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Subject: Re: [PATCH 05/18] writeback: per task dirty rate limit
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: linux-fsdevel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>,
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        Minchan Kim <minchan.kim@gmail.com>, Vivek Goyal <vgoyal@redhat.com>,
        Andrea Righi <arighi@develer.com>, linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Date: Tue, 06 Sep 2011 17:47:10 +0200
In-Reply-To: <20110904020915.240747479@intel.com>
References: <20110904015305.367445271@intel.com>
	 <20110904020915.240747479@intel.com>
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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?