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

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Date:	Wed, 07 Sep 2011 09:27:50 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jan Kara <jack@...e.cz>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/18] writeback: per task dirty rate limit

On Wed, 2011-09-07 at 01:27 +0200, Jan Kara wrote:
> 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...

Right, and I guess with a cpu that can do the fls its slightly faster
than our int_sqrt().
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