Messages in this thread Patch in this message | | | Date | Wed, 12 Jul 2023 09:33:45 +0800 | Subject | Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands during checkpoint | From | Chao Yu <> |
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On 2023/7/12 0:37, Jaegeuk Kim wrote: > On 07/06, 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. > > Could you descrbie this problem first?
Oh, alright, actually, I've described this problem literally, but maybe it's not clear, let me give some examples as below:
echo 0 > /sys/fs/f2fs/vdb/max_small_discards xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync" xfs_io /mnt/f2fs/file -c "fpunch 0 4k" sync cat /proc/fs/f2fs/vdb/discard_plist_info |head -2
echo 100 > /sys/fs/f2fs/vdb/max_small_discards rm /mnt/f2fs/file xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync" xfs_io /mnt/f2fs/file -c "fpunch 0 4k" sync cat /proc/fs/f2fs/vdb/discard_plist_info |head -2
Before the patch:
Discard pend list(Show diacrd_cmd count on each entry, .:not exist): 0 . . . . . . . .
Discard pend list(Show diacrd_cmd count on each entry, .:not exist): 0 3 1 . . . . . .
After the patch: Discard pend list(Show diacrd_cmd count on each entry, .:not exist): 0 . . . . . . . .
Discard pend list(Show diacrd_cmd count on each entry, .:not exist): 0 . . . . . . . .
So, now max_small_discards can not be used to control small discard number cached by checkpoint.
Thanks,
> >> >> 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: >> >> 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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