[Codel] fp sqrt vis int sqrt?

[Dave Taht]

The linux kernel has no floating point in it, so I'd substituted the
internal int_sqrt as a substitute,
just to get something to work.

static inline ktime_t control_law(const struct codel_sched_data *q, ktime_t t)
{
        return ktime_add_ns(t, q->interval / int_sqrt(q->count));
}

Often ns2 models use floating point. Having not seen the model I don't
know that for sure.

The series for an integer sqrt is far more 'chunky' than a fp one.

int sqrt 1,2,3,4 = 1 1 1 2
fp sqrt 1 2 3 4 = 1 1.4.1 1.73 2

and gets even more chunky as you get larger values, eg, sqrt(36
through 48) = 6, sqrt(49) = 7

So we could precalculate the interval/sqrt(count) using floating point
in the control law calculation,
early, during qdisc setup, thus neatly avoiding both the divide and
sqrt in the control law path.

Or we could do fixed point.

Kathie: In the sim, in various simulations, what is the dynamic range
of count? And is it floating point sqrt?

-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net