Re: [RFC] improve malloc for large filesystems

[Artem Blagodarenko <artem.blagodarenko@gmail.com>]

Hello Alex,

Thank you for this important topic. I am going to test your patch and give feedback. But this requires some time.

Now I want to share my thougths about this topic.
Here are slides from LAD2019 “Ldiskfs block allocator and aged file system" https://www.eofs.eu/_media/events/lad19/14_artem_blagodarenko-ldiskfs_block_allocator.pdf
There is also patch https://lists.openwall.net/linux-ext4/2019/03/11/5 that was sent here, but it require to be improved.

Best  regards,
Artem Blagodarenko

> On 20 Nov 2019, at 13:35, Alex Zhuravlev <azhuravlev@whamcloud.com> wrote:
> 
> Hi,
> 
> We’ve seen few reports where a huge fragmented filesystem spends a lot of time looking for a good chunks of free space.
> Two issues have been identified so far:
> 1) mballoc tries too hard to find the best chunk which is counterproductive - it makes sense to limit this process
> 2) during scanning the bitmaps are loaded one by one, synchronously  - it makes sense to prefetch few groups at once
> Here is a patch for comments, not really tested at scale, but it’d be great to see the comments.
> 
> Thanks in advance, Alex
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 0b202e00d93f..76547601384b 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -404,7 +404,8 @@ static int ext4_validate_block_bitmap(struct super_block *sb,
>  * Return buffer_head on success or NULL in case of failure.
>  */
> struct buffer_head *
> -ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> +ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
> +				 int ignore_locked)
> {
> 	struct ext4_group_desc *desc;
> 	struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -435,6 +436,13 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> 	if (bitmap_uptodate(bh))
> 		goto verify;
> 
> +	if (ignore_locked && buffer_locked(bh)) {
> +		/* buffer under IO already, do not wait
> +		 * if called for prefetching */
> +		err = 0;
> +		goto out;
> +	}
> +
> 	lock_buffer(bh);
> 	if (bitmap_uptodate(bh)) {
> 		unlock_buffer(bh);
> @@ -524,7 +532,7 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> 	struct buffer_head *bh;
> 	int err;
> 
> -	bh = ext4_read_block_bitmap_nowait(sb, block_group);
> +	bh = ext4_read_block_bitmap_nowait(sb, block_group, 1);
> 	if (IS_ERR(bh))
> 		return bh;
> 	err = ext4_wait_block_bitmap(sb, block_group, bh);
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 03db3e71676c..2320d7e2f8d6 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1480,6 +1480,9 @@ struct ext4_sb_info {
> 	/* where last allocation was done - for stream allocation */
> 	unsigned long s_mb_last_group;
> 	unsigned long s_mb_last_start;
> +	unsigned int s_mb_toscan0;
> +	unsigned int s_mb_toscan1;
> +	unsigned int s_mb_prefetch;
> 
> 	/* stats for buddy allocator */
> 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> @@ -2333,7 +2336,8 @@ extern struct ext4_group_desc * ext4_get_group_desc(struct super_block * sb,
> extern int ext4_should_retry_alloc(struct super_block *sb, int *retries);
> 
> extern struct buffer_head *ext4_read_block_bitmap_nowait(struct super_block *sb,
> -						ext4_group_t block_group);
> +						ext4_group_t block_group,
> +						int ignore_locked);
> extern int ext4_wait_block_bitmap(struct super_block *sb,
> 				  ext4_group_t block_group,
> 				  struct buffer_head *bh);
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a3e2767bdf2f..eac4ee225527 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -861,7 +861,7 @@ static int ext4_mb_init_cache(struct page *page, char *incore, gfp_t gfp)
> 			bh[i] = NULL;
> 			continue;
> 		}
> -		bh[i] = ext4_read_block_bitmap_nowait(sb, group);
> +		bh[i] = ext4_read_block_bitmap_nowait(sb, group, 0);
> 		if (IS_ERR(bh[i])) {
> 			err = PTR_ERR(bh[i]);
> 			bh[i] = NULL;
> @@ -2095,10 +2095,52 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
> 	return 0;
> }
> 
> +/*
> + * each allocation context (i.e. a thread doing allocation) has own
> + * sliding prefetch window of @s_mb_prefetch size which starts at the
> + * very first goal and moves ahead of scaning.
> + * a side effect is that subsequent allocations will likely find
> + * the bitmaps in cache or at least in-flight.
> + */
> +static void
> +ext4_mb_prefetch(struct ext4_allocation_context *ac,
> +		    ext4_group_t start)
> +{
> +	ext4_group_t ngroups = ext4_get_groups_count(ac->ac_sb);
> +	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> +	struct ext4_group_info *grp;
> +	ext4_group_t group = start;
> +	struct buffer_head *bh;
> +	int nr;
> +
> +	/* batch prefetching to get few READs in flight */
> +	if (group + (sbi->s_mb_prefetch >> 1) < ac->ac_prefetch)
> +		return;
> +
> +	nr = sbi->s_mb_prefetch;
> +	while (nr > 0) {
> +		if (++group >= ngroups)
> +			group = 0;
> +		if (unlikely(group == start))
> +			break;
> +		grp = ext4_get_group_info(ac->ac_sb, group);
> +		/* ignore empty groups - those will be skipped
> +		 * during the scanning as well */
> +		if (grp->bb_free == 0)
> +			continue;
> +		nr--;
> +		if (!EXT4_MB_GRP_NEED_INIT(grp))
> +			continue;
> +		bh = ext4_read_block_bitmap_nowait(ac->ac_sb, group, 1);
> +		brelse(bh);
> +	}
> +	ac->ac_prefetch = group;
> +}
> +
> static noinline_for_stack int
> ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> {
> -	ext4_group_t ngroups, group, i;
> +	ext4_group_t ngroups, toscan, group, i;
> 	int cr;
> 	int err = 0, first_err = 0;
> 	struct ext4_sb_info *sbi;
> @@ -2160,6 +2202,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 	 * cr == 0 try to get exact allocation,
> 	 * cr == 3  try to get anything
> 	 */
> +
> +	ac->ac_prefetch = ac->ac_g_ex.fe_group;
> +
> repeat:
> 	for (; cr < 4 && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
> 		ac->ac_criteria = cr;
> @@ -2169,7 +2214,15 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 		 */
> 		group = ac->ac_g_ex.fe_group;
> 
> -		for (i = 0; i < ngroups; group++, i++) {
> +		/* limit number of groups to scan at the first two rounds
> +		 * when we hope to find something really good */
> +		toscan = ngroups;
> +		if (cr == 0)
> +			toscan = sbi->s_mb_toscan0;
> +		else if (cr == 1)
> +			toscan = sbi->s_mb_toscan1;
> +
> +		for (i = 0; i < toscan; group++, i++) {
> 			int ret = 0;
> 			cond_resched();
> 			/*
> @@ -2179,6 +2232,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 			if (group >= ngroups)
> 				group = 0;
> 
> +			ext4_mb_prefetch(ac, group);
> +
> 			/* This now checks without needing the buddy page */
> 			ret = ext4_mb_good_group(ac, group, cr);
> 			if (ret <= 0) {
> @@ -2872,6 +2927,9 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
> 			bio_put(discard_bio);
> 		}
> 	}
> +	sbi->s_mb_toscan0 = 1024;
> +	sbi->s_mb_toscan1 = 4096;
> +	sbi->s_mb_prefetch = 32;
> 
> 	list_for_each_entry_safe(entry, tmp, &freed_data_list, efd_list)
> 		ext4_free_data_in_buddy(sb, entry);
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index 88c98f17e3d9..9ba5c75e6490 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -175,6 +175,7 @@ struct ext4_allocation_context {
> 	struct page *ac_buddy_page;
> 	struct ext4_prealloc_space *ac_pa;
> 	struct ext4_locality_group *ac_lg;
> +	ext4_group_t ac_prefetch;
> };
> 
> #define AC_STATUS_CONTINUE	1
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index eb1efad0e20a..4476d828439b 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -198,6 +198,9 @@ EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
> EXT4_ATTR(first_error_time, 0444, first_error_time);
> EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan0, s_mb_toscan0);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan1, s_mb_toscan1);
> +EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
> 
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -228,6 +231,9 @@ static struct attribute *ext4_attrs[] = {
> 	ATTR_LIST(first_error_time),
> 	ATTR_LIST(last_error_time),
> 	ATTR_LIST(journal_task),
> +	ATTR_LIST(mb_toscan0),
> +	ATTR_LIST(mb_toscan1),
> +	ATTR_LIST(mb_prefetch),
> 	NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
>