[lkml]   [2021]   [Jul]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy

On 7/7/21 8:07 AM, Vincent Guittot wrote:
> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <> 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.

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
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.

 \ /
  Last update: 2021-07-07 09:51    [W:0.085 / U:0.216 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site