On 2024-12-06 13:14:02 [+0100], Tomas Glozar wrote: > pá 6. 12. 2024 v 12:52 odesílatel Sebastian Andrzej Siewior > <bigeasy@xxxxxxxxxxxxx> napsal: > > What is the default behaviour and what is the intended behaviour? > > Couldn't we somehow avoid adding yet another option? > > The default behavior is to hold /dev/cpu_dma_latency at zero, which > disables all idle states on all CPUs, not only those on which > cyclictest measurements are running. This has the disadvantage of a > higher power consumption than needed in most cases. With > --deepest-idle-state, idle states are limited only on CPUs cyclictest > is running on. This is a real use case? /dev/cpu_dma_latency is a big hammer to disable everything that might cause latency. So you have 4 CPUs, CPU0 is getting idle from time to time and CPU1-3 is doing RT work so it can't take sleep? > There are two reasons why we can't just use the latter and have to > have options for both. Firstly, some hardware does not support > disabling idle states on individual CPUs via the cpuidle sysfs > interface, which is used by --deepest-idle-state. Secondly, latencies > measured with --deepest-idle-state might still be higher in some cases > compared to holding /dev/cpu_dma_latency. Yes. Especially if you find power management thingy that is covered by cpu_dma_latency but not by this new switch. > Tomas Sebastian
