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Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Li Shaohua <shaohua.li@intel.com>, Christoph Hellwig <hch@lst.de>,
        Dave Chinner <david@fromorbit.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Chris Mason <chris.mason@oracle.com>, Mel Gorman <mel@csn.ul.ie>,
        Rik van Riel <riel@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        linux-mm <linux-mm@kvack.org>, linux-fsdevel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <20101117042850.002299964@intel.com>
References: <20101117042720.033773013@intel.com>
	 <20101117042850.002299964@intel.com>
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Date: Wed, 24 Nov 2010 12:05:32 +0100
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On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
> +void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
> +                               unsigned long *bw_time,
> +                               s64 *bw_written)
> +{
> +       unsigned long written;
> +       unsigned long elapsed;
> +       unsigned long bw;
> +       unsigned long w;
> +
> +       if (*bw_written == 0)
> +               goto snapshot;
> +
> +       elapsed = jiffies - *bw_time;
> +       if (elapsed < HZ/100)
> +               return;
> +
> +       /*
> +        * When there lots of tasks throttled in balance_dirty_pages(), they
> +        * will each try to update the bandwidth for the same period, making
> +        * the bandwidth drift much faster than the desired rate (as in the
> +        * single dirtier case). So do some rate limiting.
> +        */
> +       if (jiffies - bdi->write_bandwidth_update_time < elapsed)
> +               goto snapshot;

Why this goto snapshot and not simply return? This is the second call
(bdi_update_bandwidth equivalent).

If you were to leave the old bw_written/bw_time in place the next loop
around in wb_writeback() would see a larger delta..

I guess this funny loop in wb_writeback() is also the reason you've got
a single function and not the get/update like separation

> +       written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
> +       bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
> +       w = min(elapsed / (HZ/100), 128UL);
> +       bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
> +       bdi->write_bandwidth_update_time = jiffies;
> +snapshot:
> +       *bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
> +       *bw_time = jiffies;
> +}