Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 7 Jul 2021 11:56:20 +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 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 !!!
> > 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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