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

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Date:   Tue, 10 Apr 2018 15:22:47 +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 2018/4/10 12:10, Jaegeuk Kim wrote:
> On 04/10, Chao Yu wrote:
>> On 2018/4/10 2:02, Jaegeuk Kim wrote:
>>> On 04/08, Chao Yu wrote:
>>>> 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.
>>
>> Do you mean limit discard count or discard length?
> 
> Both of them.
> 
>>
>>>>
>>>> If you are worry about I/O interference in between discard and rw, I suggest to
>>>> decrease .max_requests value.
>>>
>>> What do you mean? This will produce more pending requests in the queue?
>>
>> I mean after applying this patch, we can queue more discard IOs in plug inside
>> task, otherwise, previous issued discard in block layer can make is_idle() be false,
>> then it can stop IO awared user to issue pending discard command.
> 
> Then, unplug will issue lots of discard commands, which affects the following rw
> latencies. My preference would be issuing discard commands one by one as much as
> possible.

Hmm.. for you concern, we can turn down IO priority of discard from background?

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> 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
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
> 
> .
>