Sorry forgot the 2 gnuplot figures, attached now.

> > 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
> 
> with limit=thresh+thresh/DIRTY_SCOPE
> 
>         gnuplot> set xrange [60000:90000]
>         gnuplot> plot 1 +  (70000.0 - x)**3/(90000-70000.0)**3
> 
> Figures attached.  The latter produces reasonably flat slope and I'll
> give it a spin in the dd tests :)
>  
> > You can clamp it at [0,2] or so.
> 
> Looking at the figures, we may even do without the clamp because it's
> already inside the range [0, 2].
> 
> > The implementation wouldn't be too horrid either, something like:
> > 
> > unsigned long bdi_pos_ratio(..)
> > {
> > 	if (dirty > limit)
> > 		return 0;
> > 
> > 	if (dirty < 2*setpoint - limit)
> > 		return 2 * SCALE;
> > 
> > 	x = SCALE * (setpoint - dirty) / (limit - setpoint);
> > 	xx = (x * x) / SCALE;
> > 	xxx = (xx * x) / SCALE;
> > 
> > 	return xxx;
> > }
> 
> Looks very neat, much simpler than the three curves solution!
> 
> Thanks,
> Fengguang