C-State optimization

This proposal has been rejected.


One Line Summary

Which C-State is the most optimal in terms of energy saving for a given idle time?


Within the Linux CPU idle framework, the menu governor aims at getting maximum energy saving during CPU idle time.
An important aspect is the correct and precise assessment of the system latency constraints with respect to the expected sleep time.

During idle periods, the system will choose a power state to reach, called a C-State (Inactive, Retention, OFF).
Each C-state has an energy cost during sleep and wakeup latencies that must be balanced by the reduction of energy during idle.
Which C-state is the most optimal one to be selected in terms of energy saving for a given idle period?

The aim of this study is to answer the above question by presenting an approach that permits to define the most optimal C-State during CPU idle.
Each C-state has been characterized by a set of parameters:
– Sleep & wakeup latencies: the deeper the power state is, the longer the sleep & wakeup latencies are
– Idle energy: the deeper the power state is, the lower the consumption during idle period is
– Threshold: the minimum idle time required for a given state to start saving power

Based on above parameters, each C-State is represented graphically by a curve showing the accumulated energy over time.
The energy break even point of each C-State is determined by comparing the different curves, therefore the most optimal C-State for a given idle time is deduced.

Determining the system latency constraints linked to a given HW platform in a precise way allowed achieving significant power savings in low power use cases such as MP3 playback.


power management, C-State, cpuidle


  • Vincent Bour

    Texas Instruments


    Vincent Bour works at Texas Instruments France as a system engineer since 1999. He has been working as the power management architect of the OMAP platforms and he is now focusing on the Linux power management aspect on the OMAP platforms.

  • Nicole Chalhoub

    Texas Instruments


    Nicole Chalhoub is a system engineer working on power management at Texas Instruments France. She’s focusing on the Linux power management for the OMAP platforms especially on the cpuidle governor module. She’s working on refining and optimizing this module.

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