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Date: Thu, 14 Dec 2023 11:26:26 +0900
From: Yonggil Song <yonggil.song@...sung.com>
To: Jaegeuk Kim <jaegeuk@...nel.org>
CC: "chao@...nel.org" <chao@...nel.org>,
"linux-f2fs-devel@...ts.sourceforge.net"
<linux-f2fs-devel@...ts.sourceforge.net>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Seokhwan Kim <sukka.kim@...sung.com>,
Daejun Park <daejun7.park@...sung.com>,
Siwoo Jung <siu.jung@...sung.com>
Subject: RE:(2) [PATCH v2] f2fs: New victim selection for GC
> On 12/08, Yonggil Song wrote:
> > Overview
> > ========
> >
> > This patch introduces a new way to preference data sections when selecting
> > GC victims. Migration of data blocks causes invalidation of node blocks.
> > Therefore, in situations where GC is frequent, selecting data blocks as
> > victims can reduce unnecessary block migration by invalidating node blocks.
> > For exceptional situations where free sections are insufficient, node blocks
> > are selected as victims instead of data blocks to get extra free sections.
> >
> > Problem
> > =======
> >
> > If the total amount of nodes is larger than the size of one section, nodes
> > occupy multiple sections, and node victims are often selected because the
> > gc cost is lowered by data block migration in GC. Since moving the data
> > section causes frequent node victim selection, victim threshing occurs in
> > the node section. This results in an increase in WAF.
> >
> > Experiment
> > ==========
> >
> > Test environment is as follows.
> >
> > System info
> > - 3.6GHz, 16 core CPU
> > - 36GiB Memory
> > Device info
> > - a conventional null_blk with 228MiB
> > - a sequential null_blk with 4068 zones of 8MiB
> > Format
> > - mkfs.f2fs <conv null_blk> -c <seq null_blk> -m -Z 8 -o 3.89
> > Mount
> > - mount <conv null_blk> <mount point>
> > Fio script
> > - fio --rw=randwrite --bs=4k --ba=4k --filesize=31187m --norandommap --overwrite=1 --name=job1 --filename=./mnt/sustain --io_size=128g
> > WAF calculation
> > - (IOs on conv. null_blk + IOs on seq. null_blk) / random write IOs
> >
> > Conclusion
> > ==========
> >
> > This experiment showed that the WAF was reduced by 29% (18.75 -> 13.3) when
> > the data section was selected first when selecting GC victims. This was
> > achieved by reducing the migration of the node blocks by 69.4%
> > (253,131,743 blks -> 77,463,278 blks). It is possible to achieve low WAF
> > performance with the GC victim selection method in environments where the
> > section size is relatively small.
> >
> > Signed-off-by: Yonggil Song <yonggil.song@...sung.com>
> > ---
> > fs/f2fs/f2fs.h | 1 +
> > fs/f2fs/gc.c | 98 ++++++++++++++++++++++++++++++++++++++------------
> > 2 files changed, 77 insertions(+), 22 deletions(-)
> >
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 9043cedfa12b..578d57f6022f 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -1649,6 +1649,7 @@ struct f2fs_sb_info {
> > struct f2fs_mount_info mount_opt; /* mount options */
> >
> > /* for cleaning operations */
> > + bool need_node_clean; /* need to clean dirty nodes */
> > struct f2fs_rwsem gc_lock; /*
> > * semaphore for GC, avoid
> > * race between GC and GC or CP
> > diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> > index f550cdeaa663..682dcf0de59e 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -368,6 +368,14 @@ static inline unsigned int get_gc_cost(struct f2fs_sb_info *sbi,
> > if (p->alloc_mode == SSR)
> > return get_seg_entry(sbi, segno)->ckpt_valid_blocks;
> >
> > + /*
> > + * If we don't need to clean dirty nodes,
> > + * we just skip node victims.
> > + */
> > + if (IS_NODESEG(get_seg_entry(sbi, segno)->type) &&
> > + !sbi->need_node_clean)
> > + return get_max_cost(sbi, p);
>
> How about differentiating the gc cost between data vs. node by adding some
> weights? By default, data is preferred, while node is better in the worst case?
>
Okay, I will work on v3 with your comments
> > +
> > /* alloc_mode == LFS */
> > if (p->gc_mode == GC_GREEDY)
> > return get_valid_blocks(sbi, segno, true);
> > @@ -557,6 +565,14 @@ static void atgc_lookup_victim(struct f2fs_sb_info *sbi,
> > if (ve->mtime >= max_mtime || ve->mtime < min_mtime)
> > goto skip;
> >
> > + /*
> > + * If we don't need to clean dirty nodes,
> > + * we just skip node victims.
> > + */
> > + if (IS_NODESEG(get_seg_entry(sbi, ve->segno)->type) &&
> > + !sbi->need_node_clean)
> > + goto skip;
> > +
> > /* age = 10000 * x% * 60 */
> > age = div64_u64(accu * (max_mtime - ve->mtime), total_time) *
> > age_weight;
> > @@ -913,7 +929,21 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > goto retry;
> > }
> >
> > +
> > if (p.min_segno != NULL_SEGNO) {
> > + if (sbi->need_node_clean &&
> > + IS_DATASEG(get_seg_entry(sbi, p.min_segno)->type)) {
> > + /*
> > + * we need to clean node sections.
> > + * but, data victim cost is the lowest.
> > + * if free sections are enough, stop cleaning node victim.
> > + * if not, it goes on by GCing data victims.
> > + */
> > + if (has_enough_free_secs(sbi, prefree_segments(sbi), 0)) {
> > + p.min_segno = NULL_SEGNO;
> > + goto out;
> > + }
> > + }
> > got_it:
> > *result = (p.min_segno / p.ofs_unit) * p.ofs_unit;
> > got_result:
> > @@ -1830,8 +1860,27 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > goto stop;
> > }
> >
> > + __get_secs_required(sbi, NULL, &upper_secs, NULL);
> > +
> > + /*
> > + * Write checkpoint to reclaim prefree segments.
> > + * We need more three extra sections for writer's data/node/dentry.
> > + */
> > + if (free_sections(sbi) <= upper_secs + NR_GC_CHECKPOINT_SECS) {
> > + sbi->need_node_clean = true;
> > +
> > + if (prefree_segments(sbi)) {
> > + stat_inc_cp_call_count(sbi, TOTAL_CALL);
> > + ret = f2fs_write_checkpoint(sbi, &cpc);
> > + if (ret)
> > + goto stop;
> > + /* Reset due to checkpoint */
> > + sec_freed = 0;
> > + }
> > + }
> > +
> > /* Let's run FG_GC, if we don't have enough space. */
> > - if (has_not_enough_free_secs(sbi, 0, 0)) {
> > + if (gc_type == BG_GC && has_not_enough_free_secs(sbi, 0, 0)) {
> > gc_type = FG_GC;
> >
> > /*
> > @@ -1858,10 +1907,22 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > ret = __get_victim(sbi, &segno, gc_type);
> > if (ret) {
> > /* allow to search victim from sections has pinned data */
> > - if (ret == -ENODATA && gc_type == FG_GC &&
> > - f2fs_pinned_section_exists(DIRTY_I(sbi))) {
> > - f2fs_unpin_all_sections(sbi, false);
> > - goto retry;
> > + if (ret == -ENODATA && gc_type == FG_GC) {
> > + if (f2fs_pinned_section_exists(DIRTY_I(sbi))) {
> > + f2fs_unpin_all_sections(sbi, false);
> > + goto retry;
> > + }
> > + /*
> > + * If we have no more data victims, let's start to
> > + * clean dirty nodes.
> > + */
> > + if (!sbi->need_node_clean) {
> > + sbi->need_node_clean = true;
> > + goto retry;
> > + }
> > + /* node cleaning is over */
> > + else if (sbi->need_node_clean)
> > + goto stop;
> > }
> > goto stop;
> > }
> > @@ -1882,7 +1943,13 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > if (!gc_control->no_bg_gc &&
> > total_sec_freed < gc_control->nr_free_secs)
> > goto go_gc_more;
> > - goto stop;
> > + /*
> > + * If the need_node_clean flag is set
> > + * even though there are enough free
> > + * sections, node cleaning will continue.
> > + */
> > + if (!sbi->need_node_clean)
> > + goto stop;
> > }
> > if (sbi->skipped_gc_rwsem)
> > skipped_round++;
> > @@ -1897,21 +1964,6 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > goto stop;
> > }
> >
> > - __get_secs_required(sbi, NULL, &upper_secs, NULL);
> > -
> > - /*
> > - * Write checkpoint to reclaim prefree segments.
> > - * We need more three extra sections for writer's data/node/dentry.
> > - */
> > - if (free_sections(sbi) <= upper_secs + NR_GC_CHECKPOINT_SECS &&
> > - prefree_segments(sbi)) {
> > - stat_inc_cp_call_count(sbi, TOTAL_CALL);
> > - ret = f2fs_write_checkpoint(sbi, &cpc);
> > - if (ret)
> > - goto stop;
> > - /* Reset due to checkpoint */
> > - sec_freed = 0;
> > - }
> > go_gc_more:
> > segno = NULL_SEGNO;
> > goto gc_more;
> > @@ -1920,8 +1972,10 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
> > SIT_I(sbi)->last_victim[ALLOC_NEXT] = 0;
> > SIT_I(sbi)->last_victim[FLUSH_DEVICE] = gc_control->victim_segno;
> >
> > - if (gc_type == FG_GC)
> > + if (gc_type == FG_GC) {
> > f2fs_unpin_all_sections(sbi, true);
> > + sbi->need_node_clean = false;
> > + }
> >
> > trace_f2fs_gc_end(sbi->sb, ret, total_freed, total_sec_freed,
> > get_pages(sbi, F2FS_DIRTY_NODES),
> > --
> > 2.34.1
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