Re: [PATCH 4/6] writeback: sync expired inodes first in background writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Jul 26, 2010 at 08:59:55PM +0800, Mel Gorman wrote:
> On Mon, Jul 26, 2010 at 08:56:35PM +0800, Wu Fengguang wrote:
> > > > @@ -232,8 +232,15 @@ static void move_expired_inodes(struct l
> > > >  	while (!list_empty(delaying_queue)) {
> > > >  		inode = list_entry(delaying_queue->prev, struct inode, i_list);
> > > >  		if (expire_interval &&
> > > > -		    inode_dirtied_after(inode, older_than_this))
> > > > -			break;
> > > > +		    inode_dirtied_after(inode, older_than_this)) {
> > > > +			if (wbc->for_background &&
> > > > +			    list_empty(dispatch_queue) && list_empty(&tmp)) {
> > > > +				expire_interval >>= 1;
> > > > +				older_than_this = jiffies - expire_interval;
> > > > +				continue;
> > > > +			} else
> > > > +				break;
> > > > +		}
> > > 
> > > This needs a comment.
> > > 
> > > I think what it is saying is that if background flush is active but no
> > > inodes are old enough, consider newer inodes. This is on the assumption
> > > that page reclaim has encountered dirty pages and the dirty inodes are
> > > still too young.
> > 
> > Yes this should be commented. How about this one?
> > 
> > @@ -232,8 +232,20 @@ static void move_expired_inodes(struct l
> >         while (!list_empty(delaying_queue)) {
> >                 inode = list_entry(delaying_queue->prev, struct inode, i_list);
> >                 if (expire_interval &&
> > -                   inode_dirtied_after(inode, older_than_this))
> > +                   inode_dirtied_after(inode, older_than_this)) {
> > +                       /*
> > +                        * background writeback will start with expired inodes,
> > +                        * and then fresh inodes. This order helps reducing
> > +                        * the number of dirty pages reaching the end of LRU
> > +                        * lists and cause trouble to the page reclaim.
> > +                        */
> 
> s/reducing/reduce/
> 
> Otherwise, it's enough detail to know what is going on. Thanks

Thanks. Here is the updated patch.
---
Subject: writeback: sync expired inodes first in background writeback
From: Wu Fengguang <fengguang.wu@intel.com>
Date: Wed Jul 21 20:11:53 CST 2010

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.

The policy is
- enqueue all newly expired inodes at each queue_io() time
- enqueue all dirty inodes if there are no more expired inodes to sync

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.

CC: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c |   23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

--- linux-next.orig/fs/fs-writeback.c	2010-07-26 20:19:01.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2010-07-26 21:10:42.000000000 +0800
@@ -217,14 +217,14 @@ static void move_expired_inodes(struct l
 				struct writeback_control *wbc)
 {
 	unsigned long expire_interval = 0;
-	unsigned long older_than_this;
+	unsigned long older_than_this = 0; /* reset to kill gcc warning */
 	LIST_HEAD(tmp);
 	struct list_head *pos, *node;
 	struct super_block *sb = NULL;
 	struct inode *inode;
 	int do_sb_sort = 0;
 
-	if (wbc->for_kupdate) {
+	if (wbc->for_kupdate || wbc->for_background) {
 		expire_interval = msecs_to_jiffies(dirty_expire_interval * 10);
 		older_than_this = jiffies - expire_interval;
 	}
@@ -232,8 +232,20 @@ static void move_expired_inodes(struct l
 	while (!list_empty(delaying_queue)) {
 		inode = list_entry(delaying_queue->prev, struct inode, i_list);
 		if (expire_interval &&
-		    inode_dirtied_after(inode, older_than_this))
+		    inode_dirtied_after(inode, older_than_this)) {
+			/*
+			 * background writeback will start with expired inodes,
+			 * and then fresh inodes. This order helps reduce the
+			 * number of dirty pages reaching the end of LRU lists
+			 * and cause trouble to the page reclaim.
+			 */
+			if (wbc->for_background &&
+			    list_empty(dispatch_queue) && list_empty(&tmp)) {
+				expire_interval = 0;
+				continue;
+			}
 			break;
+		}
 		if (sb && sb != inode->i_sb)
 			do_sb_sort = 1;
 		sb = inode->i_sb;
@@ -521,7 +533,8 @@ void writeback_inodes_wb(struct bdi_writ
 
 	wbc->wb_start = jiffies; /* livelock avoidance */
 	spin_lock(&inode_lock);
-	if (!wbc->for_kupdate || list_empty(&wb->b_io))
+
+	if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
 		queue_io(wb, wbc);
 
 	while (!list_empty(&wb->b_io)) {
@@ -550,7 +563,7 @@ static void __writeback_inodes_sb(struct
 
 	wbc->wb_start = jiffies; /* livelock avoidance */
 	spin_lock(&inode_lock);
-	if (!wbc->for_kupdate || list_empty(&wb->b_io))
+	if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
 		queue_io(wb, wbc);
 	writeback_sb_inodes(sb, wb, wbc, true);
 	spin_unlock(&inode_lock);

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