linux-kernel - Re: [PATCH 7/7] writeback: timestamp based bdi dirty

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Date:	Mon, 20 Jun 2011 16:09:50 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, swhiteho@...hat.com
Subject: Re: [PATCH 7/7] writeback: timestamp based bdi dirty_exceeded state

On Sun, Jun 19, 2011 at 11:01:15PM +0800, Wu Fengguang wrote:
> When there are only one (or several) dirtiers, dirty_exceeded is always
> (or mostly) off. Converting to timestamp avoids this problem. It helps
> to use smaller write_chunk for smoother throttling.

In current mainline gfs2 has grown a non-trivial reference to
backing_dev_info.dirty_exceeded, which needs to be dealt with.

> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
> 
> Before patch, the wait time in balance_dirty_pages() are ~200ms:
> 
> [ 1093.397700] write_bandwidth: comm=swapper pages=1536 time=204ms
> [ 1093.594319] write_bandwidth: comm=swapper pages=1536 time=196ms
> [ 1093.796642] write_bandwidth: comm=swapper pages=1536 time=200ms
> 
> After patch, ~25ms:
> 
> [   90.261339] write_bandwidth: comm=swapper pages=192 time=20ms
> [   90.293168] write_bandwidth: comm=swapper pages=192 time=24ms
> [   90.323853] write_bandwidth: comm=swapper pages=192 time=24ms
> [   90.354510] write_bandwidth: comm=swapper pages=192 time=28ms
> [   90.389890] write_bandwidth: comm=swapper pages=192 time=28ms
> [   90.421787] write_bandwidth: comm=swapper pages=192 time=24ms
> 
>  include/linux/backing-dev.h |    2 +-
>  mm/backing-dev.c            |    2 --
>  mm/page-writeback.c         |   24 ++++++++++++------------
>  3 files changed, 13 insertions(+), 15 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2011-06-19 22:59:49.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-06-19 22:59:53.000000000 +0800
> @@ -483,6 +483,15 @@ unsigned long bdi_dirty_limit(struct bac
>  	return bdi_dirty;
>  }
>  
> +/*
> + * last time exceeded (limit - limit/DIRTY_BRAKE)
> + */
> +static bool dirty_exceeded_recently(struct backing_dev_info *bdi,
> +				    unsigned long time_window)
> +{
> +	return jiffies - bdi->dirty_exceed_time <= time_window;
> +}
> +
>  static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
>  				       unsigned long elapsed,
>  				       unsigned long written)
> @@ -621,7 +630,6 @@ static void balance_dirty_pages(struct a
>  	unsigned long bdi_thresh;
>  	unsigned long pages_written = 0;
>  	unsigned long pause = 1;
> -	bool dirty_exceeded = false;
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  	unsigned long start_time = jiffies;
>  
> @@ -669,14 +677,9 @@ static void balance_dirty_pages(struct a
>  		 * bdi or process from holding back light ones; The latter is
>  		 * the last resort safeguard.
>  		 */
> -		dirty_exceeded = (bdi_dirty > bdi_thresh) ||
> -				  (nr_dirty > dirty_thresh);
> -
> -		if (!dirty_exceeded)
> +		if (bdi_dirty <= bdi_thresh && nr_dirty <= dirty_thresh)
>  			break;
> -
> -		if (!bdi->dirty_exceeded)
> -			bdi->dirty_exceeded = 1;
> +		bdi->dirty_exceed_time = jiffies;
>  
>  		bdi_update_bandwidth(bdi, dirty_thresh, nr_dirty, bdi_dirty,
>  				     start_time);
> @@ -719,9 +722,6 @@ static void balance_dirty_pages(struct a
>  			pause = HZ / 10;
>  	}
>  
> -	if (!dirty_exceeded && bdi->dirty_exceeded)
> -		bdi->dirty_exceeded = 0;
> -
>  	if (writeback_in_progress(bdi))
>  		return;
>  
> @@ -775,7 +775,7 @@ void balance_dirty_pages_ratelimited_nr(
>  		return;
>  
>  	ratelimit = ratelimit_pages;
> -	if (mapping->backing_dev_info->dirty_exceeded)
> +	if (dirty_exceeded_recently(bdi, MAX_PAUSE))
>  		ratelimit = 8;
>  
>  	/*
> --- linux-next.orig/include/linux/backing-dev.h	2011-06-19 22:54:58.000000000 +0800
> +++ linux-next/include/linux/backing-dev.h	2011-06-19 22:59:53.000000000 +0800
> @@ -79,7 +79,7 @@ struct backing_dev_info {
>  	unsigned long avg_write_bandwidth;
>  
>  	struct prop_local_percpu completions;
> -	int dirty_exceeded;
> +	unsigned long dirty_exceed_time;
>  
>  	unsigned int min_ratio;
>  	unsigned int max_ratio, max_prop_frac;
> --- linux-next.orig/mm/backing-dev.c	2011-06-19 22:59:49.000000000 +0800
> +++ linux-next/mm/backing-dev.c	2011-06-19 22:59:53.000000000 +0800
> @@ -670,8 +670,6 @@ int bdi_init(struct backing_dev_info *bd
>  			goto err;
>  	}
>  
> -	bdi->dirty_exceeded = 0;
> -
>  	bdi->bw_time_stamp = jiffies;
>  	bdi->written_stamp = 0;
>  
> 
> 
> --
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