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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 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).

>
>>
>> The sum_util values that we can see for x CPUs which have scale_cap=1024
>> can be close to 800, let's use it in the example:
>> cost * sum_util = 64k * (x * 800), where
>> x=4: ~200mln
>> x=8: ~400mln
>> x=16: ~800mln
>> x=64: ~3200mln (last one which would fit in u32)
>>
>> When we increase the precision by even 100, then the above values won't
>> fit in the u32. Even a max cost of e.g. 10k mW and 100 precision has
>> issues:
>> cost * sum_util = (10k *100) * (x * 800), where
>> x=4: ~3200mln
>> x=8: ~6400mln
>>
>> For *1000 precision even a power of 1Watt becomes an issue:
>> cost * sum_util = (1k *1000) * (x * 800), where
>> x=4: ~3200mln
>> x=8: ~6400mln
>>
>> That's why to make the code safe for bigger power values, I had to use
>> the u64 on 32bit machines.

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