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

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Message-ID: <20160209204400.GC4875@tassilo.jf.intel.com>
Date:	Tue, 9 Feb 2016 12:44:00 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
	lenb@...nel.org, fengguang.wu@...el.com
Subject: Re: [PATCH] Optimize int_sqrt for small values for faster idle

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)

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only