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Intel i5 idles at 500mhz on AC and 800mhz on battery

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gi1242
Posts: 17
Joined: 2009-10-30 01:33

Intel i5 idles at 500mhz on AC and 800mhz on battery

#1 Post by gi1242 »

Hi All,

I have an X1 Yoga with an Intel Core i5-6200U CPU @ 2.30GHz, running Debian/testing on a 4.6.0-1-amd64 kernel using the intel_pstate driver.
I notice it idles at 500Mhz when plugged in, and 800Mhz on battery. (No typo! You read that right.)

I stopped laptop-mode-tools from adjusting CPU frequency scaling or the intel_pstate settings (via CONTROL_CPU_FREQUENCY=0 and CONTROL_INTEL_PSTATE="0"). So on both AC and on battery I have:

Code: Select all

$ head /sys/devices/system/cpu/intel_pstate/*
==> /sys/devices/system/cpu/intel_pstate/max_perf_pct <==
100

==> /sys/devices/system/cpu/intel_pstate/min_perf_pct <==
14

==> /sys/devices/system/cpu/intel_pstate/no_turbo <==
0

==> /sys/devices/system/cpu/intel_pstate/num_pstates <==
25

==> /sys/devices/system/cpu/intel_pstate/turbo_pct <==
85
In particular, no_turbo is 0 on both AC and on battery. But the CPU idle frequency in both cases is different. And backwards if you ask me... I would have guessed that idling at 500MHz consumes less battery than idling at 800MHz. (Of course, under load it jumps up to a higher frequency on both AC and battery, so I have no performance issues; I just wonder if I'm wasting more battery than I should because I mis-configured it...)

Does anyone have any insight?

Thanks in advance.

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bw123
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Re: Intel i5 idles at 500mhz on AC and 800mhz on battery

#2 Post by bw123 »

gi1242 wrote: I would have guessed that idling at 500MHz consumes less battery than idling at 800MHz. (Of course, under load it jumps up to a higher frequency on both AC and battery, so I have no performance issues; I just wonder if I'm wasting more battery than I should because I mis-configured it...)

Does anyone have any insight?

Thanks in advance.
I don't know for sure, but could it be that there are other reasons to idle at the higher speed on battery? Like stability? I wouldn't think that difference is going to mean an awful lot. I'm not sure how you configured it, but that looks like a reasonable setup to me.

Sounds like a very nice machine, I've seen a couple of those.
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gi1242
Posts: 17
Joined: 2009-10-30 01:33

Re: Intel i5 idles at 500mhz on AC and 800mhz on battery

#3 Post by gi1242 »

bw123 wrote:I don't know for sure, but could it be that there are other reasons to idle at the higher speed on battery? Like stability?
Really? I'm very curious about this. I know that it isn't always optimal to keep your CPU on the lowest frequency on battery, since there's a trade off: Use a higher powered state, but for shorter time and idle longer; or use the low powered state for longer time and idle less.

I assumed high frequency translates to high power...
bw123 wrote:I wouldn't think that difference is going to mean an awful lot. I'm not sure how you configured it, but that looks like a reasonable setup to me.
You're certainly right about this. I get some 12+ hours on battery in my use cases. (Drops down if I do something CPU intensive, but given that I use fvwm and vim most of the time, the CPU stays at 800mhz most of the time...)

Thanks!

GI

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Re: Intel i5 idles at 500mhz on AC and 800mhz on battery

#4 Post by stevepusser »

Have you tried fiddling around with tlp instead of laptop-tools? It's in the testing repo. It's not in jessie-backports, but the Stretch package (just a bunch of scripts) turns out to be safe to install in Jessie, or easy to backport yourself.
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