linux-kernel - Re: [linux-pm] RFC: /sys/power/policy_preference

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Date:	Thu, 17 Jun 2010 11:14:50 -0500
From:	Victor Lowther <victor.lowther@...il.com>
To:	Len Brown <lenb@...nel.org>
Cc:	Linux Power Management List <linux-pm@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [linux-pm] RFC: /sys/power/policy_preference

On Jun 16, 2010, at 4:05 PM, Len Brown <lenb@...nel.org> wrote:

> Create /sys/power/policy_preference, giving user-space
> the ability to express its preference for kernel based
> power vs. performance decisions in a single place.
>
> This gives kernel sub-systems and drivers a central place
> to discover this system-wide policy preference.
> It also allows user-space to not have to be updated
> every time a sub-system or driver adds a new power/perf knob.

I would prefer documenting all the current knobs and adding them to pm- 
utils so that pm-powersave knows about and can manage them. Once that  
is done, creating arbitrary powersave levels should be fairly simple.

> policy_preference has 5 levels, from max_performance
> through max_powersave.  Here is how 4 parts of the kernel
> might respond to those 5 levels:
>
> max_performance (unwilling to sacrifice any performance)
>    scheduler: default (optimized for performance)
>    cpuidle: disable all C-states except polling mode
>    ondemand: disable all P-states except max perf
>    msr_ia32_energy_perf_bias: 0 of 15
>
> performance (care primarily about performance)
>    scheduler: default (optimized for performance)
>    cpuidle: enable all C-states subject to QOS
>    ondemand: all P-states, using no bias
>    msr_ia32_energy_perf_bias: 3 of 15
>
> balanced (default)
>    scheduler: enable sched_mc_power_savings
>    cpuidle: enable all C-states subject to QOS
>    ondemand: all P-states, powersave_bias=5
>    msr_ia32_energy_perf_bias: 7 of 15
>
> powersave (can sacrifice measurable performance)
>    scheduler: enable sched_smt_power_savings
>    cpuidle: enable all C-states, subject to QOS
>    ondemand: disable turbo mode, powersave_bias=10
>    msr_ia32_energy_perf_bias: 11 of 15
>
> max_powersave (can sacrifice significant performance)
>    scheduler: enable sched_smt_power_savings
>    cpuidle: enable all C-states, subject to QOS
>    ondemand: min P-state (do not invoke T-states)
>    msr_ia32_energy_perf_bias: 15 of 15
>
> Note that today Linux is typically operating in the mode
> called "performance" above, rather than "balanced",
> which is proposed to be the default.  While a system
> should work well if left in "balanced" mode, it is likely
> that some users would want to use "powersave" when on
> battery and perhaps shift to "performance" on A/C.
>
> Please let me know what you think.
>
> thanks,
> Len Brown, Intel Open Source Technology Center
> _______________________________________________
> linux-pm mailing list
> linux-pm@...ts.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/linux-pm
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