Re: Issues with ondemand governor

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Vishwa,

Have you had a chance to do some usetime tests with these changes?

It would be interesting to measure the power consumption with and
without these changes.

/Amit

On Tue, Nov 23, 2010 at 5:59 PM, Vishwanath Sripathy
<vishwanath.sripathy@xxxxxxxxxx> wrote:
> Thanks David for the inputs.
> I tried your patch. In addition to that I reduced transition_latency.
> With these 2 changes, I do see much better results (worst case
> performance of ondemand is 88%).
>
> Vishwa
>
>
> On Mon, Nov 22, 2010 at 9:39 PM, David C Niemi <dniemi@xxxxxxxxxxxx> wrote:
>> The general problem here is that the ondemand governor is aimed more at
>> power savings than performance.  In cases where the ondemand governor
>> performs worse than the performance governor, the "sampling_down_factor"
>> tunable is often useful.  I submitted the patch to add this tunable a
>> few weeks ago and it was acked by Venki, but I don't know what happened
>> to it after that.  It helps in two ways:
>>
>> 1) the governor does not spend as much overhead on the governor when the
>> CPU is truly busy
>>
>> 2) the governor is a lot less eager to downshift when the CPU is busy --
>> without this patch, even on a busy system ondemand will blip down in
>> clock speed surprisingly often, hurting performance.
>>
>> This patch is all about improving peak load performance.  On quite a few
>> loads I've tried this patch with a sampling_down_factor of 100 matches
>> the performance governor quite well while the original ondemand
>> performance was poor.  On the other hand, it is not much help if you are
>> trying to minimize power consumption on light to medium loads.  If you
>> set sampling_down_factor to "1" it preserves default behavior.
>>
>> David C Niemi
>>
>> Vishwanath Sripathy wrote:
>>> Hi,
>>>
>>> I was trying to investigate performance issues that we were seeing
>>> with some usecases like Video playback on OMAP Platforms with ondemand
>>> governor.
>>> As part of this, I found a tool called cpufreq-bench
>>> (http://lwn.net/Articles/339862) which can be used determine the
>>> performance impact of ondemand governor compared to performacne
>>> governor.
>>> When I ran this tool on OMAP3 (ZOOM3) platform using 2.6.36 kernel
>>> with below command, the worstcase ondemand performance is 35% compared
>>> to performance governor.
>>> cpufreq-bench -l 50000 -s 100000 -x 50000 -y 100000 -g ondemand -r 5 -n 5 -v
>>>
>>> I tried the same on x86 platforms and there the worstcase performance
>>> is around 88%.
>>> Attached are the cpufreq-bench logs for x86 and omap3.
>>>
>>> Questions:
>>> 1. Is this is known limitaiton of ondemand governor?
>>> 2. How do we support system usecases (like video playback etc) with
>>> ondemand governor if governor is not able to scale the frequencies in
>>> realtime? Are applications expected to play with scaling_min_freq to
>>> increase mpu frequency?
>>>
>>> Regards
>>> Vishwa
>>
>>
>
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