A number of traversal changes:
- Traversal now traverses the mtree's btree (the inner btree nodes)
separately from iterating over mdirs in the mtree.
This makes resuming clobbered traversals more robust as there's less
state to worry about. It also reduces all btree traversals to a single
state which simplifies the traversal logic and _in theory_ reduces
code/RAM costs.
This does add a second O(n logbn) pass through the mtree, but this
takes the fast path since we already validated btree nodes. mtree
traversal is probably dominated by mdir fetching anyways...
- lfsr_mdir_commit no longer clobbers mid-related traversals. This was a
bit too complicated with attrs potentially inserting new mids.
Instead, it's up to upper layers to explicitly clobber traversals.
Most of these already need to update dir positions, so it's not that
much extra code, but it does add cost.
lfsr_mdir_commit still clobbers mroot/mtree related traversals.
- We now stage bshrubs in traversals during mdir compaction, so we
shouldn't need to clobber traversals when the mdir compacts.
In theory as long as we clobber traversals that reference opened
files, we should never end up being the only reference to a bshrub. So
we should be able to stage bshrubs without cost.
This is _not_ working at the moment, because we aren't updating the
actual btraversal state correctly... not sure how to fix this yet...
Code/stack changes:
code stack
before: 34682 2544
after: 34716 (+0.1%) 2648 (+4.1%)
The surprise stack cost is _very_ interesting. Where is this coming
from?
It turns out when we reduce all btree traversals to a single state, and a
single function call, GCC is happy to inline lfsr_btree_traverse
directly into lfsr_fs_traverse.
This is great for code cost, but now lfs_fs_traverse contains the entire
stack frame of lfsr_btree_traverse, which is quite large. When we called
lfsr_btree_traverse twice, this stack frame was never nested with
lfsr_mtree_lookup, but now our tools think it is...
I'm not sure how to fix this. Maybe improving our tooling to understand
shrinkwrap optimizations will find this doesn't actually cost as much?
Or maybe not since this is in a complicated switch case state machine?
We could use an explicit __attribute__((noinline)), but this sort of
heavy-handed optimization guidance has been out-of-scope for littlefs up
until now...
I'm leaving this as-is for now, but it may be worth looking this again
in the future.
Christopher Haster
4cd1f84a89