Messages in this thread | | | Subject | Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy | From | Dietmar Eggemann <> | Date | Wed, 7 Jul 2021 11:45:00 +0200 |
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On 07/07/2021 10:23, Lukasz Luba 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:
[...]
>>>> 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).
I assume the worst case is `x * 1024` (max return value of effective_cpu_util = effective_cpu_util()) so x ~ 6.7.
I'm not aware of any arm32 b.L. systems with > 4 CPUs in a PD.
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