[Xen-devel] Re: Power aware credit scheduler

[Emmanuel Ackaouy <ackaouy@xxxxxxxxx>]

On Jun 19, 2008, at 15:32 , Tian, Kevin wrote:
> > Regardless of any new knobs, a good default behavior might be
> > to only take a package out of C-state when another non-idle
> > package has had more than one VCPU active on it over some
> > reasonable amount of time.
> >
> > By default, putting multiple VCPUs on the same physical package
> > when other packages are idle is obviously not always going to
> > be optimal. Maybe it's not a bad default for VCPUs that are
> > related (same VM or qemu)? I think Ian P hinted at this. But it
> > frightens me that you would always do this by default for any set
> > of VCPUs. Power saving is good but so is memory bandwidth
> To enable this feature depends on a control command from system
> adminstrator, who knows the tradeoff. From absolute performance
> P.O.V, I believe it's not optimal. However if looking from the
> performance/watt, i.e. power efficiency angle, power saving due to
> package level idle may overwhelm performance impact by keeping
> activity in other package. Of course finally memory latency should
> be also considered in NUMA system, as you mentioned.
I'm saying something can be done to improve power saving in
the current system without adding a knob. Perhaps you can give
the admin even more power saving abilities with a knob, but it
makes sense to save power when performance is not impacted,
regardless of any knob position.

Also, note I mentioned memory BANDWIDTH and not latency.
It's not the same thing. And I wasn't just thinking about NUMA
systems.

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