in-rack-1-slot.qa-latencyplot-… https://www.osadl.org/Latency-plot-of-system-in-rack-0-slot.qa-latencyplot-… (These will cause xruns at 16/2 at 48000 sample/sec) Some slow cpus do much better: https://www.osadl.org/Latency-plot-of-system-in-rack-3-slot.qa-latencyplot-… https://www.osadl.org/Latency-plot-of-system-in-rack-1-slot.qa-latencyplot-… Two very similar CPUs can vary: https://www.osadl.org/Latency-plot-of-system-in-rack-c-slot.qa-latencyplot-… https://www.osadl.org/Latency-plot-of-system-in-rack-0-slot.qa-latencyplot-… (the second is a faster but older one) Hyperthreading does not effect latency?: https://www.osadl.org/Latency-plot-of-system-in-rack-0-slot.qa-latencyplot-… (at least this plot seems to say that) The state of HT and what effects how much it hits latency has changed. In order for HT to be worth while, any one thread needs to keep the core for a certain number of instructions. As the CPU speed goes up the same number of instructions gets done faster. I also think that the overhead for context switching has gone down. With my old P4 (single core) HT restricted my AI to 64/2 with the ocasional xrun, HT off would allow solid 16/2 performance. My new i5 does not have HT, so I am not able to compare. AMD seems to have something similar enough to hyperthreading to be wary of. A number of the AMD CPUs shows twice as many threads as cores. I know that they do double caching, I don't know if this is what the Linux kernel is seeing as shadow cpus or not. Looking through some of the AMD documentation (which is very sparse BTW) It appears to say that some of the boost and processor speed changes are not controllable by bios. On any of the Intel CPUs, both boost and HT can be turned off by any reasonable bios. With AMD I am sure that the kernel at least can be told to ignore shadow CPUs. I would like to know more about speed scaling and how much influence the the OS has on it. Another thing with CPUs that seems to becoming common is powering off parts of the CPU that are idle. The poweron time is very short, but context change time would still be there. There have been some experiments that show just keeping the cpu busy with a do-nothing-script can give lower latency times. Anyway, the latest, greatest MB and CPU is not the best bet for low latency audio. Advertised "performance" is based on throughput. Any of the benchmarks I have seen on the manufacture's sites are about throughput. Latency is not mentioned or measured. The few times it has been mentioned has been in regard to video where it is called "low latency" which probably in the audio world is noticable delay. (in most computer engineer's minds low latency is 30ms) The idea of what latency is good enough for audio work has changed with the change from mostly PCI cards to USB. Where 64/2 was the normal maximum, 128/2 or even 256/2 have become common... and seem to be accepted as "OK". (there are also USB cards that will run at 32/2 or better) Maybe there are other reasons for the change in thought on this too. Note: this was a very lite look, not in depth at all. The profiles do not match the tests always. I see that in the profiles the governor is set to OnDemand, but the test plots all say "lowest P state: performance". -- Len Ovens www.ovenwerks.net