Re: [PATCH 05/18] writeback: per task dirty rate limit

[Jan Kara]

On Tue 06-09-11 17:47:10, Peter Zijlstra wrote:
> On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> >  /*
> > + * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
> > + * will look to see if it needs to start dirty throttling.
> > + *
> > + * If dirty_poll_interval is too low, big NUMA machines will call the expensive
> > + * global_page_state() too often. So scale it near-sqrt to the safety margin
> > + * (the number of pages we may dirty without exceeding the dirty limits).
> > + */
> > +static unsigned long dirty_poll_interval(unsigned long dirty,
> > +                                        unsigned long thresh)
> > +{
> > +       if (thresh > dirty)
> > +               return 1UL << (ilog2(thresh - dirty) >> 1);
> > +
> > +       return 1;
> > +}
> 
> Where does that sqrt come from? 
  He does 2^{log_2(x)/2} which, if done in real numbers arithmetics, would
result in x^{1/2}. Given the integer arithmetics, it might be twice as
small but still it's some approximation...

									Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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