Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

[Mike Galbraith <efault@gmx.de>]

On Fri, 2013-02-22 at 14:06 +0100, Ingo Molnar wrote: 
> * Mike Galbraith <efault@gmx.de> wrote:
> 
> > > > No, that's too high, you loose too much of the pretty 
> > > > face. [...]
> > > 
> > > Then a logical proportion of it - such as half of it?
> > 
> > Hm.  Better would maybe be a quick boot time benchmark, and 
> > use some multiple of your cross core pipe ping-pong time?  
> > That we know is a complete waste of cycles, because almost all 
> > cycles are scheduler cycles with no other work to be done, 
> > making firing up another scheduler rather pointless.  If we're 
> > approaching that rate, we're approaching bad idea.
> 
> Well, one problem with such dynamic boot time measurements is 
> that it introduces a certain amount of uncertainty that persists 
> for the whole lifetime of the booted up box - and it also sucks 
> in any sort of non-deterministic execution environment, such as 
> virtualized systems.

Ok, bad idea.

> I think it might be better to measure the scheduling rate all 
> the time, and save the _shortest_ cross-cpu-wakeup and 
> same-cpu-wakeup latencies (since bootup) as a reference number. 
> 
> We might be able to pull this off pretty cheaply as the 
> scheduler clock is running all the time and we have all the 
> timestamps needed.

Yeah, that might work.  We have some quick kthreads, so saving ctx
distance may get close enough to scheduler cost to be good enough.

> Pretty quickly after bootup this 'shortest latency' would settle 
> down to a very system specific (and pretty accurate) value.
> 
> [ One downside would be an increased sensitivity to the accuracy
>   and monotonicity of the scheduler clock - but that's something 
>   we want to improve on anyway - and 'worst case' we get too 
>   short latencies and we are where we are today. So it can only 
>   improve the situation IMO. ]
> 
> Would you be interested in trying to hack on an auto-tuning 
> feature like this?

Yeah, should be easy, but rainy day has to happen so I have time to
measure twiddle measure measure <curse> tweak.. ;-)

-Mike