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qLtwgCdx92QX9fBd2qmo

Member

Registered: 2015-02-01

Posts: 3

CPU barely enters idle state while on battery

Hello all,

While trying to improve battery life using powertop I noticed that my cpu showed some behaviour I didn't expect. I'm using the Intel Core 2 Duo T5500.

When on ac the cpu is roughly always in C2, 25% idle and 75% 800Mhz.
When on battery the cpu is roughly always in C3, 2% idle and 98% 800Mhz.
The amount of wakeups/s is between 30 and 90. I'm undervolting slightly with 42, 35, 27, 23.

Why is it behaving in this way? Which state is consumes less energy?

Thanks in advance.

byte

Member

From: Düsseldorf (DE)

Registered: 2006-05-01

Posts: 2,046

Re: CPU barely enters idle state while on battery

> Why is it behaving in this way?

Check your Laptop BIOS. My old (Core Duo, so no 64-bit) ThinkPad for examples offers different presets (powersave/performance) whether it's running on AC or battery.

> Which state is consumes less energy?

More wakeups = more energy (obviuosly)
Higher C-states = less energy (not so obvious)
The CPU cores of my Haswell desktop for example are in C7 for >90%.

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1000

qLtwgCdx92QX9fBd2qmo

Member

Registered: 2015-02-01

Posts: 3

Re: CPU barely enters idle state while on battery

Unfortunately there are no BIOS settings related to power.
I know that higher C-states consume less energy. I was wondering how this tradeoff between a higher C-state and less idle would work out.
When your CPU is on C7, what percentage of the time is it idle, if ever?