Re: [PATCH 06/17] writeback: sync expired inodes first in background writeback

[Jan Kara <jack@suse.cz>]

On Fri 06-05-11 11:08:27, Wu Fengguang wrote:
> A background flush work may run for ever. So it's reasonable for it to
> mimic the kupdate behavior of syncing old/expired inodes first.
> 
> At each queue_io() time, first try enqueuing only newly expired inodes.
> If there are zero expired inodes to work with, then relax the rule and
> enqueue all dirty inodes.
> 
> This will help reduce the number of dirty pages encountered by page
> reclaim, eg. the pageout() calls. Normally older inodes contain older
> dirty pages, which are more close to the end of the LRU lists. So
> syncing older inodes first helps reducing the dirty pages reached by
> the page reclaim code.
> 
> More background: as Mel put it, "it makes sense to write old pages first
> to reduce the chances page reclaim is initiating IO."
> 
> Rik also presented the situation with a graph:
> 
> LRU head                                 [*] dirty page
> [                          *              *      * *  *  * * * * * *]
> 
> Ideally, most dirty pages should lie close to the LRU tail instead of
> LRU head. That requires the flusher thread to sync old/expired inodes
> first (as there are obvious correlations between inode age and page
> age), and to give fair opportunities to newly expired inodes rather
> than sticking with some large eldest inodes (as larger inodes have
> weaker correlations in the inode<=>page ages).
> 
> This patch helps the flusher to meet both the above requirements.
> 
> Side effects: it might reduce the batch size and hence reduce
> inode_wb_list_lock hold time, but in turn make the cluster-by-partition
> logic in the same function less effective on reducing disk seeks.
> 
> v2: keep policy changes inside wb_writeback() and keep the
> wbc.older_than_this visibility as suggested by Dave.
  The age of pages in LRU is not necessarily related with the
i_dirtied_when time stamp so I'm not sure how much this will help after all
but it makes some sense from the data integrity point of view at least. You
can add:
Acked-by: Jan Kara <jack@suse.cz>

								Honza
> 
> CC: Dave Chinner <david@fromorbit.com>
> CC: Jan Kara <jack@suse.cz>
> CC: Rik van Riel <riel@redhat.com>
> Acked-by: Mel Gorman <mel@csn.ul.ie>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  fs/fs-writeback.c |   16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2011-05-05 23:30:25.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2011-05-05 23:30:26.000000000 +0800
> @@ -718,7 +718,7 @@ static long wb_writeback(struct bdi_writ
>  		if (work->for_background && !over_bground_thresh())
>  			break;
>  
> -		if (work->for_kupdate) {
> +		if (work->for_kupdate || work->for_background) {
>  			oldest_jif = jiffies -
>  				msecs_to_jiffies(dirty_expire_interval * 10);
>  			wbc.older_than_this = &oldest_jif;
> @@ -729,6 +729,7 @@ static long wb_writeback(struct bdi_writ
>  		wbc.pages_skipped = 0;
>  		wbc.inodes_cleaned = 0;
>  
> +retry:
>  		trace_wbc_writeback_start(&wbc, wb->bdi);
>  		if (work->sb)
>  			__writeback_inodes_sb(work->sb, wb, &wbc);
> @@ -752,6 +753,19 @@ static long wb_writeback(struct bdi_writ
>  		if (wbc.inodes_cleaned)
>  			continue;
>  		/*
> +		 * background writeback will start with expired inodes, and
> +		 * if none is found, fallback to all inodes. This order helps
> +		 * reduce the number of dirty pages reaching the end of LRU
> +		 * lists and cause trouble to the page reclaim.
> +		 */
> +		if (work->for_background &&
> +		    wbc.older_than_this &&
> +		    list_empty(&wb->b_io) &&
> +		    list_empty(&wb->b_more_io)) {
> +			wbc.older_than_this = NULL;
> +			goto retry;
> +		}
> +		/*
>  		 * No more inodes for IO, bail
>  		 */
>  		if (!wbc.more_io)
> 
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR