Re: [RFC PATCH] cpufreq: intel_pstate: Change the calculation of next pstate
From: Stratos Karafotis
Date: Fri May 09 2014 - 10:57:01 EST
Hi Dirk,
On 08/05/2014 11:52 ÎÎ, Dirk Brandewie wrote:
> On 05/05/2014 04:57 PM, Stratos Karafotis wrote:
>> Currently the driver calculates the next pstate proportional to
>> core_busy factor, scaled by the ratio max_pstate / current_pstate.
>>
>> Using the scaled load (core_busy) to calculate the next pstate
>> is not always correct, because there are cases that the load is
>> independent from current pstate. For example, a tight 'for' loop
>> through many sampling intervals will cause a load of 100% in
>> every pstate.
>>
>> So, change the above method and calculate the next pstate with
>> the assumption that the next pstate should not depend on the
>> current pstate. The next pstate should only be proportional
>> to measured load. Use the linear function to calculate the load:
>>
>> Next P-state = A + B * load
>>
>> where A = min_state and B = (max_pstate - min_pstate) / 100
>> If turbo is enabled the B = (turbo_pstate - min_pstate) / 100
>> The load is calculated using the kernel time functions.
>>
Thank you very much for your comments and for your time to test my patch!
>
> This will hurt your power numbers under "normal" conditions where you
> are not running a performance workload. Consider the following:
>
> 1. The system is idle, all core at min P state and utilization is low say < 10%
> 2. You run something that drives the load as seen by the kernel to 100%
> which scaled by the current P state.
>
> This would cause the P state to go from min -> max in one step. Which is
> what you want if you are only looking at a single core. But this will also
> drag every core in the package to the max P state as well. This would be fine
I think, this will also happen using the original driver (before your
new patch 4/5), after some sampling intervals.
> if the power vs frequency cure was linear all the cores would finish
> their work faster and go idle sooner (race to halt) and maybe spend
> more time in a deeper C state which dwarfs the amount of power we can
> save by controlling P states. Unfortunately this is *not* the case,
> power vs frequency curve is non-linear and get very steep in the turbo
> range. If it were linear there would be no reason to have P state
> control you could select the highest P state and walk away.
>
> Being conservative on the way up and aggressive on way down give you
> the best power efficiency on non-benchmark loads. Most benchmarks
> are pretty useless for measuring power efficiency (unless they were
> designed for it) since they are measuring how fast something can be
> done which is measuring the efficiency at max performance.
>
> The performance issues you pointed out were caused by commit
> fcb6a15c intel_pstate: Take core C0 time into account for core busy calculation
> and the ensuing problem is caused. These have been fixed in the patch set
>
> https://lkml.org/lkml/2014/5/8/574
>
> The performance comparison between before/after this patch set, your patch
> and ondemand/acpi_cpufreq is available at:
> http://openbenchmarking.org/result/1405085-PL-C0200965993
> ffmpeg was added to the set of benchmarks because there was a regression
> reported against this benchmark as well.
> https://bugzilla.kernel.org/show_bug.cgi?id=75121
Of course, I agree generally with your comments above. But I believe that
the we should scale the core as soon as we measure high load.
I tested your new patches and I confirm your benchmarks. But I think
they are against the above theory (at least on low loads).
With the new patches I get increased frequencies even on an idle system.
Please compare the results below.
With your latest patches during a mp3 decoding (a non-benchmark load)
the energy consumption increased to 5187.52 J from 5036.57 J (almost 3%).
Thanks again,
Stratos
With my patch
-------------
[root@albert ~]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat -i 60
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 PkgWatt CorWatt GFXWatt
- - 1 0.06 1645 3392 0 0.26 0.00 99.67 0.00 32 32 0.00 0.00 0.00 0.00 20.18 2.00 0.02
0 0 2 0.10 1623 3392 0 0.63 0.01 99.26 0.00 32 32 0.00 0.00 0.00 0.00 20.18 2.00 0.02
0 4 0 0.01 1618 3392 0 0.72
1 1 1 0.03 1618 3392 0 0.03 0.00 99.94 0.00 27
1 5 0 0.01 1606 3392 0 0.05
2 2 0 0.02 1635 3392 0 0.28 0.00 99.70 0.00 22
2 6 3 0.17 1668 3392 0 0.13
3 3 2 0.12 1647 3392 0 0.08 0.00 99.80 0.00 30
3 7 0 0.02 1623 3392 0 0.18
With your latest patch
----------------------
[root@albert ~]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat -i 60
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 PkgWatt CorWatt GFXWatt
- - 1 0.05 2035 3392 0 0.28 0.01 99.66 0.00 34 34 0.00 0.00 0.00 0.00 20.20 2.01 0.02
0 0 1 0.04 1831 3392 0 0.06 0.00 99.90 0.00 34 34 0.00 0.00 0.00 0.00 20.20 2.01 0.02
0 4 0 0.01 2136 3392 0 0.09
1 1 1 0.06 1931 3392 0 0.70 0.00 99.24 0.00 31
1 5 0 0.01 2024 3392 0 0.75
2 2 1 0.03 2231 3392 0 0.21 0.03 99.73 0.00 26
2 6 2 0.09 1967 3392 0 0.15
3 3 3 0.15 2115 3392 0 0.06 0.00 99.78 0.00 34
3 7 0 0.02 2073 3392 0 0.19
With my patch:
--------------
[root@albert ~]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat mpg321 /home/stratosk/One\ Direction\ -\ Story\ of\ My\ Life.mp3
[4:05] Decoding of One Direction - Story of My Life.mp3 finished.
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 PkgWatt CorWatt GFXWatt
- - 7 0.45 1613 3392 0 14.55 0.02 84.97 0.00 35 35 0.00 0.00 0.00 0.00 20.51 2.33 0.01
0 0 16 1.01 1623 3392 0 1.06 0.04 97.89 0.00 35 35 0.00 0.00 0.00 0.00 20.51 2.33 0.01
0 4 0 0.02 1616 3392 0 2.05
1 1 3 0.16 1609 3392 0 1.61 0.00 98.22 0.00 30
1 5 13 0.80 1606 3392 0 0.97
2 2 8 0.52 1606 3392 0 38.97 0.03 60.48 0.00 26
2 6 10 0.65 1613 3392 0 38.84
3 3 7 0.42 1613 3392 0 16.28 0.01 83.29 0.00 33
3 7 1 0.05 1624 3392 0 16.65
245.566284 sec
With your patch:
----------------
[root@albert ~]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat mpg321 /home/stratosk/One\ Direction\ -\ Story\ of\ My\ Life.mp3
[4:05] Decoding of One Direction - Story of My Life.mp3 finished.
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 PkgWatt CorWatt GFXWatt
- - 7 0.27 2773 3392 0 40.05 0.01 59.67 0.00 35 35 0.00 0.00 0.00 0.00 21.11 2.93 0.01
0 0 9 0.31 2773 3392 0 82.55 0.01 17.12 0.00 35 35 0.00 0.00 0.00 0.00 21.11 2.93 0.01
0 4 5 0.15 3290 3392 0 82.71
1 1 8 0.31 2541 3392 0 26.87 0.00 72.82 0.00 30
1 5 19 0.79 2400 3392 0 26.38
2 2 8 0.23 3490 3392 0 15.43 0.00 84.34 0.00 27
2 6 1 0.04 2086 3392 0 15.62
3 3 4 0.13 2978 3392 0 35.44 0.00 64.42 0.00 31
3 7 6 0.16 3553 3392 0 35.42
245.642873 sec
With original code
-----------------
[root@albert ~]# /home/stratosk/kernels/linux-pm/tools/power/x86/turbostat/turbostat mpg321 /home/stratosk/One\ Direction\ -\ Story\ of\ My\ Life.mp3
[4:05] Decoding of One Direction - Story of My Life.mp3 finished.
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 PkgWatt CorWatt GFXWatt
- - 5 0.32 1608 3392 0 20.43 0.01 79.24 0.00 35 35 0.00 0.00 0.00 0.00 20.59 2.41 0.01
0 0 2 0.11 1621 3392 0 20.90 0.01 78.98 0.00 35 35 0.00 0.00 0.00 0.00 20.59 2.41 0.01
0 4 6 0.38 1600 3392 0 20.63
1 1 8 0.50 1603 3392 0 24.10 0.00 75.40 0.00 29
1 5 0 0.02 1611 3392 0 24.58
2 2 13 0.81 1598 3392 0 0.45 0.02 98.73 0.00 29
2 6 1 0.04 1675 3392 0 1.21
3 3 9 0.59 1603 3392 0 35.54 0.01 63.86 0.00 33
3 7 1 0.08 1749 3392 0 36.05
245.641863 sec
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