Re: [RFC PATCH] cpufreq: intel_pstate: Change the calculation of next pstate

[Stratos Karafotis]

On 09/05/2014 05:56 ÎÎ, Stratos Karafotis wrote:
> 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
> 

I would like to explain a little bit further the logic behind this patch.

The patch is based on the following assumptions (some of them are pretty
obvious but please let me mention them):

1) We define the load of the CPU as the percentage of sampling period that
CPU was busy (not idle), as measured by the kernel.

2) It's not possible to predict (with accuracy) the load of a CPU in future
sampling periods.

3) The load in the next sampling interval is most probable to be very
close to the current sampling interval. (Actually the load in the
next sampling interval could have any value, 0 - 100).

4) In order to select the next performance state of the CPU we need to
calculate the load frequently (as fast as hardware permits) and change
the next state accordingly.

5) At a given constant 0% (zero) load in a specific period, the CPU
performance state should be equal to minimum available state.

6) At a given constant 100% load in a specific period, the CPU performance
state should be equal to maximum available state.

7) Ideally, the CPU should execute instructions at maximum performance state.


According to the above if the measured load in a sampling interval is, for
example 50%, ideally the CPU should spent half of the next sampling period
to maximum pstate and half of the period to minimum pstate. Of course
it's impossible to increase the sampling frequency so much.

Thus, we consider that the best approximation would be:

Next performance state = min_perf + (max_perf - min_perf) * load / 100


Thanks again for your time,
Stratos



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