Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 7 Jul 2021 11:37:29 +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 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 sum_util values that we can see for x CPUs which have scale_cap=1024 > >> can be close to 800, let's use it in the example: > >> cost * sum_util = 64k * (x * 800), where > >> x=4: ~200mln > >> x=8: ~400mln > >> x=16: ~800mln > >> x=64: ~3200mln (last one which would fit in u32) > >> > >> When we increase the precision by even 100, then the above values won't > >> fit in the u32. Even a max cost of e.g. 10k mW and 100 precision has > >> issues: > >> cost * sum_util = (10k *100) * (x * 800), where > >> x=4: ~3200mln > >> x=8: ~6400mln > >> > >> For *1000 precision even a power of 1Watt becomes an issue: > >> cost * sum_util = (1k *1000) * (x * 800), where > >> x=4: ~3200mln > >> x=8: ~6400mln > >> > >> That's why to make the code safe for bigger power values, I had to use > >> the u64 on 32bit machines.
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