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

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Message-ID: <20160210133105.GB22162@wfg-t540p.sh.intel.com>
Date:	Wed, 10 Feb 2016 21:31:05 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
	lenb@...nel.org
Subject: Re: [PATCH] Optimize int_sqrt for small values for faster idle

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@...ux.intel.com> wrote:
> > 
> > > On Mon, Feb 01, 2016 at 10:25:17PM +0100, Rasmus Villemoes wrote:
> > >> On Thu, Jan 28 2016, Andi Kleen <andi@...stfloor.org> wrote:
> > >> 
> > >> > From: Andi Kleen <ak@...ux.intel.com>
> > >> >
> > >> > 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