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

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Date:	Mon, 1 Feb 2016 16:00:33 -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 Tue, Feb 02, 2016 at 12:08:46AM +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.
> >
> 
> Thanks. (Is there a good way to tell gcc that avg*avg is actually a
> 32x32->64 multiplication?)

I don't think there is, but you could define a custom macro with a fallback
on pure 64x64->64.

-Andi