linux-kernel - Re: [PATCH 2/2] cpufreq: ondemand: Eliminate the deadband effect

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Message-ID: <53C01F15.2090702@semaphore.gr>
Date:	Fri, 11 Jul 2014 20:29:57 +0300
From:	Stratos Karafotis <stratosk@...aphore.gr>
To:	Pavel Machek <pavel@....cz>
CC:	rjw@...ysocki.net, viresh.kumar@...aro.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpufreq: ondemand: Eliminate the deadband effect

Hi Pavel!

On 11/07/2014 07:57 μμ, Pavel Machek wrote:
> Hi!
> 
>> Tested on Intel i7-3770 CPU @ 3.40GHz and on ARM quad core 1500MHz Krait
>> (Android smartphone).
>> Benchmarks on Intel i7 shows a performance improvement on low and medium
>> work loads with lower power consumption. Specifics:
>>
>> Phoronix Linux Kernel Compilation 3.1:
>> Time: -0.40%, energy: -0.07%
>> Phoronix Apache:
>> Time: -4.98%, energy: -2.35%
>> Phoronix FFMPEG:
>> Time: -6.29%, energy: -4.02%
> 
> Hmm. Intel i7 should be race-to-idle machine. So basically rule like
> if (load > 0) go to max frequency else go to lowest frequency would do
> the right thing in your test, right?

I don't think that "if (load > 0) go to max" will work even on i7.
For low load this will have impact on energy consumption.
On my tests, a simple mp3 decoding (very low load on my machine) have no
difference with and without this patch.

> So... should we do that, or do we need better benchmark?

I'm sorry. I'm not sure I understood exactly what do you mean by "better
benchmark".
Of course, we should do as many benchmarks as we can.
I usually do these 5 sets of benchmarks on my i7 that IMHO give a good
indication about the changes in different CPU loads.

1) Linux kernel compilation (about 85% busy CPU)
2) Apache (about 32% busy CPU)
3) ffmpeg (about 24% busy CPU)
4) mp3 decoding (about 0.3% CPU)
5) Idle system (about 0.06% CPU)

The patch was also tested on a Android smartphone (kernel 3.4). The kernel
distributed to 1000+ users.
Unfortunately I have no benchmarks, but no regressions reported on
consumption. Actually, there reports for better performance and
lower power consumption, but of course we can't rely on these reports. :)

Thanks for your comments!

Stratos

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