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

From: Jaegeuk Kim
Date: Fri Jul 21 2023 - 16:24:32 EST

On 07/13, Chao Yu wrote:
> On 2023/7/12 23:55, Jaegeuk Kim wrote:
> > On 07/12, Chao Yu wrote:
> > > 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@xxxxxxxxxx>
> > > > > > > > > > ---
> > > > > > > > > >
> > > > > > > > > > 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@xxxxxxxxxxx>
> > > > > > > > > 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.
>
> Let me explain more:
>
> Previously, we have two mechanisms to cache & submit small discards:
>
> a) set max small discard number in /sys/fs/f2fs/vdb/max_small_discards, and checkpoint
> will cache small discard candidates w/ configured maximum number.
>
> b) call FITRIM ioctl, also, checkpoint in f2fs_trim_fs() will cache small discard
> candidates w/ configured discard granularity, but w/o limitation of number. FSTRIM
> interface is asynchronized, so it won't submit discard directly.
>
> Finally, discard thread will submit them in background periodically.
>
> So what I mean is the mechanism a) is broken, since no matter how we configure the
> sysfs entry /sys/fs/f2fs/vdb/max_small_discards, checkpoint will not cache small
> discard candidates any more.

Ok, it seems what I encountered before was adding this small discard even
after issuing it by checkpoint. Thoughts?

node=0], cp [data=0, node=0, meta=0], app [read=0 (direct=0, buffered=0), mapped=0], compr(buffered=0, mapped=0)], fs [data=0, (gc_data=0, cdata=0), node=0, meta=0]
f2fs_discard-25-752 [003] ..... 9744.173085: f2fs_iostat_latency: dev = (254,51), iotype [peak lat.(ms)/avg lat.(ms)/count], rd_data [0/0/0], rd_node [0/0/0], rd_meta [0/0/0], wr_sync_data [0/0/0], wr_sync_node [0/0/0], wr_sync_meta [0/0/0], wr_async_data [0/0/0], wr_async_node [0/0/0], wr_async_meta [0/0/0]
f2fs_discard-25-752 [003] ..... 9744.173089: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0c6, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173111: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173126: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c116, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173140: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c121, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173154: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c15b, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173169: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c16e, blklen = 0x1
f2fs_discard-25-752 [003] ..... 9744.173183: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c181, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175272: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0a9, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175345: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0c6, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175348: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175352: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c116, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175354: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c121, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175360: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c15b, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175362: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c16e, blklen = 0x1
f2fs_discard-25-752 [004] ..... 9744.175367: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c181, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.228775: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c197, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.228950: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1ad, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.228965: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1b1, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.228994: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1b3, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.229006: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c27a, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.229017: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c2a3, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.229028: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c2ab, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.229041: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c304, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230811: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c197, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230926: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1ad, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230929: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1b1, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230932: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1b3, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230940: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c27a, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230945: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c2a3, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230947: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c2ab, blklen = 0x1
f2fs_discard-25-752 [002] ..... 9744.230952: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c304, blklen = 0x1

>
> So, it needs to fix max_small_discards sysfs functionality? or just drop the
> functionality?
>
> >
> > Since we do not submit small discards anymore during checkpoint. Why not relying
> > on the discard thread to issue them?
>
> Sorry, I'm not sure I get your point, do you mean max_small_discards functionality
> is obsoleted, so it recommended to use fstrim to cache & submit small discards?
>
> Let me know, if I'm missing something or misunderstanding the point.
>
> Thanks,
>
> >
> > >
> > > 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@xxxxxxxxxx>
> > > > > > > > > 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@xxxxxxxxxx>
> > > > > > > > > ---
> > > > > > > > > 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,