linux-kernel - Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write more than dirtied pages

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Date:	Wed, 9 Sep 2009 17:44:13 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>, jack@...e.cz,
	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write
	more than dirtied pages

On Wed 09-09-09 22:51:48, Wu Fengguang wrote:
> Some filesystem may choose to write much more than ratelimit_pages
> before calling balance_dirty_pages_ratelimited_nr(). So it is safer to
> determine number to write based on real number of dirtied pages.
> 
> The increased write_chunk may make the dirtier more bumpy.  This is
> filesystem writers' duty not to dirty too much at a time without
> checking the ratelimit.
  I don't get this. balance_dirty_pages_ratelimited_nr() is called when we
dirty the page, not when we write it out. So a problem would only happen if
filesystem dirties pages by set_page_dirty() and won't call
balance_dirty_pages_ratelimited_nr(). But e.g. generic_perform_write()
and do_wp_page() takes care of that. So where's the problem?

								Honza
> ---
>  mm/page-writeback.c |   13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> --- linux.orig/mm/page-writeback.c	2009-09-09 21:19:21.000000000 +0800
> +++ linux/mm/page-writeback.c	2009-09-09 21:25:45.000000000 +0800
> @@ -44,12 +44,12 @@ static long ratelimit_pages = 32;
>  /*
>   * When balance_dirty_pages decides that the caller needs to perform some
>   * non-background writeback, this is how many pages it will attempt to write.
> - * It should be somewhat larger than RATELIMIT_PAGES to ensure that reasonably
> + * It should be somewhat larger than dirtied pages to ensure that reasonably
>   * large amounts of I/O are submitted.
>   */
> -static inline long sync_writeback_pages(void)
> +static inline long sync_writeback_pages(unsigned long dirtied)
>  {
> -	return ratelimit_pages + ratelimit_pages / 2;
> +	return dirtied + dirtied / 2;
>  }
>  
>  /* The following parameters are exported via /proc/sys/vm */
> @@ -476,7 +476,8 @@ get_dirty_limits(unsigned long *pbackgro
>   * If we're over `background_thresh' then pdflush is woken to perform some
>   * writeout.
>   */
> -static void balance_dirty_pages(struct address_space *mapping)
> +static void balance_dirty_pages(struct address_space *mapping,
> +				unsigned long write_chunk)
>  {
>  	long nr_reclaimable, bdi_nr_reclaimable;
>  	long nr_writeback, bdi_nr_writeback;
> @@ -484,7 +485,6 @@ static void balance_dirty_pages(struct a
>  	unsigned long dirty_thresh;
>  	unsigned long bdi_thresh;
>  	unsigned long pages_written = 0;
> -	unsigned long write_chunk = sync_writeback_pages();
>  
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  
> @@ -638,9 +638,10 @@ void balance_dirty_pages_ratelimited_nr(
>  	p =  &__get_cpu_var(bdp_ratelimits);
>  	*p += nr_pages_dirtied;
>  	if (unlikely(*p >= ratelimit)) {
> +		ratelimit = sync_writeback_pages(*p);
>  		*p = 0;
>  		preempt_enable();
> -		balance_dirty_pages(mapping);
> +		balance_dirty_pages(mapping, ratelimit);
>  		return;
>  	}
>  	preempt_enable();
> 
> -- 
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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