linux-kernel - Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands during checkpoint

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Date:   Tue, 4 Jul 2023 22:58:33 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Daejun Park <daejun7.park@...sung.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint

On 2023/7/4 18:53, Jaegeuk Kim wrote:
> On 07/03, Chao Yu wrote:
>> On 2023/6/15 0:10, Jaegeuk Kim wrote:
>>> If there're huge # of small discards, this will increase checkpoint latency
>>> insanely. Let's issue small discards only by trim.
>>>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>> ---
>>>
>>>    Change log from v1:
>>>     - move the skip logic to avoid dangling objects
>>>
>>>    fs/f2fs/segment.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index 8c7af8b4fc47..0457d620011f 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -2193,7 +2193,7 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>>>    			len = next_pos - cur_pos;
>>>    			if (f2fs_sb_has_blkzoned(sbi) ||
>>> -			    (force && len < cpc->trim_minlen))
>>> +					!force || len < cpc->trim_minlen)
>>>    				goto skip;
>>
>> Sorry for late reply.
>>
>> We have a configuration for such case, what do you think of setting
>> max_small_discards to zero? otherwise, w/ above change, max_small_discards
>> logic may be broken?
>>
>> What:           /sys/fs/f2fs/<disk>/max_small_discards
>> Date:           November 2013
>> Contact:        "Jaegeuk Kim" <jaegeuk.kim@...sung.com>
>> Description:    Controls the issue rate of discard commands that consist of small
>>                  blocks less than 2MB. The candidates to be discarded are cached until
>>                  checkpoint is triggered, and issued during the checkpoint.
>>                  By default, it is disabled with 0.
>>
>> Or, if we prefer to disable small_discards by default, what about below change:
> 
> I think small_discards is fine, but need to avoid long checkpoint latency only.

I didn't get you, do you mean we can still issue small discard by
fstrim, so small_discards functionality is fine?

Thanks,

> 
>>
>>  From eb89d9b56e817e3046d7fa17165b12416f09d456 Mon Sep 17 00:00:00 2001
>> From: Chao Yu <chao@...nel.org>
>> Date: Mon, 3 Jul 2023 09:06:53 +0800
>> Subject: [PATCH] Revert "f2fs: enable small discard by default"
>>
>> This reverts commit d618ebaf0aa83d175658aea5291e0c459d471d39 in order
>> to disable small discard by default, so that if there're huge number of
>> small discards, it will decrease checkpoint's latency obviously.
>>
>> Also, this patch reverts 9ac00e7cef10 ("f2fs: do not issue small discard
>> commands during checkpoint"), due to it breaks small discard feature which
>> may be configured via sysfs entry max_small_discards.
>>
>> Fixes: 9ac00e7cef10 ("f2fs: do not issue small discard commands during checkpoint")
>> Signed-off-by: Chao Yu <chao@...nel.org>
>> ---
>>   fs/f2fs/segment.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>> index 14c822e5c9c9..0a313368f18b 100644
>> --- a/fs/f2fs/segment.c
>> +++ b/fs/f2fs/segment.c
>> @@ -2193,7 +2193,7 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>>   			len = next_pos - cur_pos;
>>
>>   			if (f2fs_sb_has_blkzoned(sbi) ||
>> -					!force || len < cpc->trim_minlen)
>> +			    (force && len < cpc->trim_minlen))
>>   				goto skip;
>>
>>   			f2fs_issue_discard(sbi, entry->start_blkaddr + cur_pos,
>> @@ -2269,7 +2269,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
>>   	atomic_set(&dcc->queued_discard, 0);
>>   	atomic_set(&dcc->discard_cmd_cnt, 0);
>>   	dcc->nr_discards = 0;
>> -	dcc->max_discards = MAIN_SEGS(sbi) << sbi->log_blocks_per_seg;
>> +	dcc->max_discards = 0;
>>   	dcc->max_discard_request = DEF_MAX_DISCARD_REQUEST;
>>   	dcc->min_discard_issue_time = DEF_MIN_DISCARD_ISSUE_TIME;
>>   	dcc->mid_discard_issue_time = DEF_MID_DISCARD_ISSUE_TIME;
>> -- 
>> 2.40.1
>>
>>
>>
>>>    			f2fs_issue_discard(sbi, entry->start_blkaddr + cur_pos,