Re: [RFC PATCH v2 0/7] Tunable sched_mc_power_savings=n

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Tuesday 09 September 2008 18:25, Peter Zijlstra wrote:
> On Tue, 2008-09-09 at 17:59 +1000, Nick Piggin wrote:

> > But it's all per-cpu, so you'd have to iterate down other CPU's child
> > domains. Which may get dirtied by that CPU. So you get cacheline
> > bounces.
>
> Humm, are you saying each cpu has its own domain tree? My understanding
> was that its a global structure, eg. given:
>
>    domain[0-1]
>
> domain[0] domain[1]
>
> cpu0's parent domain is the same instance as cpu1's.

I haven't looked recently, but that's how I wrote it. Has that changed?


> > You also lose flexibility (although nobody really takes full advantage
> > of it) of totally arbitrary topology on a per-cpu basis.
>
> Afaict the only flexibility you loose is that you cannot make groups
> larger/smaller than the child domain - which given that the whole
> premesis of the groups existence is that the inner-group balancing
> should be done by the level below - doesn't make sense anyway.

But you *also* cannot have per-cpu domain trees.


> > > So my idea was to ditch the groups and just iterate over the child
> > > domains.
> >
> > I'm not saying you couldn't do it (reasonably well -- cacheline bouncing
> > might be a problem if you propose to traverse other CPU's domains), but
> > what exactly does that gain you?
>
> Those cacheline bounces could be mitigated by splitting sched_domain
> into two parts with a cacheline aligned dummy and keep the rarely
> modified data separate from the frequently modified data.

You could.


> As to the gains - a graph walk with a single type seems more elegant to
> me.

It's fundamentally two different things anyway though. I don't
see any theoretical improvement, and it definitely wouldn't
improve the practical side much if any because the biggest problem
I don't think is the simple walks themselves but the calculations
and stuff.

If it can yield something clearly better that is impossible using
domains and groups, I could change my mind.