On Thu, Jul 07, 2016 at 10:11:59AM +0800, Qu Wenruo wrote:
> Btrfs_read_block_groups() function is the most time consuming function
> if the fs is large and filled with small extents.
>
> For a btrfs filled with 100,000,000 16K sized files, mount the fs takes
> about 10 seconds.
>
> While ftrace shows that, btrfs_read_block_groups() takes about 9
> seconds, taking up 90% of the mount time.
> So it's worthy to speedup btrfs_read_block_groups(), to reduce the
> overall mount time.
>
> Btrfs_read_block_groups() calls btrfs_search_slot() to find block group
> items.
> However the search key is (<block group start>, BLOCK_GROUP_KEY, 0).
> This makes search_slot() always returns previous slot.
>
> Under most case, it's OK since the block group item and previous slot
> are in the same leaf.
> But for cases where block group item are the first item of a leaf, we
> must read out next leaf to get block group item.
> This needs extra IO and makes btrfs_read_block_groups() slower.
>
> In fact, before we call btrfs_read_block_groups(), we have already read
> out all chunks, which are 1:1 mapped with block group items.
> So we can get the exact block group length for btrfs_search_slot(), to
> avoid any possible btrfs_next_leaf() to speedup
> btrfs_read_block_groups().
>
> With this patch, time spent on btrfs_read_block_groups() is reduced to
> 7.56s, compared to old 8.94s, about 15% improvement.
It would be interesting to see if it helps to speed up the mount on a
large filesystem where it takes long to mount. The difference is ~1
second, this is too low to claim a 15% improvement.
>
> Reported-by: Tsutomu Itoh <t-itoh@xxxxxxxxxxxxxx>
> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
> ---
> v2:
> Update commit message
> ---
> fs/btrfs/extent-tree.c | 61 ++++++++++++++++++++------------------------------
> fs/btrfs/extent_map.h | 22 ++++++++++++++++++
> 2 files changed, 46 insertions(+), 37 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 82b912a..874f5b3 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -9648,39 +9648,20 @@ out:
> return ret;
> }
>
> -static int find_first_block_group(struct btrfs_root *root,
> - struct btrfs_path *path, struct btrfs_key *key)
> +int find_block_group(struct btrfs_root *root,
> + struct btrfs_path *path,
> + struct extent_map *chunk_em)
> {
> int ret = 0;
> - struct btrfs_key found_key;
> - struct extent_buffer *leaf;
> - int slot;
> -
> - ret = btrfs_search_slot(NULL, root, key, path, 0, 0);
> - if (ret < 0)
> - goto out;
> + struct btrfs_key key;
>
> - while (1) {
> - slot = path->slots[0];
> - leaf = path->nodes[0];
> - if (slot >= btrfs_header_nritems(leaf)) {
> - ret = btrfs_next_leaf(root, path);
> - if (ret == 0)
> - continue;
> - if (ret < 0)
> - goto out;
> - break;
> - }
> - btrfs_item_key_to_cpu(leaf, &found_key, slot);
> + key.objectid = chunk_em->start;
> + key.offset = chunk_em->len;
> + key.type = BTRFS_BLOCK_GROUP_ITEM_KEY;
>
> - if (found_key.objectid >= key->objectid &&
> - found_key.type == BTRFS_BLOCK_GROUP_ITEM_KEY) {
> - ret = 0;
> - goto out;
> - }
> - path->slots[0]++;
> - }
> -out:
> + ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> + if (ret > 0)
> + ret = -ENOENT;
> return ret;
> }
>
> @@ -9899,16 +9880,14 @@ int btrfs_read_block_groups(struct btrfs_root *root)
> struct btrfs_block_group_cache *cache;
> struct btrfs_fs_info *info = root->fs_info;
> struct btrfs_space_info *space_info;
> - struct btrfs_key key;
> + struct btrfs_mapping_tree *map_tree = &root->fs_info->mapping_tree;
> + struct extent_map *chunk_em;
> struct btrfs_key found_key;
> struct extent_buffer *leaf;
> int need_clear = 0;
> u64 cache_gen;
>
> root = info->extent_root;
> - key.objectid = 0;
> - key.offset = 0;
> - key.type = BTRFS_BLOCK_GROUP_ITEM_KEY;
> path = btrfs_alloc_path();
> if (!path)
> return -ENOMEM;
> @@ -9921,10 +9900,16 @@ int btrfs_read_block_groups(struct btrfs_root *root)
> if (btrfs_test_opt(root, CLEAR_CACHE))
> need_clear = 1;
>
> + /* Here we don't lock the map tree, as we are the only reader */
> + chunk_em = first_extent_mapping(&map_tree->map_tree);
> + /* Not really possible */
> + if (!chunk_em) {
> + ret = -ENOENT;
> + goto error;
> + }
> +
> while (1) {
> - ret = find_first_block_group(root, path, &key);
> - if (ret > 0)
> - break;
> + ret = find_block_group(root, path, chunk_em);
> if (ret != 0)
> goto error;
>
> @@ -9958,7 +9943,6 @@ int btrfs_read_block_groups(struct btrfs_root *root)
> sizeof(cache->item));
> cache->flags = btrfs_block_group_flags(&cache->item);
>
> - key.objectid = found_key.objectid + found_key.offset;
> btrfs_release_path(path);
>
> /*
> @@ -10039,6 +10023,9 @@ int btrfs_read_block_groups(struct btrfs_root *root)
> }
> spin_unlock(&info->unused_bgs_lock);
> }
> + chunk_em = next_extent_mapping(chunk_em);
> + if (!chunk_em)
> + break;
> }
>
> list_for_each_entry_rcu(space_info, &root->fs_info->space_info, list) {
> diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
> index eb8b8fa..9358e3e 100644
> --- a/fs/btrfs/extent_map.h
> +++ b/fs/btrfs/extent_map.h
> @@ -90,4 +90,26 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len, u64 gen
> void clear_em_logging(struct extent_map_tree *tree, struct extent_map *em);
> struct extent_map *search_extent_mapping(struct extent_map_tree *tree,
> u64 start, u64 len);
> +
> +static inline struct extent_map *
> +first_extent_mapping(struct extent_map_tree *tree)
> +{
> + struct rb_node *node;
> +
> + node = rb_first(&tree->map);
> + if (!node)
> + return NULL;
> + return rb_entry(node, struct extent_map, rb_node);
> +}
> +
> +static inline struct extent_map *
> +next_extent_mapping(struct extent_map *map)
> +{
> + struct rb_node *node;
> +
> + node = rb_next(&map->rb_node);
> + if (!node)
> + return NULL;
> + return rb_entry(node, struct extent_map, rb_node);
> +}
> #endif
> --
> 2.9.0
>
>
>
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