Messages in this thread | | | Subject | Re: [PATCH 3/3] PM: EM: Increase energy calculation precision | From | Lukasz Luba <> | Date | Tue, 6 Jul 2021 20:51:00 +0100 |
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On 7/5/21 1:45 PM, Dietmar Eggemann wrote: > On 25/06/2021 17:26, Lukasz Luba wrote: >> The Energy Model (EM) provides useful information about device power in >> each performance state to other subsystems like: Energy Aware Scheduler >> (EAS). The energy calculation in EAS does arithmetic operation based on >> the EM em_cpu_energy(). Current implementation of that function uses >> em_perf_state::cost as a pre-computed cost coefficient equal to: >> cost = power * max_frequency / frequency. >> The 'power' is expressed in milli-Watts (or in abstract scale). >> >> There are corner cases then the EAS energy calculation for two Performance > ^^^^^^^^^^^^ > > Again, an easy to understand example to describe in which situation this > change would bring a benefit would help. > >> Domains (PDs) return the same value, e.g. 10mW. The EAS compares these >> values to choose smaller one. It might happen that this values are equal >> due to rounding error. In such scenario, we need better precision, e.g. >> 10000 times better. To provide this possibility increase the precision on >> the em_perf_state::cost. >> >> This patch allows to avoid the rounding to milli-Watt errors, which might >> occur in EAS energy estimation for each Performance Domains (PD). The >> rounding error is common for small tasks which have small utilization >> values. > > What's the influence of the CPU utilization 'cpu_util_next()' here? > > compute_energy() > em_cpu_energy() > return ps->cost * sum_util / scale_cpu > ^^^^^^^^
This is the place where the rounding error triggers. If sum_util is small and scale_cpu is e.g. 1024, then we have a small fraction here. It depends on the EM 'cost', but for most platforms we have small power and cost values, so we suffer this rounding. The example that I gave in my response in patch 2/3 shows this.
>> The rest of the EM code doesn't change, em_perf_state::power is still >> expressed in milli-Watts (or in abstract scale). Thus, all existing >> platforms don't have to change their reported power. The same applies to > > Not only existing platforms since there are no changes. So why > highlighting `existing` here.?
I just wanted to be clear that it doesn't affect existing platforms at all. We don't require to report power in better resolution e.g. micro-Watts. Also, the clients in the kernel won't be affected, since they use EM 'power' filed, not 'cost'.
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