RE:(2) [PATCH v4] f2fs: New victim selection for GC
From: Yonggil Song
Date: Wed Jan 03 2024 - 05:43:53 EST
> On 12/28, Yonggil Song wrote:
> > >From d08b97183bc830779c82b83d94f8b75ad11cb29a Mon Sep 17 00:00:00 2001
> > From: Yonggil Song <yonggil.song@xxxxxxxxxxx>
> > Date: Thu, 7 Dec 2023 16:34:38 +0900
> > Subject: [PATCH v4] f2fs: New victim selection for GC
> >
> > 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@xxxxxxxxxxx>
> > ---
> > fs/f2fs/f2fs.h | 1 +
> > fs/f2fs/gc.c | 99 +++++++++++++++++++++++++++++++++++++++-----------
> > fs/f2fs/gc.h | 6 +++
> > 3 files changed, 85 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 9043cedfa12b..b2c0adfb2704 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 require_node_gc; /* flag for node GC */
> > 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..d8a81a6ed325 100644
> > --- a/fs/f2fs/gc.c
> > +++ b/fs/f2fs/gc.c
> > @@ -341,6 +341,14 @@ static unsigned int get_cb_cost(struct f2fs_sb_info *sbi, unsigned int segno)
> > unsigned int i;
> > unsigned int usable_segs_per_sec = f2fs_usable_segs_in_sec(sbi, segno);
> >
> > + /*
> > + * When BG_GC selects victims based on age, it prevents node victims
> > + * from being selected. This is because node blocks can be invalidated
> > + * by moving data blocks.
> > + */
> > + if (__skip_node_gc(sbi, segno))
> > + return UINT_MAX;
> > +
> > for (i = 0; i < usable_segs_per_sec; i++)
> > mtime += get_seg_entry(sbi, start + i)->mtime;
> > vblocks = get_valid_blocks(sbi, segno, true);
> > @@ -369,10 +377,24 @@ static inline unsigned int get_gc_cost(struct f2fs_sb_info *sbi,
> > return get_seg_entry(sbi, segno)->ckpt_valid_blocks;
> >
> > /* alloc_mode == LFS */
> > - if (p->gc_mode == GC_GREEDY)
> > - return get_valid_blocks(sbi, segno, true);
> > - else if (p->gc_mode == GC_CB)
> > + if (p->gc_mode == GC_GREEDY) {
> > + /*
> > + * If the data block that the node block pointed to is GCed,
> > + * the node block is invalidated. For this reason, we add a
> > + * weight to cost of node victims to give priority to data
> > + * victims during the gc process. However, in a situation
> > + * where we run out of free sections, we remove the weight
> > + * because we need to clean up node blocks.
> > + */
> > + unsigned int cost = get_valid_blocks(sbi, segno, true);
> > +
> > + if (__skip_node_gc(sbi, segno))
> > + return cost +
> > + (sbi->segs_per_sec << sbi->log_blocks_per_seg);
> > + return cost;
>
> Given the comment, can we use a weight instead of cost?
>
> - unsigned int cost = get_valid_blocks(sbi, segno, true);
> + unsigned int weight = 0;
>
> - if (__skip_node_gc(sbi, segno))
> - return cost +
> - (sbi->segs_per_sec << sbi->log_blocks_per_seg);
> - return cost;
> + if (__skip_node_gc(sbi, segno)) {
> + weight = sbi->segs_per_sec << sbi->log_blocks_per_seg;
> +
> + return get_valid_blocks(sbi, segno, true) + weight;
>
I agree with you. It looks better that there is only one return.
> > + } else if (p->gc_mode == GC_CB) {
> > return get_cb_cost(sbi, segno);
> > + }
> >
> > f2fs_bug_on(sbi, 1);
> > return 0;
> > @@ -557,6 +579,14 @@ static void atgc_lookup_victim(struct f2fs_sb_info *sbi,
> > if (ve->mtime >= max_mtime || ve->mtime < min_mtime)
> > goto skip;
> >
> > + /*
> > + * When BG_GC selects victims based on age, it prevents node victims
> > + * from being selected. This is because node blocks can be invalidated
> > + * by moving data blocks.
> > + */
> > + if (__skip_node_gc(sbi, ve->segno))
> > + goto skip;
> > +
> > /* age = 10000 * x% * 60 */
> > age = div64_u64(accu * (max_mtime - ve->mtime), total_time) *
> > age_weight;
> > @@ -913,7 +943,22 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > goto retry;
> > }
> >
> > +
>
> Unnecessary line.
OK. I'll fix it.
>
> > if (p.min_segno != NULL_SEGNO) {
>
> I'm wondering whether we need to modify all the changes below. If we added
> more cost to the node segments, how about just managing require_node_gc in
> the specific cases?
>
What about the following changes?
@@ -944,20 +944,6 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
}
if (p.min_segno != NULL_SEGNO) {
- if (sbi->require_node_gc &&
- 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)) {
- sbi->require_node_gc = false;
- p.min_segno = NULL_SEGNO;
- goto out;
- }
- }
got_it:
*result = (p.min_segno / p.ofs_unit) * p.ofs_unit;
got_result:
@@ -1929,6 +1915,18 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
goto stop;
}
+ if (sbi->require_node_gc &&
+ IS_DATASEG(get_seg_entry(sbi, 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, sec_freed, 0))
+ goto stop;
+ }
+
seg_freed = do_garbage_collect(sbi, segno, &gc_list, gc_type,
gc_control->should_migrate_blocks);
total_freed += seg_freed;
> > + if (sbi->require_node_gc &&
> > + 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.
> > + */
>
> ^-- Wrong indentation.
>
OK. I'll fix it.
> > + if (has_enough_free_secs(sbi, prefree_segments(sbi), 0)) {
> > + sbi->require_node_gc = false;
> > + p.min_segno = NULL_SEGNO;
> > + goto out;
> > + }
> > + }
> > got_it:
> > *result = (p.min_segno / p.ofs_unit) * p.ofs_unit;
> > got_result:
> > @@ -1830,8 +1875,27 @@ 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->require_node_gc = 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;
> >
> > /*
> > @@ -1882,7 +1946,13 @@ 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 require_node_gc flag is set even though there
> > + * are enough free sections, node cleaning will
> > + * continue.
> > + */
> > + if (!sbi->require_node_gc)
> > + goto stop;
> > }
> > if (sbi->skipped_gc_rwsem)
> > skipped_round++;
> > @@ -1897,21 +1967,6 @@ 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 +1975,10 @@ 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->require_node_gc = false;
> > + }
> >
> > trace_f2fs_gc_end(sbi->sb, ret, total_freed, total_sec_freed,
> > get_pages(sbi, F2FS_DIRTY_NODES),
> > diff --git a/fs/f2fs/gc.h b/fs/f2fs/gc.h
> > index 28a00942802c..cd07bf125177 100644
> > --- a/fs/f2fs/gc.h
> > +++ b/fs/f2fs/gc.h
> > @@ -166,3 +166,9 @@ static inline bool has_enough_invalid_blocks(struct f2fs_sb_info *sbi)
> > free_user_blocks(sbi) <
> > limit_free_user_blocks(invalid_user_blocks));
> > }
> > +
> > +static inline bool __skip_node_gc(struct f2fs_sb_info *sbi, unsigned int segno)
> > +{
> > + return (IS_NODESEG(get_seg_entry(sbi, segno)->type) &&
> > + !sbi->require_node_gc);
> > +}
> > --
> > 2.34.1