linux-kernel - Re: [PATCH] f2fs: enlarge block plug coverage

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Date:   Sun, 8 Apr 2018 09:29:24 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>, <chao@...nel.org>
Subject: Re: [PATCH] f2fs: enlarge block plug coverage

On 2018/4/5 11:51, Jaegeuk Kim wrote:
> On 04/04, Chao Yu wrote:
>> This patch enlarges block plug coverage in __issue_discard_cmd, in
>> order to collect more pending bios before issuing them, to avoid
>> being disturbed by previous discard I/O in IO aware discard mode.
> 
> Hmm, then we need to wait for huge discard IO for over 10 secs, which

We found that total discard latency is rely on total discard number we issued
last time instead of range or length discard covered. IMO, if we don't change
.max_requests value, we will not suffer longer latency.

> will affect following read/write IOs accordingly. In order to avoid that,
> we actually need to limit the discard size.

If you are worry about I/O interference in between discard and rw, I suggest to
decrease .max_requests value.

Thanks,

> 
> Thanks,
> 
>>
>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>> ---
>>  fs/f2fs/segment.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>> index 8f0b5ba46315..4287e208c040 100644
>> --- a/fs/f2fs/segment.c
>> +++ b/fs/f2fs/segment.c
>> @@ -1208,10 +1208,12 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>  		pend_list = &dcc->pend_list[i];
>>  
>>  		mutex_lock(&dcc->cmd_lock);
>> +
>> +		blk_start_plug(&plug);
>> +
>>  		if (list_empty(pend_list))
>>  			goto next;
>>  		f2fs_bug_on(sbi, !__check_rb_tree_consistence(sbi, &dcc->root));
>> -		blk_start_plug(&plug);
>>  		list_for_each_entry_safe(dc, tmp, pend_list, list) {
>>  			f2fs_bug_on(sbi, dc->state != D_PREP);
>>  
>> @@ -1227,8 +1229,9 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>  			if (++iter >= dpolicy->max_requests)
>>  				break;
>>  		}
>> -		blk_finish_plug(&plug);
>>  next:
>> +		blk_finish_plug(&plug);
>> +
>>  		mutex_unlock(&dcc->cmd_lock);
>>  
>>  		if (iter >= dpolicy->max_requests)
>> -- 
>> 2.15.0.55.gc2ece9dc4de6
> 
> .
>