Re: [PATCH 2/5] writeback: dirty position control

[Wu Fengguang]

> > Making our final function look like:
> > 
> >                s - x 3
> >  f(x) :=  1 + (-----)
> >                l - s
> 
> Very intuitive reasoning, thanks!
> 
> I substituted real numbers to the function assuming a mem=2GB system.
> 
> with limit=thresh:
> 
>         gnuplot> set xrange [60000:80000]
>         gnuplot> plot 1 +  (70000.0 - x)**3/(80000-70000.0)**3

I'll use the above one, which is more simple and elegant: 

        f(freerun)  = 2.0
        f(setpoint) = 1.0
        f(limit)    = 0

Code is

        unsigned long freerun = (thresh + bg_thresh) / 2;

        setpoint = (limit + freerun) / 2;
        pos_ratio = abs(dirty - setpoint);
        pos_ratio <<= BANDWIDTH_CALC_SHIFT;
        do_div(pos_ratio, limit - setpoint + 1);
        x = pos_ratio;
        pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;
        pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;
        if (dirty > setpoint)
                pos_ratio = -pos_ratio;
        pos_ratio += 1 << BANDWIDTH_CALC_SHIFT;

Thanks,
Fengguang
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