Re: [PATCH 01/17] writeback: remove the internal 5% low bound on dirty_ratio

[Mel Gorman <mel@csn.ul.ie>]

On Sun, Sep 12, 2010 at 11:49:46PM +0800, Wu Fengguang wrote:
> The dirty_ratio was siliently limited in global_dirty_limits() to >= 5%.
> This is not a user expected behavior. And it's inconsistent with
> calc_period_shift(), which uses the plain vm_dirty_ratio value.
> 
> Let's rip the arbitrary internal bound. It may impact some very weird
> user space applications. However we are going to dynamicly sizing the
> dirty limits anyway, which may well break such applications, too.
> 
> At the same time, fix balance_dirty_pages() to work with the
> dirty_thresh=0 case. This allows applications to proceed when
> dirty+writeback pages are all cleaned.
> 
> And ">" fits with the name "exceeded" better than ">=" does. Neil
> think it is an aesthetic improvement as well as a functional one :)
> 
> CC: Jan Kara <jack@suse.cz>
> Proposed-by: Con Kolivas <kernel@kolivas.org>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: Neil Brown <neilb@suse.de>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  fs/fs-writeback.c   |    2 +-
>  mm/page-writeback.c |   16 +++++-----------
>  2 files changed, 6 insertions(+), 12 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2010-08-29 08:10:30.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-08-29 08:12:08.000000000 +0800
> @@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long *
>  
>  	if (vm_dirty_bytes)
>  		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
> -	else {
> -		int dirty_ratio;
> -
> -		dirty_ratio = vm_dirty_ratio;
> -		if (dirty_ratio < 5)
> -			dirty_ratio = 5;
> -		dirty = (dirty_ratio * available_memory) / 100;
> -	}
> +	else
> +		dirty = (vm_dirty_ratio * available_memory) / 100;
>  

What kernel is this? In a recent mainline kernel and on linux-next, this
is

dirty = (dirty_ratio * available_memory) / 100;

i.e. * instead of +. With +, the value for dirty is almost always going
to be simply 1%.

>  	if (dirty_background_bytes)
>  		background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
> @@ -510,7 +504,7 @@ static void balance_dirty_pages(struct a
>  		 * catch-up. This avoids (excessively) small writeouts
>  		 * when the bdi limits are ramping up.
>  		 */
> -		if (nr_reclaimable + nr_writeback <
> +		if (nr_reclaimable + nr_writeback <=
>  				(background_thresh + dirty_thresh) / 2)
>  			break;
>  
> @@ -542,8 +536,8 @@ static void balance_dirty_pages(struct a
>  		 * the last resort safeguard.
>  		 */
>  		dirty_exceeded =
> -			(bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
> -			|| (nr_reclaimable + nr_writeback >= dirty_thresh);
> +			(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
> +			|| (nr_reclaimable + nr_writeback > dirty_thresh);
>  
>  		if (!dirty_exceeded)
>  			break;
> --- linux-next.orig/fs/fs-writeback.c	2010-08-29 08:12:51.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-08-29 08:12:53.000000000 +0800
> @@ -574,7 +574,7 @@ static inline bool over_bground_thresh(v
>  	global_dirty_limits(&background_thresh, &dirty_thresh);
>  
>  	return (global_page_state(NR_FILE_DIRTY) +
> -		global_page_state(NR_UNSTABLE_NFS) >= background_thresh);
> +		global_page_state(NR_UNSTABLE_NFS) > background_thresh);
>  }
>  
>  /*
> 
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab