Re: [PATCH 2/6] writeback: reduce calls to global_page_state in balance_dirty_pages()

[Jan Kara <jack@suse.cz>]

On Sun 11-07-10 10:06:58, Wu Fengguang wrote:
> Reducing the number of times balance_dirty_pages calls global_page_state
> reduces the cache references and so improves write performance on a
> variety of workloads.
> 
> 'perf stats' of simple fio write tests shows the reduction in cache
> access.  Where the test is fio 'write,mmap,600Mb,pre_read' on AMD
> AthlonX2 with 3Gb memory (dirty_threshold approx 600 Mb) running each
> test 10 times, dropping the fasted & slowest values then taking the
> average & standard deviation
> 
> 		average (s.d.) in millions (10^6)
> 2.6.31-rc8	648.6 (14.6)
> +patch		620.1 (16.5)
> 
> Achieving this reduction is by dropping clip_bdi_dirty_limit as it
> rereads the counters to apply the dirty_threshold and moving this check
> up into balance_dirty_pages where it has already read the counters.
> 
> Also by rearrange the for loop to only contain one copy of the limit
> tests allows the pdflush test after the loop to use the local copies of
> the counters rather than rereading them.
> 
> In the common case with no throttling it now calls global_page_state 5
> fewer times and bdi_stat 2 fewer.
> 
> Fengguang:
> 
> This patch slightly changes behavior by replacing clip_bdi_dirty_limit()
> with the explicit check (nr_reclaimable + nr_writeback >= dirty_thresh)
> to avoid exceeding the dirty limit. Since the bdi dirty limit is mostly
> accurate we don't need to do routinely clip. A simple dirty limit check
> would be enough.
> 
> The check is necessary because, in principle we should throttle
> everything calling balance_dirty_pages() when we're over the total
> limit, as said by Peter.
> 
> We now set and clear dirty_exceeded not only based on bdi dirty limits,
> but also on the global dirty limits. This is a bit counterintuitive, but
> the global limits are the ultimate goal and shall be always imposed.
  Thinking about this again - what you did is rather big change for systems
with more active BDIs. For example if I have two disks sda and sdb and
write for some time to sda, then dirty limit for sdb gets scaled down.
So when we start writing to sbd we'll heavily throttle the threads until
the dirty limit for sdb ramps up regardless of how far are we to reach the
global limit...

> We may now start background writeback work based on outdated conditions.
> That's safe because the bdi flush thread will (and have to) double check
> the states. It reduces overall overheads because the test based on old
> states still have good chance to be right.

									Honza
> 
> CC: Jan Kara <jack@suse.cz>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/page-writeback.c |   95 ++++++++++++++----------------------------
>  1 file changed, 33 insertions(+), 62 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2010-07-11 08:42:14.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-07-11 08:44:49.000000000 +0800
> @@ -253,32 +253,6 @@ static void bdi_writeout_fraction(struct
>  	}
>  }
>  
> -/*
> - * Clip the earned share of dirty pages to that which is actually available.
> - * This avoids exceeding the total dirty_limit when the floating averages
> - * fluctuate too quickly.
> - */
> -static void clip_bdi_dirty_limit(struct backing_dev_info *bdi,
> -		unsigned long dirty, unsigned long *pbdi_dirty)
> -{
> -	unsigned long avail_dirty;
> -
> -	avail_dirty = global_page_state(NR_FILE_DIRTY) +
> -		 global_page_state(NR_WRITEBACK) +
> -		 global_page_state(NR_UNSTABLE_NFS) +
> -		 global_page_state(NR_WRITEBACK_TEMP);
> -
> -	if (avail_dirty < dirty)
> -		avail_dirty = dirty - avail_dirty;
> -	else
> -		avail_dirty = 0;
> -
> -	avail_dirty += bdi_stat(bdi, BDI_RECLAIMABLE) +
> -		bdi_stat(bdi, BDI_WRITEBACK);
> -
> -	*pbdi_dirty = min(*pbdi_dirty, avail_dirty);
> -}
> -
>  static inline void task_dirties_fraction(struct task_struct *tsk,
>  		long *numerator, long *denominator)
>  {
> @@ -469,7 +443,6 @@ get_dirty_limits(unsigned long *pbackgro
>  			bdi_dirty = dirty * bdi->max_ratio / 100;
>  
>  		*pbdi_dirty = bdi_dirty;
> -		clip_bdi_dirty_limit(bdi, dirty, pbdi_dirty);
>  		task_dirty_limit(current, pbdi_dirty);
>  	}
>  }
> @@ -491,7 +464,7 @@ static void balance_dirty_pages(struct a
>  	unsigned long bdi_thresh;
>  	unsigned long pages_written = 0;
>  	unsigned long pause = 1;
> -
> +	int dirty_exceeded;
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  
>  	for (;;) {
> @@ -510,10 +483,35 @@ static void balance_dirty_pages(struct a
>  		nr_writeback = global_page_state(NR_WRITEBACK) +
>  			       global_page_state(NR_WRITEBACK_TEMP);
>  
> -		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> -		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> +		/*
> +		 * In order to avoid the stacked BDI deadlock we need
> +		 * to ensure we accurately count the 'dirty' pages when
> +		 * the threshold is low.
> +		 *
> +		 * Otherwise it would be possible to get thresh+n pages
> +		 * reported dirty, even though there are thresh-m pages
> +		 * actually dirty; with m+n sitting in the percpu
> +		 * deltas.
> +		 */
> +		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
> +			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> +			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
> +		} else {
> +			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> +			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> +		}
> +
> +		/*
> +		 * The bdi thresh is somehow "soft" limit derived from the
> +		 * global "hard" limit. The former helps to prevent heavy IO
> +		 * bdi or process from holding back light ones; The latter is
> +		 * the last resort safeguard.
> +		 */
> +		dirty_exceeded =
> +			(bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
> +			|| (nr_reclaimable + nr_writeback >= dirty_thresh);
>  
> -		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> +		if (!dirty_exceeded)
>  			break;
>  
>  		/*
> @@ -541,34 +539,10 @@ static void balance_dirty_pages(struct a
>  		if (bdi_nr_reclaimable > bdi_thresh) {
>  			writeback_inodes_wb(&bdi->wb, &wbc);
>  			pages_written += write_chunk - wbc.nr_to_write;
> -			get_dirty_limits(&background_thresh, &dirty_thresh,
> -				       &bdi_thresh, bdi);
>  			trace_wbc_balance_dirty_written(&wbc, bdi);
> +			if (pages_written >= write_chunk)
> +				break;		/* We've done our duty */
>  		}
> -
> -		/*
> -		 * In order to avoid the stacked BDI deadlock we need
> -		 * to ensure we accurately count the 'dirty' pages when
> -		 * the threshold is low.
> -		 *
> -		 * Otherwise it would be possible to get thresh+n pages
> -		 * reported dirty, even though there are thresh-m pages
> -		 * actually dirty; with m+n sitting in the percpu
> -		 * deltas.
> -		 */
> -		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
> -			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> -			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
> -		} else if (bdi_nr_reclaimable) {
> -			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> -			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> -		}
> -
> -		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> -			break;
> -		if (pages_written >= write_chunk)
> -			break;		/* We've done our duty */
> -
>  		trace_wbc_balance_dirty_wait(&wbc, bdi);
>  		__set_current_state(TASK_INTERRUPTIBLE);
>  		io_schedule_timeout(pause);
> @@ -582,8 +556,7 @@ static void balance_dirty_pages(struct a
>  			pause = HZ / 10;
>  	}
>  
> -	if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
> -			bdi->dirty_exceeded)
> +	if (!dirty_exceeded && bdi->dirty_exceeded)
>  		bdi->dirty_exceeded = 0;
>  
>  	if (writeback_in_progress(bdi))
> @@ -598,9 +571,7 @@ static void balance_dirty_pages(struct a
>  	 * background_thresh, to keep the amount of dirty memory low.
>  	 */
>  	if ((laptop_mode && pages_written) ||
> -	    (!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
> -			       + global_page_state(NR_UNSTABLE_NFS))
> -					  > background_thresh)))
> +	    (!laptop_mode && (nr_reclaimable > background_thresh)))
>  		bdi_start_background_writeback(bdi);
>  }
>  
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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