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Subject: Re: [f2fs-dev] [PATCH 3/3] f2fs: skip migration only when BG_GC is
 called
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
References: <20200214185855.217360-1-jaegeuk@kernel.org>
 <20200214185855.217360-3-jaegeuk@kernel.org>
 <9c497f3e-3399-e4a6-f81c-6c4a1f35e5bb@huawei.com>
 <20200218232714.GB10213@google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <117a927f-7128-b5a1-a961-22934bb62ec5@huawei.com>
Date:   Wed, 19 Feb 2020 11:01:21 +0800
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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.

Thanks,

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