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Subject: RE:(2) [f2fs-dev] [PATCH v1] f2fs: New victim selection for GC
Reply-To: yonggil.song@samsung.com
Sender: Yonggil Song <yonggil.song@samsung.com>
From:   Yonggil Song <yonggil.song@samsung.com>
To:     Chao Yu <chao@kernel.org>,
        "jaegeuk@kernel.org" <jaegeuk@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>
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Message-ID: <20231129004829epcms2p424135b51305a8d006835109dcfa0f1c5@epcms2p4>
Date:   Wed, 29 Nov 2023 09:48:29 +0900
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References: <f59b8c00-3628-6318-ee69-1caf79a03f2b@kernel.org>
        <20231012100345epcms2p84b95e24ee5e7965858ab8be99fbc3eff@epcms2p8>
        <20231026091838epcms2p46f14260fdaf53e27ee6374887ed87f41@epcms2p4>
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>Hi Yonggil,
>
>On 2023/10/26 17:18, 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.
>
>What about introducing parameter to adjust cost calculated by get_gc_cost()?
>
>Something like:
>
>get_gc_cost()
>
>	if (p->gc_mode == GC_GREEDY) {
>		vblocks = get_valid_blocks();
>		if (seg_type is data)
>			return vblocks * data_factor;
>		return vblock * node_factor;
>	}
>
>If we prefer to select data segment during fggc, we can config data/node factor
>as 1 and 512?
>
>Thoughts?
>
>Thanks,
>

Hi Chao.

I think that's a simpler way to do it.
I'll work on v2 with your idea.
Thanks for the feedback

>> 
>> 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.
>> 
>> 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);