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

From	Andi Kleen <ak@linux.intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] Optimize int_sqrt for small values for faster idle
Date	2016-02-02 01:10 +0100
Message-ID	<qXwHE-2lh-7@gated-at.bofh.it> (permalink)
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On Tue, Feb 02, 2016 at 12:08:46AM +0100, Rasmus Villemoes wrote:
> On Mon, Feb 01 2016, Andi Kleen <ak@linux.intel.com> wrote:
> 
> > On Mon, Feb 01, 2016 at 10:25:17PM +0100, Rasmus Villemoes wrote:
> >> On Thu, Jan 28 2016, Andi Kleen <andi@firstfloor.org> wrote:
> >> 
> >> > From: Andi Kleen <ak@linux.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

Thread

[PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 22:50 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle kbuild test robot <lkp@intel.com> - 2016-01-28 23:10 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Joe Perches <joe@perches.com> - 2016-01-28 23:20 +0100
    Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 23:50 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle kbuild test robot <lkp@intel.com> - 2016-01-28 23:20 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Joe Perches <joe@perches.com> - 2016-01-28 23:30 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <andi@firstfloor.org> - 2016-01-28 23:40 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-01-28 23:40 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-01-29 05:00 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Thomas Rohwer <trohwer85@gmail.com> - 2016-01-31 08:30 +0100
  Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-01 22:30 +0100
    Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <ak@linux.intel.com> - 2016-02-01 22:40 +0100
      Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-02 00:10 +0100
        Re: [PATCH] Optimize int_sqrt for small values for faster idle Andi Kleen <ak@linux.intel.com> - 2016-02-02 01:10 +0100
        Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-02 01:40 +0100
          Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-02 21:50 +0100
            Re: [PATCH] Optimize int_sqrt for small values for faster idle Eric Dumazet <eric.dumazet@gmail.com> - 2016-02-02 22:40 +0100
      Re: [PATCH] Optimize int_sqrt for small values for faster idle Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2016-02-07 22:40 +0100

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