# Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy

**From: **Vincent Guittot

**Date: ** Wed Jul 07 2021 - 05:56:39 EST

On Wed, 7 Jul 2021 at 11:48, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:

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>

>

>* On 7/7/21 10:37 AM, Vincent Guittot wrote:*

>* > On Wed, 7 Jul 2021 at 10:23, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:*

>* >>*

>* >>*

>* >>*

>* >> On 7/7/21 9:00 AM, Vincent Guittot wrote:*

>* >>> On Wed, 7 Jul 2021 at 09:49, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:*

>* >>>>*

>* >>>>*

>* >>>>*

>* >>>> On 7/7/21 8:07 AM, Vincent Guittot wrote:*

>* >>>>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@xxxxxxx> 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 !!!

>

>* The returned value after applying the whole patch set*

>* won't fit in u32 for such cluster.*

>

>* We might make *assumption* that the 32bit platforms will not*

>* have bigger number of CPUs in the cluster or won't report*

>* big power values. But I didn't wanted to make such assumption.*

>

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