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Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision
 of EAS estimated energy
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Chris Redpath <Chris.Redpath@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Quentin Perret <qperret@google.com>,
        "open list:THERMAL" <linux-pm@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, segall@google.com,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        CCj.Yeh@mediatek.com
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From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <e3b11a84-9a26-9a7d-cf90-12a7a8fbcc3f@arm.com>
Date:   Wed, 7 Jul 2021 12:02:19 +0100
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On 7/7/21 11:50 AM, Vincent Guittot wrote:
> On Wed, 7 Jul 2021 at 12:41, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>
>>
>>
>> On 7/7/21 11:32 AM, Vincent Guittot wrote:
>>> On Wed, 7 Jul 2021 at 12:29, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>
>>>>
>>>>
>>>> On 7/7/21 11:11 AM, Vincent Guittot wrote:
>>>>> On Wed, 7 Jul 2021 at 12:06, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>>>
>>>>
>>>> [snip]
>>>>
>>>>>> No. It's in 0.1uW scale, so 800Watts. Which is 16 CPUs * 64Watts
>>>>>
>>>>> Oh! you want 0.1uW precision .... This doesn't seem realistic at all.
>>>>> I'm not even sure that the power model can even reach an accuracy of
>>>>> 1mW
>>>>>
>>>>
>>>> True, the EM is registering platform with 1mW precision, but 1uW
>>>
>>> Do you mean 1uW or 0.1uW ?
>>
>> In this patch set I've proposed 0.1uW, but I'm open to drop one
>> order of magnitude. The 1uW still be good.
> 
> I don't want to underestimate the capabilities of the power model but
> I don't see which benefit you will get with 0.1uW precision
> With a 1uW precision the long type currently used for the returned
> value is fine for 32bits machine AFAICT
> 

For 1uW and 1.2Watts for one core, 4 CPUs in cluster we get:
(1200 * 1000) * (4 * 1024) = ~4.9bln
so it would need div 64 version