RE: [f2fs-dev] [PATCH] f2fs: reduce lock overhead of extent node releasing

[Chao Yu]

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> Sent: Saturday, July 04, 2015 3:04 PM
> To: Chao Yu
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [f2fs-dev] [PATCH] f2fs: reduce lock overhead of extent node releasing
> 
> On Thu, Jul 02, 2015 at 08:40:12PM +0800, Chao Yu wrote:
> > >From e5c6600d01c4462c4e1ee0c70ec1d9319862077d Mon Sep 17 00:00:00 2001
> > From: Chao Yu <chao2.yu@xxxxxxxxxxx>
> > Date: Thu, 2 Jul 2015 18:52:46 +0800
> > Subject: [PATCH] f2fs: reduce lock overhead of extent node releasing
> >
> > Open and close critical section for each extent node when traversing rb-tree
> > results in high overhead of cpu, slows thing down.
> >
> > This patch alternates to batch mode for removing extent nodes under spin lock.
> >
> > Signed-off-by: Chao Yu <chao2.yu@xxxxxxxxxxx>
> > ---
> >  fs/f2fs/data.c | 28 ++++++++++++++++++++--------
> >  1 file changed, 20 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index 6a706dd..7fb56a0 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -441,19 +441,31 @@ static unsigned int __free_extent_tree(struct f2fs_sb_info *sbi,
> >  	struct extent_node *en;
> >  	unsigned int count = et->count;
> >
> > -	node = rb_first(&et->root);
> > -	while (node) {
> > -		next = rb_next(node);
> > -		en = rb_entry(node, struct extent_node, rb_node);
> > +	if (!et->count)
> > +		return 0;
> > +
> > +	/* 1. remove all extent nodes in global lru list */
> > +	if (free_all) {
> > +		spin_lock(&sbi->extent_lock);
> > +		node = rb_first(&et->root);
> > +		while (node) {
> > +			next = rb_next(node);
> > +			en = rb_entry(node, struct extent_node, rb_node);
> >
> > -		if (free_all) {
> > -			spin_lock(&sbi->extent_lock);
> >  			if (!list_empty(&en->list))
> >  				list_del_init(&en->list);
> > -			spin_unlock(&sbi->extent_lock);
> > +			node = next;
> >  		}
> > +		spin_unlock(&sbi->extent_lock);
> > +	}
> > +
> > +	/* 2. release all extent nodes which are not in global lru list */
> 
> Hmm,
> Is there any overhead to traverse the rb_tree twice and 

Yes, it will cost us more time to traverse the rb tree twice, adding one more
shrink list may reduce the overhead.

> any spin_lock delay caused by contention?

Maybe, since our critical region is enlarged, but the overhead of operation
for re-entering the critical region will be decreased as we invoke spin_lock
only one time.

Thanks,

> 
> Thanks,
> 
> > +	node = rb_first(&et->root);
> > +	while (node) {
> > +		next = rb_next(node);
> > +		en = rb_entry(node, struct extent_node, rb_node);
> >
> > -		if (free_all || list_empty(&en->list)) {
> > +		if (list_empty(&en->list)) {
> >  			__detach_extent_node(sbi, et, en);
> >  			kmem_cache_free(extent_node_slab, en);
> >  		}
> > --
> > 2.4.2
> 
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