RE:(2) (2) [f2fs-dev] [PATCH v1] f2fs: New victim selection for GC

From: Yonggil Song
Date: Tue Nov 28 2023 - 19:56:45 EST

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On 11/20, Yonggil Song wrote:
> > >Hi Yonggil,
> > >
> > >On 10/26, Yonggil Song wrote:
> > >> Overview
> > >> ========
> > >>
> > >> Introduce a new way to select the data section first when selecting a
> > >> victim in foreground GC. This victim selection method works when the
> > >> prefer_data_victim mount option is enabled. If foreground GC migrates only
> > >> data sections and runs out of free sections, it cleans dirty node sections
> > >> to get more 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 foreground 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.
> > >
> > >How does that work w/ ATGC?
> > >
> >
> > Hi jaegeuk.
> >
> > I didn't consider ATGC because this feature is only supported by zoned devices(LFS).
> > I didn't add ATGC exception handling because I'm only enabling this feature when
> > it's a zoned device, but should I?
>
> I'm open to apply this to the existing flow in general. Can you take a look at
> that way?
>

I see. I'll consider applying this feature to the general GC routine, including ATGC.

thanks.

> >
> > >>
> > >> 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 -o prefer_data_victim <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>
> > >> ---
> > >> Documentation/filesystems/f2fs.rst | 3 +
> > >> fs/f2fs/f2fs.h | 2 +
> > >> fs/f2fs/gc.c | 100 +++++++++++++++++++++++------
> > >> fs/f2fs/segment.h | 2 +
> > >> fs/f2fs/super.c | 9 +++
> > >> 5 files changed, 95 insertions(+), 21 deletions(-)
> > >>
> > >> diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
> > >> index d32c6209685d..58e6d001d7ab 100644
> > >> --- a/Documentation/filesystems/f2fs.rst
> > >> +++ b/Documentation/filesystems/f2fs.rst
> > >> @@ -367,6 +367,9 @@ errors=%s Specify f2fs behavior on critical errors. This supports modes:
> > >> pending node write drop keep N/A
> > >> pending meta write keep keep N/A
> > >> ====================== =============== =============== ========
> > >> +prefer_data_victim When selecting victims in foreground GC, victims of data type
> > >> + are prioritized. This option minimizes GC victim threshing
> > >> + in the node section to reduce WAF.
> > >> ======================== ============================================================
> > >>
> > >> Debugfs Entries
> > >> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > >> index 6d688e42d89c..8b31fa2ea09a 100644
> > >> --- a/fs/f2fs/f2fs.h
> > >> +++ b/fs/f2fs/f2fs.h
> > >> @@ -108,6 +108,7 @@ extern const char *f2fs_fault_name[FAULT_MAX];
> > >> #define F2FS_MOUNT_GC_MERGE 0x02000000
> > >> #define F2FS_MOUNT_COMPRESS_CACHE 0x04000000
> > >> #define F2FS_MOUNT_AGE_EXTENT_CACHE 0x08000000
> > >> +#define F2FS_MOUNT_PREFER_DATA_VICTIM 0x10000000
> > >>
> > >> #define F2FS_OPTION(sbi) ((sbi)->mount_opt)
> > >> #define clear_opt(sbi, option) (F2FS_OPTION(sbi).opt &= ~F2FS_MOUNT_##option)
> > >> @@ -1648,6 +1649,7 @@ struct f2fs_sb_info {
> > >> struct f2fs_mount_info mount_opt; /* mount options */
> > >>
> > >> /* for cleaning operations */
> > >> + bool need_node_clean; /* only used for prefer_data_victim */
> > >> 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..8a2da808a5fb 100644
> > >> --- a/fs/f2fs/gc.c
> > >> +++ b/fs/f2fs/gc.c
> > >> @@ -752,6 +752,8 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > >> unsigned int last_segment;
> > >> unsigned int nsearched;
> > >> bool is_atgc;
> > >> + bool is_prefer_data_victim =
> > >> + test_opt(sbi, PREFER_DATA_VICTIM) && gc_type == FG_GC;
> > >> int ret = 0;
> > >>
> > >> mutex_lock(&dirty_i->seglist_lock);
> > >> @@ -767,6 +769,11 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > >> p.oldest_age = 0;
> > >> p.min_cost = get_max_cost(sbi, &p);
> > >>
> > >> + if (is_prefer_data_victim) {
> > >> + p.node_min_cost = p.min_cost;
> > >> + p.node_min_segno = p.min_segno;
> > >> + }
> > >> +
> > >> is_atgc = (p.gc_mode == GC_AT || p.alloc_mode == AT_SSR);
> > >> nsearched = 0;
> > >>
> > >> @@ -884,9 +891,25 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > >>
> > >> cost = get_gc_cost(sbi, segno, &p);
> > >>
> > >> - if (p.min_cost > cost) {
> > >> - p.min_segno = segno;
> > >> - p.min_cost = cost;
> > >> + if (is_prefer_data_victim) {
> > >> + if (IS_DATASEG(get_seg_entry(sbi, segno)->type)) {
> > >> + /* update data segments victim */
> > >> + if (p.min_cost > cost) {
> > >> + p.min_segno = segno;
> > >> + p.min_cost = cost;
> > >> + }
> > >> + } else {
> > >> + /* update node segments victim */
> > >> + if (p.node_min_cost > cost) {
> > >> + p.node_min_segno = segno;
> > >> + p.node_min_cost = cost;
> > >> + }
> > >> + }
> > >> + } else {
> > >> + if (p.min_cost > cost) {
> > >> + p.min_segno = segno;
> > >> + p.min_cost = cost;
> > >> + }
> > >> }
> > >> next:
> > >> if (nsearched >= p.max_search) {
> > >> @@ -901,6 +924,25 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
> > >> }
> > >> }
> > >>
> > >> + if (is_prefer_data_victim && sbi->need_node_clean) {
> > >> + /* we need to clean node sections */
> > >> + if (p.min_cost > p.node_min_cost) {
> > >> + p.min_segno = p.node_min_segno;
> > >> + p.min_cost = p.node_min_cost;
> > >> + } else {
> > >> + /*
> > >> + * 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->need_node_clean = false;
> > >> + p.min_segno = NULL_SEGNO;
> > >> + goto out;
> > >> + }
> > >> + }
> > >> + }
> > >> +
> > >> /* get victim for GC_AT/AT_SSR */
> > >> if (is_atgc) {
> > >> lookup_victim_by_age(sbi, &p);
> > >> @@ -1830,8 +1872,27 @@ 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) {
> > >> + if (test_opt(sbi, PREFER_DATA_VICTIM)) {
> > >> + sbi->need_node_clean = true;
> > >> + }
> > >> + if (prefree_segments(sbi)) {
> > >> + 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 +1943,17 @@ 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 (test_opt(sbi, PREFER_DATA_VICTIM)) {
> > >> + /*
> > >> + * 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;
> > >> + } else {
> > >> + goto stop;
> > >> + }
> > >> }
> > >> if (sbi->skipped_gc_rwsem)
> > >> skipped_round++;
> > >> @@ -1897,21 +1968,6 @@ 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 +1976,10 @@ 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),
> > >> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
> > >> index 2ca8fb5d0dc4..d55fa1fee2e0 100644
> > >> --- a/fs/f2fs/segment.h
> > >> +++ b/fs/f2fs/segment.h
> > >> @@ -197,8 +197,10 @@ struct victim_sel_policy {
> > >> unsigned int offset; /* last scanned bitmap offset */
> > >> unsigned int ofs_unit; /* bitmap search unit */
> > >> unsigned int min_cost; /* minimum cost */
> > >> + unsigned int node_min_cost; /* minimum cost of node type section */
> > >> unsigned long long oldest_age; /* oldest age of segments having the same min cost */
> > >> unsigned int min_segno; /* segment # having min. cost */
> > >> + unsigned int node_min_segno; /* node segment # having min. cost */
> > >> unsigned long long age; /* mtime of GCed section*/
> > >> unsigned long long age_threshold;/* age threshold */
> > >> };
> > >> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > >> index a8c8232852bb..133137dd6fd0 100644
> > >> --- a/fs/f2fs/super.c
> > >> +++ b/fs/f2fs/super.c
> > >> @@ -165,6 +165,7 @@ enum {
> > >> Opt_memory_mode,
> > >> Opt_age_extent_cache,
> > >> Opt_errors,
> > >> + Opt_prefer_data_victim,
> > >> Opt_err,
> > >> };
> > >>
> > >> @@ -245,6 +246,7 @@ static match_table_t f2fs_tokens = {
> > >> {Opt_memory_mode, "memory=%s"},
> > >> {Opt_age_extent_cache, "age_extent_cache"},
> > >> {Opt_errors, "errors=%s"},
> > >> + {Opt_prefer_data_victim, "prefer_data_victim"},
> > >> {Opt_err, NULL},
> > >> };
> > >>
> > >> @@ -1286,6 +1288,13 @@ static int parse_options(struct super_block *sb, char *options, bool is_remount)
> > >> }
> > >> kfree(name);
> > >> break;
> > >> + case Opt_prefer_data_victim:
> > >> + if (!f2fs_sb_has_blkzoned(sbi)) {
> > >> + f2fs_err(sbi, "prefer_data_victim is only allowed with zoned block device feature");
> > >> + return -EINVAL;
> > >> + }
> > >> + set_opt(sbi, PREFER_DATA_VICTIM);
> > >> + break;
> > >> default:
> > >> f2fs_err(sbi, "Unrecognized mount option \"%s\" or missing value",
> > >> p);
> > >> --
> > >> 2.34.1

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