Re: [Announce] [patch] Modular Scheduler Core and Completely FairScheduler [CFS]

[Peter Williams]

Nick Piggin wrote:
> On Tue, Apr 17, 2007 at 02:25:39PM +1000, Peter Williams wrote:
> > Nick Piggin wrote:
> > > On Mon, Apr 16, 2007 at 04:10:59PM -0700, Michael K. Edwards wrote:
> > > > On 4/16/07, Peter Williams <pwil3058@xxxxxxxxxxxxxx> wrote:
> > > > > Note that I talk of run queues
> > > > > not CPUs as I think a shift to multiple CPUs per run queue may be a good
> > > > > idea.
> > > > This observation of Peter's is the best thing to come out of this
> > > > whole foofaraw.  Looking at what's happening in CPU-land, I think it's
> > > > going to be necessary, within a couple of years, to replace the whole
> > > > idea of "CPU scheduling" with "run queue scheduling" across a complex,
> > > > possibly dynamic mix of CPU-ish resources.  Ergo, there's not much
> > > > point in churning the mainline scheduler through a design that isn't
> > > > significantly more flexible than any of those now under discussion.
> > > Why? If you do that, then your load balancer just becomes less flexible
> > > because it is harder to have tasks run on one or the other.
> > >
> > > You can have single-runqueue-per-domain behaviour (or close to) just by
> > > relaxing all restrictions on idle load balancing within that domain. It
> > > is harder to go the other way and place any per-cpu affinity or
> > > restirctions with multiple cpus on a single runqueue.
> > Allowing N (where N can be one or greater) CPUs per run queue actually 
> > increases flexibility as you can still set N to 1 to get the current 
> > behaviour.
>
> But you add extra code for that on top of what we have, and are also
> prevented from making per-cpu assumptions.
>
> And you can get N CPUs per runqueue behaviour by having them in a domain
> with no restrictions on idle balancing. So where does your increased
> flexibilty come from?
>
> > One advantage of allowing multiple CPUs per run queue would be at the 
> > smaller end of the system scale i.e. a PC with a single hyper threading 
> > chip (i.e. 2 CPUs) would not need to worry about load balancing at all 
> > if both CPUs used the one runqueue and all the nasty side effects that 
> > come with hyper threading would be minimized at the same time.
>
> I don't know about that -- the current load balancer already minimises
> the nasty multi threading effects. SMT is very important for IBM's chips
> for example, and they've never had any problem with that side of it
> since it was introduced and bugs ironed out (at least, none that I've
> heard).

There's a lot of ugly code in the load balancer that is only there to 
overcome the side effects of SMT and dual core.  A lot of it was put 
there by Intel employees trying to make load balancing more friendly to 
their systems.  What I'm suggesting is that an N CPUs per runqueue is a 
better way of achieving that end.  I may (of course) be wrong but I 
think that the idea deserves more consideration than you're willing to 
give it.

Peter
--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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