todo

1 files changed, 12 insertions, 11 deletions

diff --git a/Todo.mdwn b/Todo.mdwn
index e6d2d72..e5c286d 100644
--- a/Todo.mdwn
+++ b/Todo.mdwn
@@ -1,16 +1,17 @@
 bcache/bcachefs todo list:
 
- * Asynchronous btree node splits
-
-   The last source of tail latency in the index update path is btree node
-   splits/compacts - a btree node split has to write out the new node(s) and
-   then update the parent node, all synchronously.
-
-   This is particularly painful with bcachefs, as we end up doing a lot more
-   index updates (dirents/inodes/xattrs) as a proportion of total IO.
-
-   Need to get the design for this written down. Slava and I had it worked out,
-   but we didn't write anything down so we'll have to go over it again.
+ * Implement epochs, and get rid of read retry path
+   
+   The idea is a refcount on allocator cycles - "epochs" - if something (e.g.
+   copygc) is buffering extents where GC can't find them (for recalculating the
+   oldest gen), it can take a ref on the current epoch; current epoch - oldest
+   existing epoch = how much more out of date pointer gens can be, for keys gc
+   can't find - then the allocator can wait if this is too big.
+
+   For foreground reads, the trick is the read takes a (percpu) refcount on the
+   current epoch, and releases it when the read completes; then when the
+   allocator wants to start another cycle, it bumps the epoch, then waits for
+   all the references from foreground reads to go to 0 on the previous epoch.
 
  * Lockless btree lookups