From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 21/45] writeback: estimate bdi write bandwidth
Date: Wed, 07 Oct 2009 10:53:19 +0200
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	 <20091007074903.867171763@intel.com>
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Cc: Andrew Morton <akpm@linux-foundation.org>,
	Theodore Tso <tytso@mit.edu>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Chris Mason <chris.mason@oracle.com>,
	Li Shaohua <shaohua.li@intel.com>,
	Myklebust Trond <Trond.Myklebust@netapp.com>,
	"jens.axboe@oracle.com" <jens.axboe@oracle.com>,
	Jan Kara <jack@suse.cz>, Nick Piggin <npiggin@suse.de>,
	linux-fsdevel@vger.kernel.org
To: Wu Fengguang <fengguang.wu@intel.com>
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On Wed, 2009-10-07 at 15:38 +0800, Wu Fengguang wrote:
> +static void bdi_calc_write_bandwidth(struct backing_dev_info *bdi,
> +                                    unsigned long nr_pages,
> +                                    unsigned long time)
> +{
> +       unsigned long bw;
> +
> +       bw = HZ * nr_pages / (time | 1);
> +       bdi->write_bandwidth = (bdi->write_bandwidth * 63 + bw) / 64;
> +}

If you have block times < 1 jiffy this all falls apart quite quickly.
You could perhaps try to use cpu_clock() for ns resolution timestamps if
this is a real issue.

(I could imagine fast arrays with huge throughput causing small sleeps,
resulting in underestimates of their bandwidth)

Also, 63/64 seems rather slow progress.. maybe that's good.