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SubjectRe: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy
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On 7/7/21 10:56 AM, Vincent Guittot wrote:
> On Wed, 7 Jul 2021 at 11:48, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>
>>
>>
>> On 7/7/21 10:37 AM, Vincent Guittot wrote:
>>> On Wed, 7 Jul 2021 at 10:23, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>
>>>>
>>>>
>>>> On 7/7/21 9:00 AM, Vincent Guittot wrote:
>>>>> On Wed, 7 Jul 2021 at 09:49, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 7/7/21 8:07 AM, Vincent Guittot wrote:
>>>>>>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>>>>>
>>>>>>>> The Energy Aware Scheduler (EAS) tries to find best CPU for a waking up
>>>>>>>> task. It probes many possibilities and compares the estimated energy values
>>>>>>>> for different scenarios. For calculating those energy values it relies on
>>>>>>>> Energy Model (EM) data and em_cpu_energy(). The precision which is used in
>>>>>>>> EM data is in milli-Watts (or abstract scale), which sometimes is not
>>>>>>>> sufficient. In some cases it might happen that two CPUs from different
>>>>>>>> Performance Domains (PDs) get the same calculated value for a given task
>>>>>>>> placement, but in more precised scale, they might differ. This rounding
>>>>>>>> error has to be addressed. This patch prepares EAS code for better
>>>>>>>> precision in the coming EM improvements.
>>>>>>>
>>>>>>> Could you explain why 32bits results are not enough and you need to
>>>>>>> move to 64bits ?
>>>>>>>
>>>>>>> Right now the result is in the range [0..2^32[ mW. If you need more
>>>>>>> precision and you want to return uW instead, you will have a result in
>>>>>>> the range [0..4kW[ which seems to be still enough
>>>>>>>
>>>>>>
>>>>>> Currently we have the max value limit for 'power' in EM which is
>>>>>> EM_MAX_POWER 0xffff (64k - 1). We allow to register such big power
>>>>>> values ~64k mW (~64Watts) for an OPP. Then based on 'power' we
>>>>>> pre-calculate 'cost' fields:
>>>>>> cost[i] = power[i] * freq_max / freq[i]
>>>>>> So, for max freq the cost == power. Let's use that in the example.
>>>>>>
>>>>>> Then the em_cpu_energy() calculates as follow:
>>>>>> cost * sum_util / scale_cpu
>>>>>> We are interested in the first part - the value of multiplication.
>>>>>
>>>>> But all these are internal computations of the energy model. At the
>>>>> end, the computed energy that is returned by compute_energy() and
>>>>> em_cpu_energy(), fits in a long
>>>>
>>>> Let's take a look at existing *10000 precision for x CPUs:
>>>> cost * sum_util / scale_cpu =
>>>> (64k *10000) * (x * 800) / 1024
>>>> which is:
>>>> x * ~500mln
>>>>
>>>> So to be close to overflowing u32 the 'x' has to be > (?=) 8
>>>> (depends on sum_util).
>>>
>>> Sorry but I don't get your point.
>>> This patch is about the return type of compute_energy() and
>>> em_cpu_energy(). And even if we decide to return uW instead of mW,
>>> there is still a lot of margin.
>>>
>>> It's not because you need u64 for computing intermediate value that
>>> you must returns u64
>>
>> The example above shows the need of u64 return value for platforms
>> which are:
>> - 32bit
>> - have e.g. 16 CPUs
>> - has big power value e.g. ~64k mW
>> Then let's to the calc:
>> (64k * 10000) * (16 * 800) / 1024 = ~8000mln = ~8bln
>
> so you return a power consumption of 8kW !!!
>

No. It's in 0.1uW scale, so 800Watts. Which is 16 CPUs * 64Watts
each at max freq and 80% load.

Max power can be < 64Watts, which is 64k milli-Watts (< EM_MAX_POWER)
64k mW * 10000 --> is the 0.1uW precision

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