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Date:	Mon, 21 Jul 2014 08:41:58 +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

On 21/07/2014 12:51 πμ, 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.
>>>
>>> Are you sure? CPU frequency should not matter on idle CPU.
>>
>> Even on a totally idle CPU there will be a small impact because of leakage
>> current (thanks to Dirk Brandewie for this info).
> 
> Are you sure? IIRC Intel cpus will automatically lower CPU frequency
> in deep C states..

I'm sorry. I don't know further details about the leakage current
in deeper C states.

>> This simple test on a nearly idle system shows this:
>>
>> [root@...ert cpufreq]# for CPUFREQ in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do [ -f $CPUFREQ ] || continue; echo -n performance > $CPUFREQ; done
>> [root@...ert cpufreq]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat -J sleep 20
>>     Core     CPU Avg_MHz   %Busy Bzy_MHz TSC_MHz     SMI  CPU%c1  CPU%c3  CPU%c6  CPU%c7 CoreTmp  PkgTmp Pkg%pc2 Pkg%pc3 Pkg%pc6 Pkg%pc7   Pkg_J   Cor_J   GFX_J   time
>>        -       -       2    0.06    2712    3392       0    0.30    0.00   99.63    0.00      34      34    8.09    0.00   81.94    0.00  380.41   14.51    1.64   20.00
>>        0       0       0    0.02    1891    3392       0    0.09    0.00   99.88    0.00      34      34    8.09    0.00   81.94    0.00  380.41   14.51    1.64   20.00
>>        0       4       1    0.04    3006    3392       0    0.07
>>        1       1       1    0.04    2501    3392       0    0.62    0.00   99.33    0.00      34
>>        1       5       0    0.01    2346    3392       0    0.66
>>        2       2       0    0.01    1996    3392       0    0.44    0.00   99.55    0.00      34
>>        2       6       4    0.18    2278    3392       0    0.26
>>        3       3       5    0.15    3449    3392       0    0.07    0.01   99.77    0.00      34
>>        3       7       0    0.01    1839    3392       0    0.21
>> 20.000899 sec
>> [root@...ert cpufreq]# ^C
>> [root@...ert cpufreq]# for CPUFREQ in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do [ -f $CPUFREQ ] || continue; echo -n ondemand > $CPUFREQ; done
>> [root@...ert cpufreq]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat -J sleep 20
>>     Core     CPU Avg_MHz   %Busy Bzy_MHz TSC_MHz     SMI  CPU%c1  CPU%c3  CPU%c6  CPU%c7 CoreTmp  PkgTmp Pkg%pc2 Pkg%pc3 Pkg%pc6 Pkg%pc7   Pkg_J   Cor_J   GFX_J   time
>>        -       -       2    0.09    1693    3392       0    0.35    0.01   99.55    0.00      35      36    8.33    0.00   84.31    0.00  377.68   12.23    1.15   20.00
>>        0       0       1    0.08    1603    3392       0    0.13    0.00   99.79    0.00      35      36    8.33    0.00   84.31    0.00  377.68   12.23    1.15   20.00
>>        0       4       1    0.08    1646    3392       0    0.13
>>        1       1       1    0.06    1647    3392       0    0.66    0.00   99.28    0.00      35
>>        1       5       0    0.01    1611    3392       0    0.71
>>        2       2       0    0.02    1617    3392       0    0.50    0.02   99.46    0.00      35
>>        2       6       4    0.22    1764    3392       0    0.30
>>        3       3       4    0.25    1701    3392       0    0.07    0.00   99.68    0.00      35
>>        3       7       0    0.01    1602    3392       0    0.31
>> 20.001580 sec
>>
>>
>> So, for low loads the impact will be higher.
> 
> So it seems ondemand saves cca 1% of energy?

Yes, in this small test, on my nearly "idle" system.


Stratos
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