Re: [RFC PATCH v1 0/3] Saving power by cpu evacuation usingsched_mc=n

[Ingo Molnar]

* Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxxxxxxx> wrote:

> > > --------------------------------------------------------
> > > sched_mc	No Cores	Performance	AvgPower	
> > > 		used		Records/sec	(Watts)
> > > --------------------------------------------------------
> > > 0		8		1.00x		1.00y
> > > 1		8		1.02x		1.01y
> > > 2		8		0.83x		1.01y
> > > 3		7		0.86x		0.97y
> > > 4		6		0.76x		0.92y
> > > 5		4		0.72x		0.82y
> > > --------------------------------------------------------
> > 
> > Looks like we want the kernel default to be sched_mc=1 ?
> 
> Hi Ingo,
> 
> Yes, sched_mc wins for a simple cpu bound workload like this.  But 
> the challenge is that the best settings depends on the workload 
> and the system configuration.  This leads me to think that the 
> default setting should be left with the distros where we can 
> factor in various parameters and choose the right default from 
> user space.
> 
> 
> > Regarding the values for 2...5 - is the AvgPower column time 
> > normalized or workload normalized?
> 
> The AvgPower is time normalised, just the power value divided by 
> the baseline at sched_mc=0.
>  
> > If it's time normalized then it appears there's no power win 
> > here at all: we'd be better off by throttling the workload 
> > directly (by injecting sleeps or something like that), right?
> 
> Yes, there is no power win when comparing with peak benchmark 
> throughput in this case.  However more complex workload setup may 
> not show similar characteristics because they are not dependent 
> only on CPU bandwidth for their peak performance.
> 
> * Reduction in cpu bandwidth may not directly translate to performance
>   reduction on complex workloads
> * Even if there is degradation, the system may still meet the design
>   objectives.  20-30% increase in response time over a 1 second
>   nominal value may be acceptable in most cases

But ... we could probably get a _better_ (near linear) slowdown by 
injecting wait cycles into the workload.

I.e. we should only touch balancing if there's a _genuine_ power 
saving: i.e. less power is used for the same throughput.

The numbers in the table show a plain slowdown: doing fewer 
transactions means less power used. But that is trivial to achieve 
for a CPU-bound workload: throttle the workload. I.e. inject less 
work, save power.

And if we want to throttle 'transparently', from the kernel, we 
should do it not via an artificial open-ended scale of 
sched_mc=2,3,4,5... - we should do it via a _percentage_ value.

I.e. a system setting that says "at most utilize the system 80% of 
its peak capacity". That can be implemented by the kernel injecting 
small delays or by intentionally not scheduling on certain CPUs (but 
not delaying tasks - forcing them to other cpus in essence).

	Ingo
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