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

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Message-ID: <20160201213622.GA15302@tassilo.jf.intel.com>
Date:	Mon, 1 Feb 2016 13:36:22 -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, davidlohr.bueso@...com,
	rafael.j.wysocki@...el.com, lenb@...nel.org
Subject: Re: [PATCH] Optimize int_sqrt for small values for faster idle

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