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Date: Tue, 11 Jul 2023 16:00:25 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: linux-kernel@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
during checkpoint
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@...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?
>>
>> 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@...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,
>
>
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