Messages in this thread | | | Subject | Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy | From | Lukasz Luba <> | Date | Wed, 7 Jul 2021 11:06:28 +0100 |
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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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