linux-kernel - Re: [f2fs-dev] [PATCH 3/3] f2fs: skip migration only when BG

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Message-ID: <20200218232714.GB10213@google.com>
Date:   Tue, 18 Feb 2020 15:27:14 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 3/3] f2fs: skip migration only when BG_GC is
 called

On 02/17, Chao Yu wrote:
> On 2020/2/15 2:58, Jaegeuk Kim wrote:
> > FG_GC needs to move entire section more quickly.
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >  fs/f2fs/gc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index bbf4db3f6bb4..1676eebc8c8b 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -1203,7 +1203,7 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
> >  
> >  		if (get_valid_blocks(sbi, segno, false) == 0)
> >  			goto freed;
> > -		if (__is_large_section(sbi) &&
> > +		if (gc_type == BG_GC && __is_large_section(sbi) &&
> >  				migrated >= sbi->migration_granularity)
> 
> I knew migrating one large section is a more efficient way, but this can
> increase long-tail latency of f2fs_balance_fs() occasionally, especially in
> extreme fragmented space.

FG_GC requires to wait for whole section migration which shows the entire
latency.

> 
> Thanks,
> 
> >  			goto skip;
> >  		if (!PageUptodate(sum_page) || unlikely(f2fs_cp_error(sbi)))
> >