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

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Date:	Thu, 28 Jan 2016 14:22:57 -0800
From:	Joe Perches <joe@...ches.com>
To:	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	Anshul Garg <aksgarg1989@...il.com>
Cc:	linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
	lenb@...nel.org, Andi Kleen <ak@...ux.intel.com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH] Optimize int_sqrt for small values for faster idle

(resending with email addresses that shouldn't bounce)
(adding Anshul Garg)
(fixed Davidlohr Bueso's address)

On Thu, 2016-01-28 at 13:42 -0800, Andi Kleen 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.
> 
> I instrumented the function on my workstation and most values are
> 16bit only and most others 32bit (50% percentile is 122094,
> 75% is 3699533).
> 
> sqrt is implemented by starting with an initial estimation,
> and then iterating. int_sqrt currently only uses a fixed
> estimating which is good for 64bits worth of input.
> 
> This patch adds some checks at the beginning to start with
> a better estimate for values fitting in 8, 16bit and 32bit.
> This makes int_sqrt between 60+% faster for values in 16bit,
> and still somewhat faster (between 10 and 30%) for larger values
> upto 32bit. Full 64bit is slightly slower.
> 
> This optimizes the short idle calls and does not hurt the
> long sleep (which probably do not care) much.
> 
> An alternative would be a full table drive approach, or
> trying some inverted sqrt optimization, but this simple change
> already seems to have a good payoff.

This thread might be relevant:

https://lkml.org/lkml/2015/2/2/600

and perhaps using fls might still be a good approach.