Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 7 Jul 2021 12:11:28 +0200 | Subject | Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy |
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On Wed, 7 Jul 2021 at 12:06, Lukasz Luba <lukasz.luba@arm.com> wrote: > > > > 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
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
> 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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