- lfs3_mdir_lookupnext
- lfs3_mdir_lookup
Like btree lookups, mdir lookups are also tag-inspection heavy, so we
see some nice savings:
code stack ctx
before: 36520 2352 656
after: 36348 (-0.5%) 2336 (-0.7%) 656 (+0.0%)
lfs3_mdir_lookup also highlights how tag-returns help reduce API noise
around the tag mask bits. lfs3_mdir_lookup's tag out-pointer doesn't
really make sense with the default non-masked tags, and moving it to the
return position hides it aways a bit.
- lfs3_btree_lookupleaf
- lfs3_btree_lookupnext
- lfs3_btree_lookup
- lfs3_btree_traverse
- NOT lfs3_btree_namelookup
Looks like we're starting to claw back stack usage a bit. This makes
sense as the btree logic involves the most layers -- with out-pointers
it needs more temporary copies to inspect tags along the way:
code stack ctx
before: 36576 2376 656
after: 36520 (-0.2%) 2352 (-1.0%) 656 (+0.0%)
This is where I would've adopted tag-returns in lfs3_rbyd_namelookup,
but it turns out this isn't possible. We are already muxing error codes
with compare flags (lfs3_scmp_t)!
In theory we could merge err + lfs3_cmp_t + lfs3_tag_t into one big
16-bit ordered tag mux abomination, but I decided that was probably
overkill for now.
As a plus this avoids an awkward temporary tag copy in
lfs3_rbyd_namelookup as we search for a better tag. Turns out the
out-pointers in lfs3_rbyd_namelookup are quite useful for staging
things.
No code changes.
This is the start of a big refactor to try to move tag out-pointers into
the return position of functions, muxing with error codes via the
sign-bit when necessary.
So instead of:
lfs3_tag_t tag_;
lfs3_data_t data_;
int err = lfs3_rbyd_lookup(&lfs3, &rbyd, rid, tag,
&tag_, &data_);
if (err) {
return err;
}
We now do:
lfs3_data_t data_;
lfs3_stag_t tag_ = lfs3_rbyd_lookup(&lfs3, &rbyd, rid, tag,
&data_);
if (tag_ < 0) {
return tag_;
}
In theory, removing an out-pointer saves both code and stack, though it
will be interesting to actually see how much of an affect this has after
the dust has settled.
littlefs v2 used this technique heavily for its 32-bit tags, but we
never did a comparison with/without tags in the return position.
This is a big rewrite in the test code, so hopefully this ends up worth
it :)
Lots of regex.
Note this implicitly limits error codes to 16-bits, but supported error
codes are already a bit limited because we're using int everywhere
(instead of int32_t). If we need 32-bit error codes we can always add
another type to represent the mux in the future (lfs3_etag_t?).
---
So far the code savings look promising:
code stack ctx
before: 36828 2368 656
after: 36576 (-0.7%) 2376 (+0.3%) 656 (+0.0%)
Stack usage is a big disappointing, but hopefully that is just a
temporary cost due to the internal scaffolding between different API
types while the refactor is ongoing.
This use of 0 here as a no-height indicator is probably not a good idea.
If this loop ever encounters height=0, it will reset progress, and
possibly calculate an incorrect min_height.
Fortunately this can't actually happen with our current rbyds. Only the
single element rbyd has height=0, and, lacking a tree, it is trivially
balanced. But this is the sort of sleeping bug that risks becoming a
real bug in the future, so might as well fix.
Using -1 (UINT_MAX) as a default value for min_height avoids this.
The child rbyd inherits all of the btree's root state when we hit the
root, including the shrub bit. This means we don't need to check
child.block == btree.block, since only the btree root can be shrubbed
(how would non-root shrubs even work? wait... they could work, but I
think it would just end up a worse balanced binary tree? anyways).
This lets us reorder things into the rare 3-case if statement, which
helps a bit with readability:
- !lfs3_rbyd_trunk(&child) || lfs3_rbyd_isshrub(&child) => need root
- child.blocks[0] == btree->blocks[0] => is root
- otherwise => not root
Shaved off some code:
code stack ctx
before: 36832 2368 656
after: 36828 (-0.0%) 2368 (+0.0%) 656 (+0.0%)
- LFS3_ERR_RANGE => need to split btree
- LFS3_ERR_EXIST => hit a shrub root
The distinct "hit shrub root" vs "split btree" error codes are a bit
more self documenting and let us assert during test time that we never
actually split bshrub roots.
Maybe this will be reverted after some use, but in the short term better
safe than sorry.
---
This comes at a small code cost, I guess loading from constant pools is
expensive (though, tbf, lfs3_btree_commit_ is a _big_ function, maybe
the size makes constant pools trickier?). I'm guessing it's the constant
pools because the changes in lfs3_bshrub_commit had no effect:
code stack ctx
before: 36800 2368 656
after: 36832 (+0.1%) 2368 (+0.0%) 656 (+0.0%)
Based on a few observations:
- Bshrubs never go straight to splitting.
Bshrubs are always converted to btrees first, which can't fail (shrub
< 1/2 block + commit < 1/2 block).
- This means we can rely on just the current shrub bit to determine if a
commitroot_ operation is a btree split or bshrub migration.
- This fully deduplicates the split/migrate logic, so we don't even need
LFS3_ERR_EXIST anymore. LFS3_ERR_RANGE now indicates both "split
btree" and "migrate bshrub".
There is an argument for keeping LFS3_ERR_EXIST around, as "migrate
bshrub" _is_ a conceptually distinct case from "split btree". But at
least this means one less error code that could be confusing.
Saves a nice bit of code and stack:
code stack ctx
before: 36832 2384 656
after: 36800 (-0.1%) 2368 (-0.7%) 656 (+0.0%)
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.
Helps with readability when we want to mutably slice a bptr.
Also saves a bit of code:
code stack ctx
before: 36936 2384 652
after: 36860 (-0.2%) 2384 (+0.0%) 652 (+0.0%)
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.
This use to save code/stack, but apparently not anymore:
code stack ctx
before: 36960 2392 652
after: 36936 (-0.1%) 2384 (-0.3%) 652 (+0.0%)
code stack ctx
ckdatacksumreads before: 38368 2720 660
ckdatacksumreads after: 38024 (-0.9%) 2624 (-3.5%) 660 (+0.0%)
The stack hot-path has changed significantly since then, with many
functions adopting LFS3_NOINLINE to get off the stack hot-path. Not sure
if that's related.
I'm also starting to think LFS3_FORCEINLINE is a symptom of
over-optimization. We shouldn't be doing the compilers job, if it can't
figure out the best inlining strategy so be it.
Maybe it's because they are relatively new, but compound literals seem
to do more harm then good.
I'm still keeping the LFS3_DATA_SLICE macro around in case it's useful
(for tests?), but now prefering lfs3_data_slice where possible.
---
This doesn't really impact the default build, but it saves a big chunk
of code/stack when compiling with LFS3_CKDATACKSUMREADS:
code stack ctx
before: 36956 2392 652
after: 36960 (+0.0%) 2392 (+0.0%) 652 (+0.0%)
code stack ctx
ckdatacksumreads before: 38576 2744 660
ckdatacksumreads after: 38368 (-0.5%) 2720 (-0.9%) 660 (+0.0%)
LFS3_CKDATACKSUMREADS adds cksize/cksum fields to lfs3_data_t, so it's
very sensitive lfs3_data_t function changes.
Though to be far, at 5-words, lfs3_data_t really shouldn't be a
pass-by-value struct. We only keep lfs3_data_t a pass-by-value struct
because LFS3_CKDATACKSUMREADS is low-priority/best-effort and it would
make the codebase a mess.
These can be accomplished with LFS3_DATA_SLICE, and I think the
TRUNCATE/FRUNCATE variants just muddy things and make the math harder to
read.
LFS3_DATA_TRUNCATE is already basically a noop. The only non-trivial
transformation is LFS3_DATA_FRUNCATE, and LFS3_DATA_FRUNCATE is the
confusing one.
---
I have no idea why _removing_ code is adding so much stack. This needs
investigation:
code stack ctx
before: 36944 2384 652
after: 36956 (+0.0%) 2392 (+0.3%) 652 (+0.0%)
I think these were copied from the initial fragment slice calculation,
but we're already checking for <=fragment_size, so the extra lfs3_min is
unnecessary.
Saves a bit of code:
code stack ctx
before: 36952 2376 652
after: 36944 (-0.0%) 2384 (+0.3%) 652 (+0.0%)
Not sure why this added stack, compiler noise?
This was missed when reverting the trailing underscores_ in other
unconditional out-pointers.
The trailing underscore now just hints at the parameter being an
out-pointer, optionality is no longer implied.
This tweaks LFS3_DBGRBYDBALANCE to be a bit less intrusive, by putting
the relevant heights in the single lfs3_rheight_t struct.
Also added ifdefs to lfs3_rbyd_lookupnext_ just to make it clear this
code is opt-in.
No code changes.
This may have changed during some refactor, but we can reuse the entire
right branch logic, and at least deduplicate the lfs3_data_frombranch
call on the left branch.
Saves a nice bit of code:
code stack ctx
before: 37020 2392 652
after: 36952 (-0.2%) 2376 (-0.7%) 652 (+0.0%)
Also deduplicating the lfs3_data_t allocations saved stack, though that
is more concerning than anything else...
Also adopted l/r_buf names in lfs3_bcommit_t. This better matches names
in lfs3_file_graft_ and elsewhere.
A bit of an abuse of this error code, but this is more explicit than the
previous rattr_count > 0 condition.
Forgetting to set rattr_count=0 on a normal exit has introduced bugs
before.
---
Though I'm not sure why this adds code. Somehow, _removing_ the
rattr_count=0 statements when lfs3_btree_commit_ collapses the root
added code?
code stack ctx
before: 36996 2392 652
after: 37020 (+0.1%) 2392 (+0.0%) 652 (+0.0%)
Seriously, add bcommit->rattr_count = 0 to lfs3_btree_commit_ and the
lfs3_btree_commit_'s code cost shrinks by 8 bytes. Is the compiler
hiding stuff in bcommit?
I'm just going to chalk this up to compiler noise for now...
This somewhat replaces lfs3_bctx_t. Really lfs3_bctx_t consumed the
previously separate bid, rattr, and rattr_count out-pointers and
underwent a slight name change. The previous contents of lfs3_bctx_t are
all available under bcommit.ctx, with some minor tweaks.
The main motivation for this was to get rid of the mess that was the
bid/rattr out-pointers. They represent a side-channel of internal btree
state that is probably better implemented as a single struct.
Hopefully this makes the logic of lfs3_btree_commit_ callers -- and
expected action on non-zero rattr_count -- more obvious.
---
Some other tweaks:
- Separated ctx.buf into bcommit.ctx.branch_l_buf/branch_r_buf.
I realized this informs the compiler that the lfs3_data_frombranch
calls should not overflow.
This may need to be reverted if we ever commit different data types in
lfs3_btree_commit_, but that's not the end of the world. Right now
this is bound to whatever split needs (2 branches + name).
- Added rattr_count <= rattrs assert after each btree commit builder.
These asserts were just adopted after the btree code was written. The
extra safeguards are good to have in case of future refactor.
Shaves off a bit more code/stack while also (hopefully) improving code
readability:
code stack ctx
before: 37048 2416 652
after: 36996 (-0.1%) 2392 (-1.0%) 652 (+0.0%)
This moves the default recurse logic (previously the commit label) back
up before the compact/relocate/split/merge branches.
I know the general rule is to try to limit gotos to foward jumps, but in
this case, placing the default recurse logic at the end of
lfs3_btree_commit_ disrupts the default "happy" path and makes
refactoring more difficult than it needs to be.
Contextually, the default recurse logic is a part of the default commit
logic, and split, merge, etc, are exceptional branches that just happen
to sometimes converge.
---
I think the real problem is that all of the gotos in lfs3_btree_commit_
are modeling mutually recursive functions, but in a context where we
can't actually recurse.
_Technically_, it is possible to transform any tail-recursive function
into loops and if statements (structured program theorem), but doing so
risks significant code duplication. We could duplicate this recurse
logic everywhere it's needed for example. But this is also something we
want to avoid in littlefs.
So goto soup it is.
---
Some code changes, but probably just compiler noise:
code stack ctx
before: 37052 2416 652
after: 37048 (-0.0%) 2416 (+0.0%) 652 (+0.0%)
I also added some more informative-only labels now that we've adopted
-Wno-unused-label. These are useful for documenting independent chunks
of logic in a large function like this, and as debugging targets.
Not sure how this was missed. The whole tradeoff of shrinking
rattr.count was that by default lfs3_rattr_t would take up less space,
but user-provided buffers would need an indirect lfs3_data_t to support
arbitrary buffer sizes.
This managed to scrape by with a 16-bit count (15-bit really), but
fortunately failed test_attrs_fattr_resync_receive with an 8-bit count.
And only barely! 256 is the smallest possible custom attr that
overflows.
I guess a point towards making internal limitation as tight as possible
to catch mistakes like these earlier.
---
Added test_attrs_setattr_big and test_attrs_fattr_big to catch this in
the future.
Note that while this added some code, stack is unaffected. This is
because custom attribute handling is off the hot-path, which is why the
lfs3_rattr_t -> lfs3_rattr_t+lfs3_data_t split is worth it:
code stack ctx
before: 37016 2416 652
after: 37052 (+0.1%) 2416 (+0.0%) 652 (+0.0%)
This breaks down the previously 16-bit rattr.count field into two 8-bit
rattr.from and rattr.count fields. Now, instead of using a mixture of
rattr.tag and sign(rattr.count) to determine rattr encoding, we just
jump based on rattr.from:
lfs3_rattr_t:
.---+---+---+---.
| tag |frm|cnt| -+-> 16-bit tag - on-disk encoding + rbyd flags
+---+---+---+---+ +-> 8-bit from - in-RAM encoding
| weight | '-> 8-bit count - from-specific count
+---+---+---+---+
| ptr |
'---+---+---+---'
The internal appendrattr_ ctx also saw a bit of rework, and now uses a
big union with multiple buffers instead of stacking a ridiculous number
of LFS_MAX calls. Expanding the LFS_MAX stack grows O(n^2), so this is
probably good for compile times.
And all rattr.from branches now generate an lfs3_data_t*. This was
already a side-effect of all the internal lfs3_data_from* functions, and
it simplifies the tail end of appendrattr_. No more relying on
data_count's sign bit.
Also rearranged rattr.from encoders to match source code order.
---
Unfortunately, while this did simplify the source code, it didn't really
lead to much improvement in code size:
code stack ctx
before: 37024 2416 652
after: 37016 (-0.0%) 2416 (+0.0%) 652 (+0.0%)
I guess jump tables are more a performance optimization than a code size
one. That and the benefit of cheaper appendrattr_ logic is likely
overshadowed by the extra constants needed to populate rattr.from in
every LFS3_RATTR_* macro.
Also test_attrs_fattr_resync_receive is now failing, but I think that's
just because of an unrelated bug exposed by the shrinking count field.
In theory rattr.count should be limited to internal fixed-size buffers.
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
This goes ahead and makes all dbg scripts available in dbg.gdb.py, via
the magic of globbing __file__ relative, and dynamic python class
generation.
Probably one of the more evil scripts I've written, but this means we
don't need to worry about dbg.gdb.py falling out-of-date when adding new
dbg scripts.
Not all of the dbg scripts are useful inside gdb, but most of them are.
After all, what's cooler than this!
(gdb) dbgrbyd -b4096 "disk" -t \
file->b.shrub.blocks[0] \
--trunk lfs3_rbyd_trunk(&file->b.shrub)
rbyd 0x46.23a w2048, rev 00000000, size 629, cksum 8f5169e1
00000004: .-> 0-334 data w335 0
00000009: .-+-> 335 data w1 1 71
0000000e: | .-> 336 data w1 1 67
00000013: .-+-+-> 337 data w1 1 66
...
00000144: | | | | .-> 350 data w1 1 74
0000019a: | | | | .-+-> 351 data w1 1 78
000001f5: | | | | | .-> 352-739 data w388 1 76
00000258: +-+-+-+-+-+-> 740-2047 data w1308 1 6c
Note some tricks to help interact with bash and gdb:
- Flags are passed as is (-b4096, -t, --trunk)
- All non-flags are parsed as expressions (file->b.shrub.blocks[0])
- String expressions may be useful for paths and stuff ("./disk")
So instead of:
$ ./scripts/dbgflags.py o 0x10000003
The filter is now specified as a normal(ish) argparse flag:
$ ./scripts/dbgflags.py --o 0x10000003
This is a bit easier to interop with in dbg.gdb.py, and I think a bit
more readable.
Though -a and --a now do _very_ different things. I'm sure that won't
confuse anyone...
Like test.py --gdb-script, being able to specify multiple header files
seems useful and is easy enough to add.
---
Note that the default is only used if no other header files are
specified, so this _replaces_ the default header file:
$ ./scripts/test.py --include=my_header.h
If you don't want to replace the default header file, you currently need
to specify it explicitly:
$ ./scripts/test.py \
--include=runners/test_runner.h \
--include=my_header.h
These just invoke the existing dbg*.py python scripts, but allow quick
references to variables in the debugginged process:
(gdb) dbgflags o file->b.o.flags
LFS3_O_RDWR 0x00000002 Open a file as read and write
LFS3_o_REG 0x10000000 Type = regular-file
LFS3_o_UNSYNC 0x01000000 File's metadata does not match disk
Quite neat and useful!
This works by injecting dbg.gdb.py via gdb -x, which includes the
necessary python hooks to add these commands to gdb. This can be
overridden/extended with test.py/bench.py's --gdb-script flag.
Currently limited to scripts that seem the most useful for process
internals:
- dbgerr - Decode littlefs error codes
- dbgflags - Decode littlefs flags
- dbgtag - Decode littlefs tags
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 was the main culprit behind our stack increase. Inlining
lfs3_file_crystallize into lfs3_file_flush_ adds a bit of code, but as a
tradeoff:
- Keeps all lfs3_file_crystallization_ calls at the same abstraction
level, which is generally easier to reason about and avoids issues
with things like lfs3_alloc_ckpoints.
- Makes some low-level interactions, such as LFS3_o_UNCRYST masking,
more obvious.
- Reduces the stack hot-path by the cost of lfs3_file_flush_
Saves some stack at a code cost:
code stack ctx
before: 37492 2464 656
after: 37504 (+0.0%) 2448 (-0.6%) 656 (+0.0%)
Now that the dust has settled a bit, we can also compare the lazy
grafting vs lazy crystallization builds:
code stack ctx
lazy-graft: 38020 2456 656
lazycryst: 37504 (-1.4%) 2448 (-0.3%) 656 (+0.0%)
It worked well for file leaves, so we might as well adopt the same
post-truncate/fruncate logic for caches.
This moves checks for cache.size==0 from lfs3_file_write into
lfs3_file_truncate/fruncate.
Note that lfs3_file_truncate/fruncate are the only functions (for now)
that can reduce the size of a file.
Adds a bit of code, which is probably why this wasn't adopted earlier,
but it reduces the state we need to worry about and makes things easier
to understand:
code stack ctx
before: 37468 2464 656
after: 37492 (+0.1%) 2464 (+0.0%) 656 (+0.0%)
Now that we don't need to worry about losing data due to ungrafted
state, we can decide whether or not to discard leaves after
truncate/fruncate.
This simplifies lfs3_file_truncate/fruncate (and makes them much more
readable as a plus), but also lets us simplify lfs3_file_crystallize
since we no longer need to worry about implicit flushes.
lfs3_file_crystallize's call sites:
- lfs3_file_flush_ - We've already committed to flushing, so
opportunistically clearing LFS3_o_UNFLUSH has no effect.
lfs3_file_flush_'s logic should already take advantage of possible
flushes anyways.
- lfs3_file_flush - We only call lfs3_file_crystallize _after_
lfs3_file_flush_, so this has no effect.
This saves a bit more code and stack:
code stack ctx
before: 37588 2472 656
after: 37468 (-0.3%) 2464 (-0.3%) 656 (+0.0%)
Seeing as the generalized lfs3_file_crystallize_ API had a much lower
cost than I thought, we might as well keep it around a bit longer.
Though I at least tweaked it to hopefully be easier for compilers to
optimize: By accepting crystal_min=-1 as an alias for
crystal_min=crystal_max, compilers should always by able to const
propagate this.
---
Note sure why this still adds 8 bytes of code, it just looks like
compiler noise in lfs3_file_crystallize__? Is the LFS3_NOINLINE
attribute messing with compiler optimizations?
code stack ctx
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Eventually the generalized crystallize API may be useful again for the
"eager crystallization" write strategy, but the codebase has drifted
apart enough already that this will require some reimplementation
anyways (review the commit history!).
Might as well clean up API weirdness we're not using.
Saves surprisingly little code. I guess the compiler was able to
optimize out the duplicated args once the logic was a bit simpler?
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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!
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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.
The motivation for this comes from the observation that lfs3_file_flush
already implies lfs3_file_crystallize, so most of the time the isuncryst
check in lfs3_file_readnext is useless.
We _do_ hit the isuncryst check when bypassing the cache, but the
situation where we bypass the cache, on a read-write file, _and_ can
avoid crystallization, seems too niche to care about.
So this reworks lfs3_file_read to prevent cache bypassing until pending
data is at least crystallized. This mirrors how we force flushing in
lfs3_file_write.
lfs3_file_read:
|<------------------------------------------------------------.
v |
data in cache? --> read from cache -------------------------------->|
| n y |
v |
data in btree? --> crystallized? --> bypass? --> read from disk --->|
| n y | n y | n y |
| | v |
| | flushed? --> read into cache ->|
| | | n y |
| | v |
| '-------> flush cache -------------------->|
v |
fill with zeros ----------------------------------------------------'
lfs3_file_write:
|<------------------------------------.
v |
flushed? --> bypass? --> write to disk ->|
| n y | n y |
| v |
| move cache |
v v |
aligned? --> write into cache ---------->|
| n y |
v |
flush cache -----------------------------'
---
As a part of the rework, I also manually inlined lfs3_file_readnext into
lfs3_file_readget_. This duplicates some logic (not code cost!), but
helps clean up some of the ifdef soup in lfs3_file_readnext.
I also tried to refactor lfs3_file_readnext to better match
lfs3_file_read and lfs3_file_write's logic, but I'm not it actually
gained us anything.
lfs3_file_readnext:
|<----------------------------.
v |
data in leaf? --> read from leaf |
| n y | |
v v |
data in hole? --> fill with zeros |
| n y | |
v | |
fetch leaf -------------|----------'
v
done!
Saves a bit of code:
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after: 38020 (-0.1%) 2456 (+0.0%) 656 (+0.0%)
Also likely eliminates lfs3_file_readnext from ever becoming the stack
hot-path again.
This drops the LFS3_TSTATE_GRAFT state for just explicitly iterating
over graft state in lfs3_alloc. This is cheaper as long as lfs3_alloc is
the only traversal we trigger while grafting.
We already rely on the lfs3_alloc-specific behavior of never touching
cksize/cksum fields anyways.
Note both lfs3_alloc_markinuse and lfs3_alloc_markinuse_ already have
multiple call sites and can't be inlined due to lookahead population in
lfs3_mtree_gc. We also don't need to worry about graft state there as
incremental traversals only make progress when bshrubs are at rest.
Saves a bit of code:
code stack ctx
before: 38092 2456 656
after: 38060 (-0.1%) 2456 (+0.0%) 656 (+0.0%)
before graft: 37936 2456 636
after graft: 38060 (+0.3%) 2456 (+0.0%) 656 (+3.1%)
Actually, surprisingly little code, but anything that simplifies
lfs3_mtree_traverse_ is welcome.
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:
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This applies the same pattern of taking both the old + staging btree as
arguments to try to avoid redundant stack allocations.
Extra appealing is being able to reuse the staging shrubs in bshrubs for
btree commits.
However, it doesn't work out so well for the btree logic:
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A couple reasons:
- Passing staging references limits what the compiler can optimize,
compilers aren't great at cross-function optimization
- These staging references push struct allocation upwards, which risks
pushing them onto the stack hot-path.
Gah, again this is likely not a real issue, just a failure of our
tooling to take stack shrinkwrapping into account.
- The extra arguments adds stack overhead to the call frame. It's just
one word, but this can add up.
I should probably revert this, but I'm going to keep it around for a
bit:
- It's only 32 bytes (1 rbyd + 1 pointer + compiler noise). Is 32 bytes
enough to really care about?
- I'm not sure how much weight to put into our stack measurements at the
moment. They don't take shrinkwrapping into account that create a
weird bias.
- This internal API better conveys how it behaves w.r.t. atomic updates
and errors.
- The API may also lead to better stack usage in the future.
I've noticed a common pattern where we tend to create copies in multiple
function frames in order to allow fallback in case of errors. This risks
redundant stack allocations across layers.
To avoid this, this commit adopts old + staging arguments for most of
the internal mdir commit functions:
static int lfs3_mdir_commit_(lfs3_t *lfs3,
lfs3_mdir_t *mdir_, lfs3_mdir_t *mdir,
...);
We already needed this for lfs3_mdir_compact__, so hey, points for
consistency.
Saves a tiny bit of code:
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Not sure why we weren't already, it doesn't really make sense to return
bid without weight, and this matches lfs3_btree/bshrub_lookupnext.
Sure we don't need weight currently, but this is useful to include in
case we need it in the future (lfs3_bptr_fetch during traversal?).
And while we're not using it, the compiler is happy to optimize it out,
so no code changes:
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after 37964 (+0.0%) 2424 (+0.0%) 636 (+0.0%)
Like the bshrub/btree dedup, this add lfs3_bptr_fetch to help dedup
bptr/data fetching.
The original plan was to eliminate bptrs from lfs3_file_lookupnext and
lfs3_file_traverse, and just return tagged data like the other
lookup/traverse functions. But this didn't work out very well. We return
arbitrary attrs from lfs3_file_traverse, so all this would've
accomplished is making every lfs3_file_lookupnext call messier.
But I think I'm still going to keep lfs3_bptr_fetch around as it
provides a nice place to deduplicate some other bits of logic:
- It makes sense to limit bptrs to compressed weights here, as opposed
to the somewhat arbitrary lfs3_file_lookupnext function.
- And it would be a bit silly to not put the bptr's LFS3_CKFETCHES logic
in lfs3_bptr_fetch.
This may fetch more than previously (during crystallization pokes?),
but better safe than sorry. LFS3_CKFETCHES will likely be a relatively
niche feature anyways.
As for lfs3_file_traverse, I got rid of it completely.
We already have special logic in lfs3_mtree_traverse_ and lfs3_file_ck
for bptrs anyways, since bptrs, unlike data fragments, reference actual
blocks. And this disentangles lfs3_mtree_traverse_ from the file APIs,
which was a bit of an awkward design.
---
This adds a bit of code to the default build, but I think it's worth it
for the better code organization:
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after: 37964 (+0.2%) 2424 (+0.0%) 636 (+0.0%)
It also saves some code in LFS3_CKFETCHES mode, thanks to deduping all
the fetch ckfetches fetch checkhes:
code stack ctx
ckfetches before: 38144 2464 636
ckfetches after: 38072 (-0.2%) 2472 (+0.3%) 636 (+0.0%)
This adds lfs3_bshrub_fetch to better deduplicate the common pattern of
fetching either a bshrub or btree based on tag.
The API ends up a bit funny because of how mdirs are attached to
specific mids. All we need is the relevant mdir object, and we can do a
single masked mdir lookup to find any bshrubs/btrees.
Saves a little bit of code:
code stack ctx
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after: 37896 (-0.1%) 2424 (+0.0%) 636 (+0.0%)
Also flipped around some lfs3_data_read* parameters to better match
common tag+weight+data ordering in lfs3_*_lookup functions.
This tries to call lfs3_alloc_ckpoint in more correct positions, and
fixes a bug where we _never_ called lfs3_alloc_ckpoint before
finishing crystallization in lfs3_file_readnext and
lfs3_file_truncate/fruncate:
- lfs3_file_crystallize now implicitly calls lfs3_alloc_ckpoint before
both finishing crystallization and grafting.
- lfs3_file_flush_ and lfs3_file_flushonce_ now call lfs3_alloc_ckpoint
at the beginning of each loop iteration.
This may be redundant on some iterations but that's ok.
- lfs3_file_write does _not_ call lfs3_alloc_ckpoint, this is all
handled in lfs3_file_flush_ now.
- lfs3_file_truncate/fruncate still call lfs3_alloc_ckpoint, but just
before lfs3_file_graft.
This matches the lfs3_alloc_ckpoint pattern used for most
lfs3_mdir_commit calls, i.e. checkpoint just before to make it easier
to audit the logic.
- Also moved the pre-fragment crystallization out of the fragment loop,
we should only crystallize once and this makes the code a bit more
readable.
I think this is the source of the extra 8 bytes of stack, but that's
small enough to consider compiler noise.
It's not the biggest problem to not call lfs3_alloc_ckpoint everytime
all blocks are at rest, but it does risk a premature ENOSPC error when
it's still possible to make progress.
This gets more complicated with lazy crystallization/grafting, as block
allocations can end up deferred to operations you might not expect
(lfs3_file_read for example).
Adds a bit of code, but is in theory more correct:
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This actually binds our custom write/writeln functions as methods to the
file object:
def writeln(self, s=''):
self.write(s)
self.write('\n')
f.writeln = writeln.__get__(f)
This doesn't really gain us anything, but is a bit more correct and may
be safer if other code messes with the file's internals.
This was modified incorrectly for LFS3_2BONLY. We do actually end up
with non-bptr non-data tags here when we encounter btree inner nodes.
Code changes:
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In lfs3_mdir_namelookup, when compiling with LFS3_2BLOCK, there was an
uninitialized variable warning that just wouldn't go away (temporarily
disabled with the x=x hack).
So, giving up on the err < 0 compiler guidance since it apparently
doesn't work. Instead lfs3_rbyd_namelookup and lfs3_btree_namelookupleaf
unconditionally initialize the problematic variables before their main
loops.
This adds a bit of code, but fighting the compiler just isn't worth the
headache:
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after: 37864 (+0.1%) 2416 (+0.0%) 636 (+0.0%)
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?
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Christopher Haster