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

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Date:	Tue, 06 Sep 2011 17:47:10 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, 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 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? 
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