On Mon, 28 Jul 2014 01:07:52 +0100 James Stone <jamesmstone@xxxxxxxxx> wrote: > I've just been playing around with this, and can confirm that a > hand-built -rt kernel has lower max sched latencies than a generic > lowlatency kernel in ubuntu on my system (<100us compared to 1500+). > However, I noticed a really weird thing - that when running the test > using sudo cyclictest -m -n -Sp99 -i100 -d0, the reported DSP load on > qjackctl is reduced (by around 50%), and there are fewer xruns at low > latencies. As soon as I stop cyclictest, the xruns (at a rather low > jack frames/period of 32) come back.. Any idea why this should be??? I think this is due to running cyclictest as root, as it then opens /dev/cpu_dma_latency and writes 0 into it. This has the effect of disabling cpu powersaving. The cpu will no longer use P or C states and runs full out. The result is that it finishes processing the audio threads faster, thus lowering the DSP load shown in qjackctl (which is a measure of how much of the available time until the deadline did the audio processing take). The documentation is in Documentation/power/pm_qos_interface.txt Note that it's not enough to just write a value to it, the file needs to be kept open too, once it's closed the cpu goes back to it's normal powersaving. On my i7 desktop I take another approach. I echo 100 to /sys/devices/system/cpu/intel_pstate/min_perf_pct which makes it run full out, but still go into C1 powersaving. This keeps the temperature down. If you have a recent Intel cpu, you can observe the change with the i7z utility. I don't know how this works on AMD cpus though. -- Joakim _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
