Re: [f2fs-dev] [PATCH 3/3] f2fs: skip migration only when BG_GC is called

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 02/19, Chao Yu wrote:
> On 2020/2/19 11:04, Jaegeuk Kim wrote:
> > 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
> 
> Yup, that's a problem, if there are more dirty datas being made, reserved segments
> may be ran out during FG_GC.
> 
> > to block everything and make min. free segments as a best shot.
> 
> I just try to simulate write GC's logic in FTL to mitigate single op's max latency,
> otherwise benchmark looks hang during FG_GC (in a 500mb+ section).

Hmm, I think we may need to think another way like doing BG_GC more aggressively.

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