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 <dni...@verisign.com> 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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