Re: [RFC][PATCH v4 3/3] sched: Periodically decay max cost of idlebalance

[Jason Low]

On Mon, 2013-09-09 at 13:44 +0200, Peter Zijlstra wrote:
> On Tue, Sep 03, 2013 at 11:02:59PM -0700, Jason Low wrote:
> > On Fri, 2013-08-30 at 12:29 +0200, Peter Zijlstra wrote:
> > >  	rcu_read_lock();
> > >  	for_each_domain(cpu, sd) {
> > > +		/*
> > > +		 * Decay the newidle max times here because this is a regular
> > > +		 * visit to all the domains. Decay ~0.5% per second.
> > > +		 */
> > > +		if (time_after(jiffies, sd->next_decay_max_lb_cost)) {
> > > +			sd->max_newidle_lb_cost =
> > > +				(sd->max_newidle_lb_cost * 254) / 256;
> > 
> > I initially picked 0.5%, but after trying it out, it appears to decay very
> > slowing when the max is at a high value. Should we increase the decay a
> > little bit more? Maybe something like:
> > 
> > sd->max_newidle_lb_cost = (sd->max_newidle_lb_cost * 63) / 64;
> 
> So the half-life in either case is is given by:
> 
>   n = ln(1/2) / ln(x)
> 
> which gives 88 seconds for x := 254/256 or 44 seconds for x := 63/64.
> 
> I don't really care too much, but obviously something like:
> 
>  256*exp(ln(.5)/60) ~= 253
> 
> Is attractive ;-)
> 
> > > +		/*
> > > +		 * Stop the load balance at this level. There is another
> > > +		 * CPU in our sched group which is doing load balancing more
> > > +		 * actively.
> > > +		 */
> > > +		if (!continue_balancing) {
> > 
> > Is "continue_balancing" named "balance" in older kernels?
> 
> Yeah, this patch crossed paths with a series remodeling the
> load-balancer a bit, that should all be pushed-out to tip/master.
> 
> In particular see commit: 
>   23f0d20 sched: Factor out code to should_we_balance()
> 
> > Here are the AIM7 results with the other 2 patches + this patch with the
> > slightly higher decay value.
> 
> Just to clarify, 'this patch' is the one I sent?

Yes, I was referring to the one you sent out. Also, I did the
s/smp_processor_id()/this_cpu/ on patch 2.


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