Re: [PATCH] Optimize int_sqrt for small values for faster idle

[Fengguang Wu]

On Tue, Feb 09, 2016 at 12:44:00PM -0800, Andi Kleen wrote:
> On Sun, Feb 07, 2016 at 10:32:26PM +0100, Rasmus Villemoes wrote:
> > On Mon, Feb 01 2016, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> > 
> > > On Mon, Feb 01, 2016 at 10:25:17PM +0100, Rasmus Villemoes wrote:
> > >> On Thu, Jan 28 2016, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> > >> 
> > >> > From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > >> >
> > >> > The menu cpuidle governor does at least two int_sqrt() each time
> > >> > we go into idle in get_typical_interval to compute stddev
> > >> >
> > >> > int_sqrts take 100-120 cycles each. Short idle latency is important
> > >> > for many workloads.
> > >> >
> > >> 
> > >> If you want to optimize get_typical_interval(), why not just take the
> > >> square root out of the equation (literally)?
> > >> 
> > >> Something like
> > >
> > > Looks good. Yes that's a better fix.
> > >
> > 
> > Andi, did you have a way to measure the impact, and if so, could I get
> > you to run the numbers again with my patch?
> 
> I got the numbers from the 0day runs (AIM7 gets faster)
> In theory if you post the patch that should happen automatically
> (checking with Fengguang)

Yes we bisect and report a lot of runtime performance changes.
On the other hand, some few will be missed, too, due to various
unstableness issues. Anyway if you would like to study performance
impacts of a patch, feel free to send us requests to gather the
numbers and do comparison.

Thanks,
Fengguang