Btrfs: delayed-refs: use rb_first_cached for href_root
rb_first_cached() trades an extra pointer "leftmost" for doing the same
job as rb_first() but in O(1).
Functions manipulating href_root need to get the first entry, this
converts href_root to use rb_first_cached().
This patch is first in the sequenct of similar updates to other rbtrees
and this is analysis of the expected behaviour and improvements.
There's a common pattern:
while (node = rb_first) {
entry = rb_entry(node)
next = rb_next(node)
rb_erase(node)
cleanup(entry)
}
rb_first needs to traverse the tree up to logN depth, rb_erase can
completely reshuffle the tree. With the caching we'll skip the traversal
in rb_first. That's a cached memory access vs looped pointer
dereference trade-off that IMHO has a clear winner.
Measurements show there's not much difference in a sample tree with
10000 nodes: 4.5s / rb_first and 4.8s / rb_first_cached. Real effects of
caching and pointer chasing are unpredictable though.
Further optimzations can be done to avoid the expensive rb_erase step.
In some cases it's ok to process the nodes in any order, so the tree can
be traversed in post-order, not rebalancing the children nodes and just
calling free. Care must be taken regarding the next node.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog from mail discussions ]
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index 62ff545..f07952e 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -101,14 +101,15 @@ static int comp_refs(struct btrfs_delayed_ref_node *ref1,
}
/* insert a new ref to head ref rbtree */
-static struct btrfs_delayed_ref_head *htree_insert(struct rb_root *root,
+static struct btrfs_delayed_ref_head *htree_insert(struct rb_root_cached *root,
struct rb_node *node)
{
- struct rb_node **p = &root->rb_node;
+ struct rb_node **p = &root->rb_root.rb_node;
struct rb_node *parent_node = NULL;
struct btrfs_delayed_ref_head *entry;
struct btrfs_delayed_ref_head *ins;
u64 bytenr;
+ bool leftmost = true;
ins = rb_entry(node, struct btrfs_delayed_ref_head, href_node);
bytenr = ins->bytenr;
@@ -117,16 +118,18 @@ static struct btrfs_delayed_ref_head *htree_insert(struct rb_root *root,
entry = rb_entry(parent_node, struct btrfs_delayed_ref_head,
href_node);
- if (bytenr < entry->bytenr)
+ if (bytenr < entry->bytenr) {
p = &(*p)->rb_left;
- else if (bytenr > entry->bytenr)
+ } else if (bytenr > entry->bytenr) {
p = &(*p)->rb_right;
- else
+ leftmost = false;
+ } else {
return entry;
+ }
}
rb_link_node(node, parent_node, p);
- rb_insert_color(node, root);
+ rb_insert_color_cached(node, root, leftmost);
return NULL;
}
@@ -164,10 +167,11 @@ static struct btrfs_delayed_ref_node* tree_insert(struct rb_root *root,
* If return_bigger is given, the next bigger entry is returned if no exact
* match is found.
*/
-static struct btrfs_delayed_ref_head *
-find_ref_head(struct rb_root *root, u64 bytenr,
- int return_bigger)
+static struct btrfs_delayed_ref_head* find_ref_head(
+ struct btrfs_delayed_ref_root *dr, u64 bytenr,
+ int return_bigger)
{
+ struct rb_root *root = &dr->href_root.rb_root;
struct rb_node *n;
struct btrfs_delayed_ref_head *entry;
@@ -187,7 +191,7 @@ find_ref_head(struct rb_root *root, u64 bytenr,
if (bytenr > entry->bytenr) {
n = rb_next(&entry->href_node);
if (!n)
- n = rb_first(root);
+ n = rb_first_cached(&dr->href_root);
entry = rb_entry(n, struct btrfs_delayed_ref_head,
href_node);
return entry;
@@ -357,12 +361,12 @@ btrfs_select_ref_head(struct btrfs_trans_handle *trans)
again:
start = delayed_refs->run_delayed_start;
- head = find_ref_head(&delayed_refs->href_root, start, 1);
+ head = find_ref_head(delayed_refs, start, 1);
if (!head && !loop) {
delayed_refs->run_delayed_start = 0;
start = 0;
loop = true;
- head = find_ref_head(&delayed_refs->href_root, start, 1);
+ head = find_ref_head(delayed_refs, start, 1);
if (!head)
return NULL;
} else if (!head && loop) {
@@ -903,7 +907,7 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
struct btrfs_delayed_ref_head *
btrfs_find_delayed_ref_head(struct btrfs_delayed_ref_root *delayed_refs, u64 bytenr)
{
- return find_ref_head(&delayed_refs->href_root, bytenr, 0);
+ return find_ref_head(delayed_refs, bytenr, 0);
}
void __cold btrfs_delayed_ref_exit(void)