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 10:48:06 +0100 |
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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@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
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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