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

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Date:	Fri, 11 Jul 2014 22:37:01 +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 11/07/2014 09:34 μμ, Pavel Machek wrote:
> On Fri 2014-07-11 20:29:57, Stratos Karafotis wrote:
>> 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.
> 
> 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).

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.
This is the reason that the intel_pstate driver don't use 'performance'
and try to request a low P state when there is no load.

> (Can you try to modify your code and rerun for example the apache
> test?)

Yes, I can do the apache test if the above example is not enough.

>>> 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".
> 
> I believe that any increase of frequency in frequency will make the
> benchmarks you qouted better (on i7). Actually, you can probably just
> select performance governor...?

Maybe in benchmarks where the CPU load is high. But definitely not, in mp3
decoding and idle system test.

The point is, as you mentioned, more tests and of course on other CPUs.
Unfortunately, I can test only on i7 and krait as mentioned in changelog.
I will happily run any test you would like for more info.


Thanks,
Stratos
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