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

[Joe Perches <joe@perches.com>]

(adding Anshul Garg)

On Thu, 2016-01-28 at 13:42 -0800, Andi Kleen 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.
> 
> 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.