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Date:   Tue, 11 Jul 2023 16:00:25 +0800
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Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint
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From:   Chao Yu <chao@kernel.org>
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
References: <20230613203947.2745943-1-jaegeuk@kernel.org>
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On 2023/7/6 8:46, Chao Yu wrote:
> On 2023/7/6 1:30, Jaegeuk Kim wrote:
>> On 07/04, Chao Yu wrote:
>>> 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@kernel.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@samsung.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?
>>
>> You got the point.
> 
> Well, actually, what I mean is max_small_discards sysfs entry's functionality
> is broken. Now, the entry can not be used to control number of small discards
> committed by checkpoint.
> 
> I think there is another way to achieve "avoid long checkpoint latency caused
> by committing huge # of small discards", the way is we can set max_small_discards
> to small value or zero, w/ such configuration, it will take checkpoint much less
> time or no time to committing small discard due to below control logic:

Jaegeuk, any comments?

Thanks,

> 
> f2fs_flush_sit_entries()
> {
> ...
>              if (!(cpc->reason & CP_DISCARD)) {
>                  cpc->trim_start = segno;
>                  add_discard_addrs(sbi, cpc, false);
>              }
> ...
> }
> 
> add_discard_addrs()
> {
> ...
>      while (force || SM_I(sbi)->dcc_info->nr_discards <=
>                  SM_I(sbi)->dcc_info->max_discards) {
> 
> It will break the loop once nr_discards is larger than max_discards, if
> max_discards is set to zero, checkpoint won't take time to handle small discards.
> 
> ...
>          if (!de) {
>              de = f2fs_kmem_cache_alloc(discard_entry_slab,
>                          GFP_F2FS_ZERO, true, NULL);
>              de->start_blkaddr = START_BLOCK(sbi, cpc->trim_start);
>              list_add_tail(&de->list, head);
>          }
> ...
>      }
> ...
> 
> Thanks,
> 
>>
>>>
>>> Thanks,
>>>
>>>>
>>>>>
>>>>>   From eb89d9b56e817e3046d7fa17165b12416f09d456 Mon Sep 17 00:00:00 2001
>>>>> From: Chao Yu <chao@kernel.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@kernel.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,
> 
> 
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