Re: Balancing leaves when walking from top to down (was Btrfs:...)

[Chris Mason]

On Mon, Jun 21, 2010 at 09:15:28AM -0400, Chris Mason wrote:
> 
> I'll reproduce from your test case and provide a fix.  mount -o
> max_inline=1500 would give us 50% usage in the worst case (minus the
> balancing bug you hit).

Ok, the balancing bug was interesting.  What happens is we create all
the inodes and directory items nicely packed into the leaves.

Then delayed allocation kicks in and starts inserting the big fat inline
extents.  This often forces the balancing code to split a leaf twice in
order to make room for the new inline extent.  The double split code
wasn't balancing the bits that were left over on either side of our
desired slot.

The patch below fixes the problem for me, reducing the number of leaves
that have more than 2K of free space down from 6% of the total to about
74 leaves.  It could be zero, but the balancing code doesn't push
items around if our leaf is in the first or last slot of the parent
node (complexity vs benefit tradeoff).

With the patch, I'm able to create 106894 files (2K each) on a 1GB FS.
That doesn't sound like a huge improvement, but the default from
mkfs.btrfs is to duplicate metadata.  After duplication, that's about
417MB or about 40% of the overall space.

When you factor in the space that we reserve to avoid exploding on
enospc and the space that we have allocated for data extents, that's not
a very surprising number.

I'm still putting this patch through more testing, the double split code
is used in some difficult corners and I need to make sure I've tried
all of them.

-chris

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0d1d966..1f393b0 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2304,12 +2304,17 @@ noinline int btrfs_leaf_free_space(struct btrfs_root *root,
 	return ret;
 }
 
+/*
+ * min slot controls the lowest index we're willing to push to the
+ * right.  We'll push up to and including min_slot, but no lower
+ */
 static noinline int __push_leaf_right(struct btrfs_trans_handle *trans,
 				      struct btrfs_root *root,
 				      struct btrfs_path *path,
 				      int data_size, int empty,
 				      struct extent_buffer *right,
-				      int free_space, u32 left_nritems)
+				      int free_space, u32 left_nritems,
+				      u32 min_slot)
 {
 	struct extent_buffer *left = path->nodes[0];
 	struct extent_buffer *upper = path->nodes[1];
@@ -2327,7 +2332,7 @@ static noinline int __push_leaf_right(struct btrfs_trans_handle *trans,
 	if (empty)
 		nr = 0;
 	else
-		nr = 1;
+		nr = max_t(u32, 1, min_slot);
 
 	if (path->slots[0] >= left_nritems)
 		push_space += data_size;
@@ -2469,10 +2474,13 @@ out_unlock:
  *
  * returns 1 if the push failed because the other node didn't have enough
  * room, 0 if everything worked out and < 0 if there were major errors.
+ *
+ * this will push starting from min_slot to the end of the leaf.  It won't
+ * push any slot lower than min_slot
  */
 static int push_leaf_right(struct btrfs_trans_handle *trans, struct btrfs_root
 			   *root, struct btrfs_path *path, int data_size,
-			   int empty)
+			   int empty, u32 min_slot)
 {
 	struct extent_buffer *left = path->nodes[0];
 	struct extent_buffer *right;
@@ -2515,7 +2523,7 @@ static int push_leaf_right(struct btrfs_trans_handle *trans, struct btrfs_root
 		goto out_unlock;
 
 	return __push_leaf_right(trans, root, path, data_size, empty,
-				right, free_space, left_nritems);
+				right, free_space, left_nritems, min_slot);
 out_unlock:
 	btrfs_tree_unlock(right);
 	free_extent_buffer(right);
@@ -2525,12 +2533,17 @@ out_unlock:
 /*
  * push some data in the path leaf to the left, trying to free up at
  * least data_size bytes.  returns zero if the push worked, nonzero otherwise
+ *
+ * max_slot can put a limit on how far into the leaf we'll push items.  The
+ * item at 'max_slot' won't be touched.  Use (u32)-1 to make us do all the
+ * items
  */
 static noinline int __push_leaf_left(struct btrfs_trans_handle *trans,
 				     struct btrfs_root *root,
 				     struct btrfs_path *path, int data_size,
 				     int empty, struct extent_buffer *left,
-				     int free_space, int right_nritems)
+				     int free_space, u32 right_nritems,
+				     u32 max_slot)
 {
 	struct btrfs_disk_key disk_key;
 	struct extent_buffer *right = path->nodes[0];
@@ -2549,9 +2562,9 @@ static noinline int __push_leaf_left(struct btrfs_trans_handle *trans,
 	slot = path->slots[1];
 
 	if (empty)
-		nr = right_nritems;
+		nr = min(right_nritems, max_slot);
 	else
-		nr = right_nritems - 1;
+		nr = min(right_nritems - 1, max_slot);
 
 	for (i = 0; i < nr; i++) {
 		item = btrfs_item_nr(right, i);
@@ -2712,10 +2725,14 @@ out:
 /*
  * push some data in the path leaf to the left, trying to free up at
  * least data_size bytes.  returns zero if the push worked, nonzero otherwise
+ *
+ * max_slot can put a limit on how far into the leaf we'll push items.  The
+ * item at 'max_slot' won't be touched.  Use (u32)-1 to make us push all the
+ * items
  */
 static int push_leaf_left(struct btrfs_trans_handle *trans, struct btrfs_root
 			  *root, struct btrfs_path *path, int data_size,
-			  int empty)
+			  int empty, u32 max_slot)
 {
 	struct extent_buffer *right = path->nodes[0];
 	struct extent_buffer *left;
@@ -2762,7 +2779,8 @@ static int push_leaf_left(struct btrfs_trans_handle *trans, struct btrfs_root
 	}
 
 	return __push_leaf_left(trans, root, path, data_size,
-			       empty, left, free_space, right_nritems);
+			       empty, left, free_space, right_nritems,
+			       max_slot);
 out:
 	btrfs_tree_unlock(left);
 	free_extent_buffer(left);
@@ -2855,6 +2873,59 @@ static noinline int copy_for_split(struct btrfs_trans_handle *trans,
 }
 
 /*
+ * double splits happen when we need to insert a big item in the middle
+ * of a leaf.  A double split can leave us with 3 mostly empty leaves:
+ * leaf: [ slots 0 - N] [ our target ] [ N + 1 - total in leaf ]
+ *          A                 B                 C
+ *
+ * We avoid this by trying to push the items on either side of our target
+ * into the adjacent leaves.  If all goes well we can avoid the double split
+ * completely.
+ */
+static noinline int push_for_double_split(struct btrfs_trans_handle *trans,
+					  struct btrfs_root *root,
+					  struct btrfs_path *path)
+{
+	int ret;
+	int progress = 0;
+	int slot;
+
+	slot = path->slots[0];
+
+	/*
+	 * try to push all the items after our slot into the
+	 * right leaf
+	 */
+	ret = push_leaf_right(trans, root, path, 1, 0, slot);
+	if (ret < 0)
+		return ret;
+
+	if (ret == 0)
+		progress++;
+
+	/*
+	 * our goal is to get our slot at the start or end of a leaf.  If
+	 * we've done so we're done
+	 */
+	if (path->slots[0] == 0 ||
+	    path->slots[0] == btrfs_header_nritems(path->nodes[0]))
+		return 0;
+
+	/* try to push all the items before our slot into the next leaf */
+	slot = path->slots[0];
+	ret = push_leaf_left(trans, root, path, 1, 0, slot);
+	if (ret < 0)
+		return ret;
+
+	if (ret == 0)
+		progress++;
+
+	if (progress)
+		return 0;
+	return 1;
+}
+
+/*
  * split the path's leaf in two, making sure there is at least data_size
  * available for the resulting leaf level of the path.
  *
@@ -2876,6 +2947,7 @@ static noinline int split_leaf(struct btrfs_trans_handle *trans,
 	int wret;
 	int split;
 	int num_doubles = 0;
+	int tried_avoid_double = 0;
 
 	l = path->nodes[0];
 	slot = path->slots[0];
@@ -2884,12 +2956,13 @@ static noinline int split_leaf(struct btrfs_trans_handle *trans,
 		return -EOVERFLOW;
 
 	/* first try to make some room by pushing left and right */
-	if (data_size && ins_key->type != BTRFS_DIR_ITEM_KEY) {
-		wret = push_leaf_right(trans, root, path, data_size, 0);
+	if (data_size) {
+		wret = push_leaf_right(trans, root, path, data_size, 0, 0);
 		if (wret < 0)
 			return wret;
 		if (wret) {
-			wret = push_leaf_left(trans, root, path, data_size, 0);
+			wret = push_leaf_left(trans, root, path,
+					      data_size, 0, (u32)-1);
 			if (wret < 0)
 				return wret;
 		}
@@ -2923,6 +2996,12 @@ again:
 				if (mid != nritems &&
 				    leaf_space_used(l, mid, nritems - mid) +
 				    data_size > BTRFS_LEAF_DATA_SIZE(root)) {
+					if (!tried_avoid_double) {
+						push_for_double_split(trans,
+							      root, path);
+						tried_avoid_double = 1;
+						goto again;
+					}
 					split = 2;
 				}
 			}
@@ -2939,6 +3018,12 @@ again:
 				if (mid != nritems &&
 				    leaf_space_used(l, mid, nritems - mid) +
 				    data_size > BTRFS_LEAF_DATA_SIZE(root)) {
+					if (!tried_avoid_double) {
+						push_for_double_split(trans,
+							      root, path);
+						tried_avoid_double = 1;
+						goto again;
+					}
 					split = 2 ;
 				}
 			}
@@ -3915,13 +4000,14 @@ int btrfs_del_items(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 			extent_buffer_get(leaf);
 
 			btrfs_set_path_blocking(path);
-			wret = push_leaf_left(trans, root, path, 1, 1);
+			wret = push_leaf_left(trans, root, path, 1, 1, (u32)-1);
 			if (wret < 0 && wret != -ENOSPC)
 				ret = wret;
 
 			if (path->nodes[0] == leaf &&
 			    btrfs_header_nritems(leaf)) {
-				wret = push_leaf_right(trans, root, path, 1, 1);
+				wret = push_leaf_right(trans, root, path,
+						       1, 1, 0);
 				if (wret < 0 && wret != -ENOSPC)
 					ret = wret;
 			}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/