Re: [RFC] scheduler: improve SMP fairness in CFS

["Chris Friesen" <cfriesen@nortel.com>]

Chris Snook wrote:

> A fraction of *each* CPU, or a fraction of *total* CPU?  Per-cpu 
> granularity doesn't make anything more fair.

Well, our current solution uses per-cpu weights, because our vendor 
couldn't get the load balancer working accurately enough.  Having 
per-cpu weights and cpu affinity gives acceptable results for the case 
where we're currently using it.

If the load balancer is good enough, per-system weights would be fine. 
It would have to play nicely with affinity though, in the case where it 
makes sense to lock tasks to particular cpus.

> If I have two threads with the same priority, and two CPUs, the 
> scheduler will put one on each CPU, and they'll run happily without any 
> migration or balancing.

Sure.  Now add a third thread.  How often do you migrate?  Put another 
way, over what time quantum do we ensure fairness?

Chris