bcachefs: Eytzinger accumulation for accounting keys

The btree write buffer takes as input keys from the journal, sorts them,
deduplicates them, and flushes them back to the btree in sorted order.

The disk space accounting rewrite is moving accounting to normal btree
keys, with update (in this case deltas) accumulated in the write buffer
and then flushed to the btree; but this is going to increase the number
of keys handled by the write buffer by perhaps as much as a factor of
3x-5x.

The overhead from copying around and sorting this many keys would cause
a significant performance regression, but: there is huge locality in
updates to accounting keys that we can take advantage of.

Instead of appending accounting keys to the list of keys to be sorted,
this patch adds an eytzinger search tree of recently seen accounting
keys. We look up the accounting key in the eytzinger search tree and
apply the delta directly, adding it if it doesn't exist, and
periodically prune the eytzinger tree of unused entries.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/bcachefs/btree_write_buffer_types.h b/fs/bcachefs/btree_write_buffer_types.h
index 9b9433de9c36..e9e76e20f43b 100644
--- a/fs/bcachefs/btree_write_buffer_types.h
+++ b/fs/bcachefs/btree_write_buffer_types.h
@@ -52,6 +52,8 @@ struct btree_write_buffer {
 	struct btree_write_buffer_keys	inc;
 	struct btree_write_buffer_keys	flushing;
 	struct work_struct		flush_work;
+
+	DARRAY(struct btree_write_buffered_key) accounting;
 };
 
 #endif /* _BCACHEFS_BTREE_WRITE_BUFFER_TYPES_H */