[PATCH v4] f2fs: New victim selection for GC

From: Yonggil Song
Date: Thu Dec 07 2023 - 02:34:38 EST

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;
+ } 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;
}

+
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:
@@ -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