Re: [PATCH 3/5] writeback: dirty rate control

Wu Fengguang
2011-08-10T14:15:49Z

On Wed, Aug 10, 2011 at 01:02:02AM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
>
> > + pos_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT;
> > + pos_bw++; /* this avoids bdi->dirty_ratelimit get stuck in 0 */
> > +
>
> > + pos_ratio *= bdi->avg_write_bandwidth;
> > + do_div(pos_ratio, dirty_bw | 1);
> > + ref_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT;
>
> when written out that results in:
>
> bw * pos_ratio * bdi->avg_write_bandwidth
> ref_bw = -----------------------------------------
> dirty_bw
>
> which would suggest you write it like:
>
> ref_bw = div_u64((u64)pos_bw * bdi->avg_write_bandwidth, dirty_bw | 1);
>
> since pos_bw is already bw * pos_ratio per the above.

Good point. Oopse I even wrote a comment for the over complex calculation:

* balanced_rate = pos_rate * write_bw / dirty_rate

Thanks,
Fengguang