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Message-ID: <8738E8FF-820F-48A5-9150-7FF64219ED42@whamcloud.com>
Date: Wed, 20 Nov 2019 10:35:38 +0000
From: Alex Zhuravlev <azhuravlev@...mcloud.com>
To: "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: [RFC] improve malloc for large filesystems
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);
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