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

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Date:   Tue, 25 Feb 2020 10:24:17 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
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 2020/2/25 5:53, Jaegeuk Kim wrote:
> 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@...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.
>>>>
>>>> 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).

Oh, if we want to FG_GC only migrate on segment on time, it needs to change
has_not_enough_free_secs() condition as well, so previous logic is broken.

Nvm, please add:

Reviewed-by: Chao Yu <yuchao0@...wei.com>

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

Agreed, I guess SMR scenario may need such policy: higher trigger frequency on bggc
when free space decreases, but less block migration for each bggc cycle.

Thanks,

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