- lfs3_omdir_t -> lfs3_handle_t
- lfs3.omdirs -> lfs3.handles
- o -> h
- lfs3_omdir_* -> lfs3_handle_*
- lfs3_omdir_ismidopen -> lfs3_mid_isopen
From conversations with users, the term "handle" or "file handle" seems
to be the most common/easily understood term for the lfs3_file_t struct
itself. It makes sense to adopt this in our codebase.
I usually dislike inventing new names for things when prefixes can imply
a relationship (size -> ssize, cache -> rcache, shrub -> bshrub, etc),
but lfs3_omdirs_t was probably a bit much.
This replaces the `bool align` parameter that goes through all the prog
layers with an optional prog-aligned cksum stored in the lfs3_t struct.
Normally ignored, this prog-aligned cksum can be requested by setting
cksum=&lfs3->pcksum in any prog call.
Does this work? Yes. Is it a great solution? Ehhhh...
I've been tinkering with other solutions that avoid the `bool align`
parameter, but with no luck.
- `bool align`, or previously two cksum arguments, work, but create a
bit of a messy API. I'd like to find an alternative solution.
- Changing the cksum pointer to a richer lfs3_cksum_t struct with flags
also works, but would be an even messier API.
- Adding an lfs3_t side-channel, lfs3->pcache could include a pointer to
an optional prog-aligned cksum. But this would be the same/more cost
as just storing the pcksum in lfs3_t. And then we'd need to worry
about disentangling the cksum pointer on errors, etc.
- We could set a flag in lfs3->flags for alignment. This avoids the
extra 4 bytes of ctx, but still suffers from the risk of entangled
state on errors, etc.
- We could unconditionally calculate lfs3->pcksum. But then we'd be
calculating a lot of cksums we don't use (every metadata commit), and
still using the extra 4 bytes of ctx.
Lacking a good solution, using cksum=&lfs3->pcksum to indicate a
prog-aligned cksum is at least an ok solution.
I will happily change this if an alternative comes up in the future.
Another way of viewing this is that `&lfs3->pcksum` acts as a special
magic pointer value to tell the prog layers to calculate lfs3->pcksum.
A different non-NULL constant value could have worked just as well, but
those are a bit trickier to create in C.
---
Actually, there is a "better" cursed solution:
- Rely on pointer alignment to sneak a flag into the cksum pointer's
lower bits.
But, while clever, this is is outside of C's machine model and would
limit portability.
---
This trades 4 bytes of ctx for 58 bytes of code and simpler (debatable)
internal prog APIs:
code stack ctx
before: 36860 2384 652
after: 36832 (-0.1%) 2384 (+0.0%) 656 (+0.6%)
In theory this also saves stack in all the prog APIs, but none of prog
APIs end up on the stack hot-path. In our codebase the read APIs
dominate the stack thanks to block allocator traversals.
LFS3_CKDATACKSUMREADS is just too much.
The downside is it may not be clear how LFS3_CKDATACKSUMREADS interacts
with the future planned LFS3_CKREADS (LFS3_CKREADS implies
LFS3_CKDATACKSUMS + LFS3_CKMETAREDUND), but on the flip side you may
actually be able to type LFS3_CKDATACKSUMS on the first try.
Limited to nested struct fields where the names don't really matter:
- bptr.data -> bptr.d
- mdir.rbyd -> mdir.r
Ok it actually just ended up those two.
This is on the tail end of some optimization work that ended up
abandoned because of maintainability concerns. But it did highlight that
struct nesting gets a bit out-of-control when trying to both optimize
stack allocations and respect C99's strict aliasing.
Consider further fragmenting lfs3_rbyd_t for fine-grain stack
allocations:
typedef struct lfs3_rbyd {
struct lfs3_rtrunkcksum {
struct lfs3_rtrunk {
lfs3_rid_t weight;
struct lfs3_rtrunktrunk {
lfs3_block_t blocks[2];
lfs3_size_t trunk;
} rtrunktrunk;
} rtrunk;
uint32_t cksum;
} rtrunkcksum;
lfs3_size_t eoff;
} lfs3_rbyd_t;
Accessing fields just starts to get silly:
rbyd.rtrunkcksum.rtrunk.trunktrunk.trunk
At least single-char field names keeps a little bit of readability:
rbyd.ck.t.t.trunk
Or for some real examples:
- file->b.o.mdir.rbyd.weight -> file->b.o.mdir.r.weight
- bptr->data.u.disk.block -> bptr->d.u.disk.block
So we now keep blocks around until they can be replaced with a single
fragment. This is simpler, cheaper, and reduces the number of commits
needed to graft (though note arbitrary range removals still keep this
unbounded).
---
So, this is a delicate tradeoff.
On one hand, not fully fragmenting blocks risks keeping around bptrs
containing very little data, depending on fragment_size.
On the other hand:
- It's expensive, and disk utilization during random _deletes_ is not
the biggest of concerns.
Note our crystallization algorithm should still clean up partial
blocks _eventually_, so this doesn't really impact random writes.
The main concerns are lfs3_file_truncate/fruncate, and in the future
collapserange/punchhole.
- Fragmenting bptrs introduces more commits, which have their own
prog/erase cost, and it's unclear how this impacts logging operations.
There's no point in fragmenting blocks at the head of a log if we're
going to fruncate them eventually.
I figure lets err on minimizing complexity/code size for now, and if
this turns out to be a mistake, we can always revert or introduce
fragmenting >1 fragment blocks as an optional feature in the future.
---
Saves a big chunk of code, stack, and even some ctx (no more
fragment_thresh):
code stack ctx
before: 37504 2448 656
after: 37024 (-1.3%) 2416 (-1.3%) 652 (-0.6%)
This merges LFS3_o_GRAFT into LFS3_o_UNCRYST, simplifying the file write
path and avoiding the mess that is ungrafted leaves.
---
This goes for a different lazy crystallization/grafting strategy that
was overlooked before. Instead of requiring all leaves to be both
crystallized and grafted, we allow leaves to be uncrystallied, but they
_must_ be grafted (in-tree) at all times.
This gets us most of the rewrite preformance of lazy-crystallization,
without needing to worry about out-of-date file leaves.
Out-of-date file leaves were a headache for both code cost and concerns
around confusing filesystem states and related bugs.
Note LFS3_o_UNCRYST gets some extra behavior here:
- LFS3_o_UNCRYST indicates when crystallization is _necessary_, and no
longer when crystallization is _possible_.
We already keep track of when crystallization is _possible_ via bptr's
erased-state, and this lets us control recrystallization in
lfs3_file_flush_ without erased-state-clearing hacks (which probably
wouldn't work with the future ddtree).
- We opportunistically clear the UNCRYST flag if it's not possible for
future lfs3_file_crystallize_ calls to make progress:
- When we crystallize a full block
- When we hit the end of the file
- When we hit a hole
- When we hit an unaligned block
---
Note this does impact performance!
Unlike true lazy grafting, eagerly grafting means we're always
committing to the bshrub/btree more than is strictly necessary, and this
translates to more frequent btree node erases/compactions.
Current simulated benchmarks show a ~3x increase (~20us -> ~60us) in
write times for linear file writes on NOR flash.
However:
- The moment you need unaligned progs, this performance optimization
goes out the window, as we need to graft bptrs before any padding
fragments.
- This only kicks in once we start crystallizing. So any writes <
crystal_thresh (both in new files and in between blocks) are forced
to commit to the bshrub/btree every flush.
This risks a difficult to predict performance characteristic.
- If you sync frequently (logging), we're forced to crystallize/graft
anyways.
- The performance hit can be alleviated with either larger writes or
larger caches, though I realize this goes against littlefs's
"RAM-not-required" mantra.
Worst case, we can always bring back "lazy grafting" as a
high-performance option in the future.
Though note the above concerns around in-between/pre crystallization
performance. This may only make sense when cache_size >= both prog_size
and crystal_thresh.
And of course, there's a significant code tradeoff!
code stack ctx
before: 38020 2456 656
after: 37588 (-1.1%) 2472 (+0.7%) 656 (+0.0%)
Uh, ignore that stack cost. The simplified logic leads to more functions
being inlined, which makes a mess of our stack measurements because we
don't take shrinkwrapping into account.
This was quite a deep bug.
We don't track the original bshrub when grafting, so it was possible to
realloc those blocks even when we need their contents to finish the
graft operation.
This was found while experimenting with eager leaf grafting, but can
also occur when grafting data fragments.
---
In theory, the block allocator's checkpoint mechanism protects against
this.
Before we alloc, we set a checkpoint with lfs3_alloc_ckpoint. This marks
the position of the block allocator before allocation, so if we loop
around the entire block device we don't double alloc any in-flight
blocks:
ckpoint lookahead
v .---'---.
[mm---ddd-d---d-------|dd--d-ddd|--------d-----d-]
'---.---'
in-flight allocations
But this only protects _new_ blocks, _old_ blocks can be anywhere on
disk and are unprotected.
In theory again, old blocks are always tracked via copy-on-write
snapshots, but this is not the case for bshrubs while grafting!
Grafting is unfortunately a multi-commit operation (we may remove
multiple fragments that span different btree nodes), and each bshrub
commit discards the old snapshot. This creates a window where old blocks
can be double alloced _while grafting_, leading to corrupted data.
You may wonder why are we discarding the old snapshot? Why not keep
track of it until the grafting completes?
The problem there is that we need the intermediate snapshot in order for
shrubs to survive compactions. We really have 3 states:
old -> mid-graft -> new
And the only one we don't need to fallback to is the old state.
---
A couple solutions:
1. Track all three states
This would add complexity increase the cost of every lfs3_file_t.
2. Open a temporary file to track the old state
This would add complexity and a big chunk of stack to what is already
one of the critical functions on our stack hot-path.
3. Carefully make sure graft commits don't lose track of in-flight data
until an atomic commit
This doesn't work when you're trying to coalesce two data fragments
in two different btree nodes. At least not without completely
restructuring the btree commit logic.
4. Just explicitly track in-flight graft state out-of-band
This goes with option no 4., adding lfs3->graft and lfs3->graft_count to
track in-flight graft state when we're grafting. lfs3_mtree_traverse_
can include the relevant blocks during traversals, effectively masking
out graft state from the lookahead buffer.
This adds a bit of code/ctx, but is probably the cheapest option:
code stack ctx
before: 37936 2456 636
after: 38092 (+0.4%) 2456 (+0.0%) 656 (+3.1%)
Like LFS3_RDONLY and LFS3_KVONLY, LFS3_2BONLY opts-out of all of the
logic necessary for filesystems larger than 2-blocks (the mimimum size
of a mutable littlefs image).
This has potential for some pretty big savings:
- No block allocation
- No lookahead buffer
- No btrees (but yes bshrubs)
- No bptrs
- No mtree traversal
Which is I guess ~1/4 of the codebase:
code stack ctx
default: 37836 2416 636
2bonly: 27704 (-26.8%) 1872 (-22.5%) 592 (-6.9%)
This can be combined with LFS3_KVONLY for a small key-value store
compatible with the full littlefs driver:
code stack ctx
default: 37836 2416 636
kvonly: 30792 (-18.6%) 2168 (-10.3%) 636 (+0.0%)
kvonly+2bonly: 22900 (-39.5%) 1736 (-28.1%) 592 (-6.9%)
It may be possible to optimize this further, but, as is the case with
LFS3_KVONLY, balancing config-specific optimization vs maintainability
is tricky.
---
I'm not sure why, but this also reduced the default build's size a bit.
Compiler noise?
code stack ctx
before: 37860 2416 636
after: 37836 (-0.1%) 2416 (+0.0%) 636 (+0.0%)
One of the ideas behind the key-value API is that it is potentially much
cheaper than a full file API. With the key-value API, we get the
guarantee that all data must fit in RAM, and avoid headaches like
random reads/writes and needing to broadcast file state.
For an example of just how much complexity is avoided, the see the
difference between lfs3_file_flushonce_ vs the mess that is
lfs3_file_flush_ + lfs3_file_crystallize + lfs3_file_graft.
However, littlefs is designed around files, and a couple design
decisions hold back how much code saving is possible:
1. littlefs's shrubs are designed around being enrolled in the omdir
linked-list, so internally we still have most of the file open/close
code lumbering around.
2. Directories and traversals still exist, so we'd need the omdir
linked-list anyways, and we still need to broadcast _some_ changes.
3. Despite being intended for small amounts of data, lfs3_set/get can
still be used to create arbitrarily large files. So we still need all
of the bshrub/btree logic.
Which we still need for the mtree anyways, so this isn't really that
much of a downside.
It also may be possible to save more code by aggressively rewriting the
_entire_ read/write path for lfs3_set/get, to not reuse any of the
existing file logic in LFS3_KVONLY mode. But I decided against this due
to concerns around maintainability.
The duplicate lfs3_file_read + lfs3_file_readonce and lfs3_file_flush_ +
lfs3_file_flushonce_ are already enough of a concern.
Anyways, here's LFS3_KVONLY:
code stack ctx
default: 37824 2416 636
kvonly: 30936 (-18.2%) 2168 (-10.3%) 636 (+0.0%)
LFS3_RDONLY + LFS3_KVONLY is also interesting:
code stack ctx
rdonly: 10776 856 508
rdonly+kvonly: 9904 (-8.1%) 888 (+3.7%) 508 (+0.0%)
---
This also added some noise to the default build's code, mainly due to
tweaks in lfs3_file_readnext to allow better reuse in LFS3_KVONLY:
code stack ctx
before: 37824 2416 636
after: 37860 (+0.1%) 2416 (+0.0%) 636 (+0.0%)
This adds LFS3_o_WRSET as an internal-only 3rd file open mode (I knew
that missing open mode would come in handy) that has some _very_
interesting behavior:
- Do _not_ clear the configured file cache. The file cache is prefilled
with the file's data.
- If the file does _not_ exist and is small, create it immediately in
lfs3_file_open using the provided file cache.
- If the file _does_ exist or is not small, do nothing and open the file
normally. lfs3_file_close/sync can do the rest of the work in one
commit.
This makes it possible to implement one-commit lfs3_set on top of the
file APIs with minimal code impact:
- All of the metadata commit logic can be handled by lfs3_file_sync_, we
just call lfs3_file_sync_ with the found did+name in lfs3_file_opencfg
when WRSET.
- The invariant that lfs3_file_opencfg always reserves an mid remains
intact, since we go ahead and write the full file if necessary,
minimizing the impact on lfs3_file_opencfg's internals.
This claws back most of the code cost of the one-commit key-value API:
code stack ctx
before: 38232 2400 636
after: 37856 (-1.0%) 2416 (+0.7%) 636 (+0.0%)
before kv: 37352 2280 636
after kv: 37856 (+1.3%) 2416 (+6.0%) 636 (+0.0%)
---
I'm quite happy how this turned out. I was worried there for a bit the
key-value API was going to end up an ugly wart for the internals, but
with LFS3_o_WRSET this integrates quite nicely.
It also raises a really interesting question, should LFS3_o_WRSET be
exposed to users?
For now I'm going to play it safe and say no. While potentially useful,
it's still a pretty unintuitive API.
Another thing worth mentioning is that this does have a negative impact
on compile-time gc. Duplication adds code cost when viewing the system
as a whole, but tighter integration can backfire if the user never calls
half the APIs.
Oh well, compile-time opt-out is always an option in the future, and
users seem to care more about pre-linked measurements, probably because
it's an easier thing to find. Still, it's funny how measuring code can
have a negative impact on code. Something something Goodhart's law.
This adds a couple functions that treat files as simple key-value pairs:
- lfs3_get - Read a file
- lfs3_size - Get the size of a file
- lfs3_set - Write a file
- lfs3_remove - Remove a file (this one already exists!)
The idea is the only real difference between a filesystem and key-value
store in the microcontroller space is the API, and the key-value API
_is_ much easier to use.
It also opens the door to making the file API opt-out in the future to
trade code cost for feature set. littlefs will probably never be
competitive with other microcontroller-scale key-value stores, but it
may be interesting for systems already using littlefs for other storage.
And don't worry, these are still files, so they can always be opened
with the full file API when more advanced operations are needed.
These APIs also matches the custom attribute APIs, which makes sense
because they're both key-values. Any mismatch should be considered an
API bug, because the best user interface is a consistent one.
This new API is tested in tests/test_kv.toml.
---
At the moment the implementation is naive, just sitting on top of the
file API. This works remarkably well thanks to littlefs's cache
bypassing logic, but does have some downsides:
- lfs3_set always writes two commits: one for the stickynote and one for
the file sync.
Unfortunately this is a fundamental limitation of littlefs's file API.
One nice benefit of lfs3_set is in theory we can bypass this
limitation, but not if we just sit on top of the file API.
- There may be code savings from more tightly integrating the key-value
code.
This also highlighted an awkward corner case with per-file cache
configuration in which the buffer needs to be non-null even if zero. Not
the end of the world, but just a bit awkward. Maybe this deserves
revisiting in the config API rework?
---
Code changes were relatively minimal given that this is a whole new API,
unfortunately the stack took quite a hit:
code stack ctx
before: 37352 2280 636
after: 37644 (+0.8%) 2448 (+7.4%) 636 (+0.0%)
The stack surprised me, but in hindsight it makes sense. In sitting on
top of the reset of the codebase, the key-value API adds very little
code, but every stack allocation in these functions add to the stack
hot-path.
This isn't the end of the world, and it's actually probably a good thing
to have an lfs3_file_t allocated in the stack hot-path. lfs3_file_t's
size has been a bit difficult to track thanks to struct lfs3_info
dominating ctx measurements...
So:
// blablabla this is my cool function
#ifdef LFS3_COOL
int lfs3_cool(lfs3_t *lfs3);
#endif
Mainly because this reads better and moves the compilation conditions
closer to the actual declaration.
One concern is if this will interfere with future doxygen/documentation
generation, but I think we can expect future scripts to be able to parse
relevant ifdefs. For one, we want to make sure to include any required
ifdefs in generated documentation, so if a script can't even parse
ifdefs, uhhhhh...
No code changes.
rbyd.eoff has the relatively unique property of only being useful in
rdwr mode. In rdonly mode we don't care where the next erased-state
starts because we're never going to use it.
Since rbyds are used everywhere, dropping rbyd.eoff has the potential to
save a significant amount of RAM.
---
At least on paper. We were using the field in lfs3_rbyd_fetch to keep
track of the most recent valid commit perturb/eoff, which was a bit
tricky to disentangle.
Disentangling lfs3_rbyd_fetch does add a bit of code to the default
build, but saves code, stack, and ctx in the rdonly mode:
code stack ctx
rdonly before: 10640 816 524
rdonly after: 10616 (-0.2%) 808 (-1.0%) 508 (-3.1%)
default before: 37320 2280 636
default after: 37352 (+0.1%) 2280 (+0.0%) 636 (+0.0%)
In theory we could ifdef the crap out of lfs3_rbyd_fetch to claw back
this code, but 1. 32 bytes of code is really not that much code, 2. the
more rdonly and default diverge the more likely rdonly breaks, and 3. I
think the new code is a bit more readable since it avoids masking
perturb/eoff together until the last minute.
We don't need the staging shrub if we never stage shrubs!
The only hangup was reuse of the staging shrub to load bshrubs/btrees in
lfs3_file_fetch (we need to be able to fallback to the previous shrub if
we error in lfs3_file_resync), but this can be handled with a stack
allocated shrub.
If btree-leaf-caches make a return, we would need to stack allocate this
anyways due to the lopsided cost of the main/staging btrees/bshrubs
introduced to avoid wasting space on the useless
staging-shrub-leaf-cache.
This saves some code in LFS3_RDONLY, and apparently an instruction or
two in the default build (I guess stack loads/stores are cheaper?):
code stack ctx
rdonly before: 10680 840 524
rdonly after: 10640 (-0.4%) 816 (+0.0%) 524 (+0.0%)
default before: 37324 2280 636
default after: 37320 (-0.0%) 2280 (+0.0%) 636 (+0.0%)
It's not apparent in ctx because lfs3_info.name dominates (guh), but
this does save some RAM in lfs3_file_t:
rdonly ctx
lfs3_file_t before: 136
lfs3_file_t after: 112 (-17.6%)
It does add some stack cost to lfs3_file_fetch, but because this isn't
on the stack hot-path in either build, we don't really care:
default code stack ctx
lfs3_file_fetch before: 372 416 0
lfs3_file_fetch after: 368 (-1.1%) 440 (+5.8%) 0 (+0.0%)
This is the new readonly flag, to be consistent with LFS3_M_RDONLY and
friends.
Note this overlaps with LFS3_YES_RDONLY in a weird way, where
LFS3_YES_RDONLY is basically just an alias for LFS3_RDONLY. For most
flags, LFS3_THING enables the _option_ of using LFS3_M_THING, with
LFS3_YES_THING implying LFS3_M_THING in all mount calls. But
LFS3_RDONLY _disables_ the option of using LFS3_M_RDWR, so it's a bit
different...
Do we really need two flags for the same thing? Not sure. But most users
probably expect LFS3_RDONLY coming from other filesystems.
Worst case this can be revisited in the planned config API rework.
---
As for the readonly code size, this is just the first draft and limited
to mostly ifdefing out all prog/write logic paths. There's some TODOs in
the code that may save a bit more (rbyd.eoff, file.b.shrub_ for
example). But the results are looking ok:
code stack ctx
v2.11.0 rdonly: 6270 448 580
v3-alpha rdonly: 10776 (+71.9%) 840 (+87.5%) 524 (-9.7%)
It's interesting to note most of the additional code/stack cost come
from filesystem traversal. In v2, the threaded linked-list made rdonly
traversal _incredibly_ cheap. But the extra rdwr baggage of turning
littlefs into a fully connected graph made it something to be avoided
in v3.
This hits v3 with the double whammy of:
1. Filesystem traversal is more complicated since we need to keep track
of which btree and where in the btree we are
2. Everything needs to be tracked explicitly due to the new inverted
state-machine driven API (no callbacks)
Note that even if we disabled the traversal APIs, lfs3_fs_usage, cksum
checking, etc, we'd still need to traverse to rebuild gstate. Otherwise
we risk showing grmed files after a powerloss.
---
This did affect the default build a little bit, due to moving things
around for nicer ifdef groupings:
code stack ctx
default before: 37300 2280 636
default after: 37304 (+0.0%) 2280 (+0.0%) 636 (+0.0%)
Christopher Haster