[Cerowrt-devel] [Bloat] capacity awareness for the deadline scheduler

Dave Taht dave.taht at gmail.com
Mon Jun 1 16:31:05 EDT 2020

If you want consistent low latency dropping in and out of sleep state
does not help. But putting a deadline oriented task - like networking
- on a smaller processor, seems like it might work.

On Mon, Jun 1, 2020 at 9:02 AM Michael Richardson <mcr at sandelman.ca> wrote:
> article points out:
> > The work of the deadline scheduler becomes more complicated in asymmetric
> > CPU configurations, like big.LITTLE or DynamIQ. Such systems include
> > different types of CPUs, with higher and lower performance. The same task
> > running on a higher-performance ("big") CPU will take less time than when
> > run on a lower-performance ("little") one. The deadline scheduler in
> > current kernels does not take that difference into account, with the result
> > that it can over-allocate the CPU time on lower-performance CPUs. Deadline
> > tasks could end up on a little CPU, scheduled in such a way that they are
> > unable to finish before their deadlines, while they would be able to do so
> > on a higher-performance CPU. On such systems, the admission-control
> > algorithm, which assumes that all CPUs perform at the level of the big
> > ones, could overcommit the system with deadline tasks, making the system
> > unusable.
> and I wonder if in some cases it is better to keep a "little" CPU (which
> presumably draws a lot less power) running continuously to deal with
> deadlines than it is to wake up the "big" CPU to do stuff.
> I understand that we learnt the opposite in the early days of Mobile CPUs: it
> was best to run the CPU as fast (and hot) as possible to finish early and
> suspend.  Sometimes it was even power-wise to turn the fan on.
> But CPU-fast/sleep-long-time would lead to a high jitter on events that one
> might want to be more regular.
> --
> ]               Never tell me the odds!                 | ipv6 mesh networks [
> ]   Michael Richardson, Sandelman Software Works        |    IoT architect   [
> ]     mcr at sandelman.ca  http://www.sandelman.ca/        |   ruby on rails    [

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dave at taht.net <Dave Täht> CTO, TekLibre, LLC Tel: 1-831-435-0729

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