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Date:   Tue, 28 Nov 2023 11:01:26 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Yonggil Song <yonggil.song@samsung.com>, gg@google.com
Cc:     "chao@kernel.org" <chao@kernel.org>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "linux-f2fs-devel@lists.sourceforge.net" 
        <linux-f2fs-devel@lists.sourceforge.net>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Seokhwan Kim <sukka.kim@samsung.com>,
        Daejun Park <daejun7.park@samsung.com>,
        Siwoo Jung <siu.jung@samsung.com>
Subject: Re: (2) [f2fs-dev] [PATCH v1] f2fs: New victim selection for GC
Message-ID: <ZWY5Bntn42SnsclR@google.com>
References: <ZVekM0GmX-Jz4T0C@google.com>
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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?

> 
> >> 
> >> 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@samsung.com>
> >> ---
> >>  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