Re: scheduling latency & CONFIG_HZ

[Tim Blechmann <tim@klingt.org>]

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>> i am curious, do the values of CONFIG_HZ and CONFIG_NO_HZ somehow affect the
>> scheduling latency (as reported by cyclictest) of a real-time/vanilla kernel,
>> especially when CONFIG_HIGH_RES_TIMERS is enabled?
> 
> CONFIG_HZ should be pretty irrelevant, but CONFIG_NO_HZ=y especially
> when the CPU does support deeper C-States and the ACPI cpuidle support
> is active may influence it simply because the wake up from deeper
> c-states can take significantly longer than waking up from C1 (the
> default halt state). Note that the wakeup latency from C-States
> depends on the CPU model, chipset, available timer hardware ...

interesting ... from my understanding, the wakeup latency is known to
the kernel (/sys/devices/system/cpu/cpuX/cpuidle/stateY/latency) and is
taken into account when governor selects the next state ... is it
possible to tweak the governor by setting the latency requirement
(pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY)) from userspace?

thanks, tim

-- 
tim@klingt.org
http://tim.klingt.org

Lesser artists borrow, great artists steal.
  Igor Stravinsky


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