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    SubjectRe: [discussion]sched: a rough proposal to enable power saving in scheduler

    * Matthew Garrett <mjg59@srcf.ucam.org> wrote:

    > On Tue, Aug 21, 2012 at 11:42:04AM +0200, Ingo Molnar wrote:
    > > * Matthew Garrett <mjg59@srcf.ucam.org> wrote:
    > > > [...] Putting this kind of policy in the kernel is an awful
    > > > idea. [...]
    > >
    > > A modern kernel better know what state the system is in: on
    > > battery or on AC power.
    >
    > That's a fundamentally uninteresting thing for the kernel to
    > know about. [...]

    I disagree.

    > [...] AC/battery is just not an important power management
    > policy input when compared to various other things.

    Such as?

    The thing is, when I use Linux on a laptop then AC/battery is
    *the* main policy input.

    > > > [...] It should never be altering policy itself, [...]
    > >
    > > The kernel/scheduler simply offers sensible defaults where
    > > it can. User-space can augment/modify/override that in any
    > > which way it wishes to.
    > >
    > > This stuff has not been properly sorted out in the last 10+
    > > years since we have battery driven devices, so we might as
    > > well start with the kernel offering sane default behavior
    > > where it can ...
    >
    > Userspace has been doing a perfectly reasonable job of
    > determining policy here.

    Has it properly switched the scheduler's balancing between
    power-effient and performance-maximizing strategies when for
    example a laptop's AC got unplugged/replugged?

    > > > [...] because it'll get it wrong and people will file bugs
    > > > complaining that it got it wrong and the biggest case
    > > > where you *need* to be able to handle switching between
    > > > performance and power optimisations (your rack management
    > > > unit just told you that you're going to have to drop power
    > > > consumption by 20W) is one where the kernel doesn't have
    > > > all the information it needs to do this. So why bother at
    > > > all?
    > >
    > > The point is to have a working default mechanism.
    >
    > Your suggestions aren't a working default mechanism.

    In what way?

    Thanks,

    Ingo


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