Date:	Wed, 05 Jun 2013 12:58:33 -0400
From:	David C Niemi <dniemi@...isign.com>
To:	Borislav Petkov <bp@...e.de>
CC:	Stratos Karafotis <stratosk@...aphore.gr>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-pm@...r.kernel.org,
	cpufreq@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] cpufreq: ondemand: Change the calculation of target
 frequency

When you are doing a locally-originated truly CPU-bound task, "race to idle" does make some sense.  But I can think of a couple of caveats.

1) If you care about power consumption, you want to avoid super-power-hungry turbo states, as you get less done per watt-hour than in some of the middle states.

2) CPU usage that is related to I/O (network, disk, video) doesn't necessarily let you go to idle sooner if at all.  In this case if you want to minimize power consumption you may want to use middle states a lot.  But if you care more about responsiveness or latency than power consumption, you might want to go to a high state anyway; that is why we have tunables -- so we can configure based on the actual priorities for the machine.

DCN

On 06/05/13 12:17, Borislav Petkov wrote:
> On Wed, Jun 05, 2013 at 07:01:25PM +0300, Stratos Karafotis wrote:
>> Ondemand calculates load in terms of frequency and increases it only
>> if the load_freq is greater than up_threshold multiplied by current
>> or average frequency. This seems to produce oscillations of frequency
>> between min and max because, for example, a relatively small load can
>> easily saturate minimum frequency and lead the CPU to max. Then, the
>> CPU will decrease back to min due to a small load_freq.
> Right, and I think this is how we want it, no?
>
> The thing is, the faster you finish your work, the faster you can become
> idle and save power.
>
> If you switch frequencies in a staircase-like manner, you're going to
> take longer to finish, in certain cases, and burn more power while doing
> so.
>
> Btw, racing to idle is also a good example for why you want boosting:
> you want to go max out the core but stay within power limits so that you
> can finish sooner.
>
>> This patch changes the calculation method of load and target frequency
>> considering 2 points:
>> - Load computation should be independent from current or average
>> measured frequency. For example an absolute load 80% at 100MHz is not
>> necessarily equivalent to 8% at 1000MHz in the next sampling interval.
>> - Target frequency should be increased to any value of frequency table
>> proportional to absolute load, instead to only the max. Thus:
>>
>> Target frequency = C * load
>>
>> where C = policy->cpuinfo.max_freq / 100
>>
>> Tested on Intel i7-3770 CPU @ 3.40GHz and on Quad core 1500MHz Krait.
>> Phoronix benchmark of Linux Kernel Compilation 3.1 test shows an
>> increase ~1.5% in performance. cpufreq_stats (time_in_state) shows
>> that middle frequencies are used more, with this patch. Highest
>> and lowest frequencies were used less by ~9%
> I read this as "the workload takes longer to complete" which means
> higher power consumption and longer execution times which means less
> time spent in idle. And I don't think we want that.
>
> Yes, no?
>
> Thanks.
>

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