Re: KS/Plumbers: c-state governor BOF

[Rik van Riel]

On 08/24/2012 01:01 PM, Rik van Riel wrote:

> Since a good number of us will be in San Diego next week, at
> Kernel Summit / Plumbers / etc, I will organize a c-state
> governor BOF for those who are interested.

Here are the note from today's c-state governor BOF session:

- idle time estimator can be improved to return more
  conservative values
- have a trace point to verify how well idle time estimation works,
  returning expected idle time for the current algorithm, expected
  idle time for a test algorithm, and actually measured idle time
- we can objectively measure whether one idle time estimation
  algorithm is better than another
- example: last 8 idle times are 180 200 30 210 1000 150 10000 190
  - average idle time around 1500, but not typical
  - typical idle time just under 200
  - almost all idle time spent in one interval
- scheduler and cpufreq people also want idle time
  statistics, idea is to move the idle time statistics
  to the scheduler
- having idle time statistics in the scheduler allows
  the stats to be migrated when the scheduler migrates
  a task
- interface: expected idle time, demotion time
  - if we sleep much more than the expected idle time, put
    CPU into a deeper c-state
  - on x86 timed mwait could be used
  - generic timer code would be usable too
  - take into account both the idle statistics and power break-even




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