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

[Dipankar Sarma <dipankar@in.ibm.com>]

On Wed, Jun 16, 2010 at 05:05:26PM -0400, Len Brown 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.
> 
> 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:

In theory this makes sense. We have been toying with something
like this, but the difficulty is that outside of benchmarking
environment, it is hard to figure out what mode to set when.
Also, the impact could be different for different workloads.
We should probably have a broader discussion around this
with data - I will share some measurements on impact
of such power modes.


> 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

Would there be sufficient difference between performance
and balanced ?


> 
> 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


Thanks
Dipankar