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Date:   Tue, 18 Feb 2020 19:04:25 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 3/3] f2fs: skip migration only when BG_GC is
 called
Message-ID: <20200219030425.GA102063@google.com>
References: <20200214185855.217360-1-jaegeuk@kernel.org>
 <20200214185855.217360-3-jaegeuk@kernel.org>
 <9c497f3e-3399-e4a6-f81c-6c4a1f35e5bb@huawei.com>
 <20200218232714.GB10213@google.com>
 <117a927f-7128-b5a1-a961-22934bb62ec5@huawei.com>
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On 02/19, Chao Yu wrote:
> On 2020/2/19 7:27, Jaegeuk Kim wrote:
> > 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@kernel.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.
> 
> That will cause long-tail latency for single f2fs_balance_fs() procedure,
> which it looks a very long hang when userspace call f2fs syscall, so why
> not splitting total elapsed time into several f2fs_balance_fs() to avoid that.

Then, other ops can easily make more dirty segments. The intention of FG_GC is
to block everything and make min. free segments as a best shot.

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