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

[Matthew Garrett <mjg59@srcf.ucam.org>]

On Tue, Aug 21, 2012 at 05:59:08PM +0200, Ingo Molnar wrote:
> * Matthew Garrett <mjg@redhat.com> wrote:
> > The scheduler's behaviour is going to have a minimal impact on 
> > power consumption on laptops. Other things are much more 
> > important - backlight level, ASPM state, that kind of thing. 
> > So why special case the scheduler? [...]
> 
> I'm not special casing the scheduler - but we are talking about 
> scheduler policies here, so *if* it makes sense to handle this 
> dynamically then obviously the scheduler wants to use system 
> state information when/if the kernel can get it.
> 
> Your argument is as if you said that the shape of a car's side 
> view mirrors is not important to its top speed, because the 
> overall shape of the chassis and engine power are much more 
> important.
> 
> But we are desiging side view mirrors here, so we might as well 
> do a good job there.

If the kernel is going to make power choices automatically then it 
should do it everywhere, not piecemeal.

> > [...] This is going to be hugely more important on 
> > multi-socket systems, where your policy is usually going to be 
> > dictated by the specific workload that you're running at the 
> > time. [...]
> 
> If only we had some kernel subsystem that is intimiately familar 
> with the workloads running on the system and could act 
> accordingly and with low latency.
> 
> We could name that subsystem it in some intuitive fashion: it 
> switches and schedules workloads, so how about calling it the 
> 'scheduler'?

The scheduler is unaware of whether I care about a process finishing 
quickly or whether I care about it consuming less power.

> > [...] The exception is in cases where your rack is 
> > overcommitted for power and your rack management unit is 
> > telling you to reduce power consumption since otherwise it's 
> > going to have to cut the power to one of the machines in the 
> > rack in the next few seconds.
> 
> ( That must be some ACPI middleware driven crap, right? Not 
>   really the Linux kernel's problem. )

It's as much the Linux kernel's problem as AC/battery decisions are - 
ie, it's not.

> > > The thing is, when I use Linux on a laptop then AC/battery 
> > > is *the* main policy input.
> > 
> > And it's already well handled from userspace, as it has to be.
> 
> Not according to the developers switching away from Linux 
> desktop distros in droves, because MacOSX or Win7 has 30%+ 
> better battery efficiency.

Ok so what you're actually telling me here is that you don't understand 
anything about power management and where our problems are.

> The scheduler might be a small part of the picture, but it's 
> certainly a part of it.

It's in the drivers, which is where it has been since we went tickless. 

> > No, because sched_mt_powersave usually crippled performance 
> > more than it saved power and nobody makes multi-socket 
> > laptops.
> 
> That's a user-space policy management fail right there: why 
> wasn't this fixed? If the default policy is in the kernel we can 
> at least fix it in one place for the most common cases. If it's 
> spread out amongst multiple projects then progress only happens 
> at glacial speed ...

sched_mt_powersave was inherently going to have a huge impact on 
performance, and with modern chips that would result in the platform 
consuming more power. It was a feature that was useful for a small 
number of generations of desktop CPUs - I don't think it would ever skew 
the power/performance ratio in a useful direction on mobile hardware. 
But feel free to blame userspace for hardware design.

-- 
Matthew Garrett | mjg59@srcf.ucam.org