haswell i7 cpu frequency

adax · 2014-07-25 23:13:50

Hi,

I have an i7-4700MQ laptop with a cpu frequency range of 800mhz to 2.4ghz (3.4ghz with turbo).
When using kernel 3.12 it would stay at 2.4ghz, but after the update to 3.14 it would clock down to 800mhz.

Now since the release of 3.15 it seems that it will never go down to 800mhz again, even if left idle for a long time.
Is this a known issue? is there something i need to do?
It does fluctuate down to about 1.3ghz for very short periods but i havent seen it go lower.
i can force it down to lower frequencies by changing
/sys/devices/system/cpu/intel_pstate/max_perf_pct
but that is a bit of a hack and not exactly dynamic.

any advice would be welcome.
Thanks.

Gusar · 2014-07-26 09:52:50

This comes up a lot, so by now a forum search should easily find it.

Anyway... https://bbs.archlinux.org/viewtopic.php … 3#p1433513

adax · 2014-08-05 09:39:20

Thanks for the reply and links to info. I read the links and
I still don't understand how the same machine running windows will scale down to 800 but in linux does not.
The implication is that either linux is using more power (higher freq) or that windows is scaling down but with no benefit to the system.

graysky · 2014-08-05 10:10:21

It's the later as I understand it. Pstates is a modern implementation of the governor based on new hardware. If you want the older inefficient one, manually force it if the module still exists. I think it was called power-save or powersave.

EDIT: https://bugzilla.kernel.org/show_bug.cgi?id=93521#c10

Last edited by graysky (2015-03-14 11:47:30)

Gusar · 2014-08-05 12:22:52

As graysky said, it's the latter. Quoting myself from the thread I linked to:

Ondemand was doing unnecessary work, it's not needed to scale the freq all the time, because it's the C-states that power saving comes from. So the "my CPU is always at max freq" thing is indeed not an issue. The CPU is *not* at the max freq all the time, it's only at the shown freq in the C0 state. And on a working system, the C0 state will be used less than 1% of the time.

If you want to do some tests, add intel_pstate=disable to the kernel commandline, so that the acpi_cpufreq governor will be used instead of intel_pstate. Then use turbostat (it's in AUR) to compare how the CPU behaves with each of them.

Arch Linux

#1 2014-07-25 23:13:50

haswell i7 cpu frequency

#2 2014-07-26 09:52:50

Re: haswell i7 cpu frequency

#3 2014-08-05 09:39:20

Re: haswell i7 cpu frequency

#4 2014-08-05 10:10:21

Re: haswell i7 cpu frequency

#5 2014-08-05 12:22:52

Re: haswell i7 cpu frequency

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