[Codel] fp sqrt vis int sqrt?

[Eric Dumazet]

On Fri, 2012-05-04 at 11:39 -0700, Dave Taht wrote:
> Heh. I'd only gotten as far as
> 
> #include <stdio.h>
> #include <math.h>
> main() {
> 
> 
> 
> 
> 
> before your mail arrived.

Yep, so maybe we can use a precomputed table of 25 values (100 bytes),
using the reciprocal trick to get precise integer approximation.

(interval/sqrt(count)). 

So that control_law() has no multiply or divide to do, just a lookup in
the array.

Then for count values >= 25, either we can afford an error less than 20%
using the q->sqrt_count maintained in // with q->count, either we
compute the reciprocal each time we change q->count. 

(cost : one divide + int_sqrt() call )
plus one multiply in control_law()

Not sure we need to be ultra precise.