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
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without needing to worry about out-of-date file leaves.
Out-of-date file leaves were a headache for both code cost and concerns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 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.
As you might expect, this is the inverse of ifdef, and is useful for
supporting opt-out flags.
I don't think ifdef + ifndef is powerful enough to handle _all_
compile-time corner cases, but they at least provide convenient handling
for the most common flags. Worst case, tests/benches can always include
explicit #if/#ifdef/#ifndef statements in the code itself.
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.
Kinda. It's actually only 3 scripts. These have been replaced with the
new dbg*.py scripts:
- readblock.py -> dbgblock.py
- readmdir.py -> dbgrbyd.py
- readtree.py -> dbglfs.py
Not sure what the point of this was, I think it was copied from a d3
example svg at some point. But it forces the svg to always fit in the
window, even if this makes the svg unreadable.
These svgs tend to end up questionably large in order to fit in the most
info, so the unreadableness ends up a real problem for even modest
window sizes.
These mimic the relevant LFS_O_* flags, and allow users to assert
whether or not a traversal will mutate the filesystem:
LFS_T_MODE 0x00000001 The traversal's access mode
LFS_T_RDWR 0x00000000 Open traversal as read and write
LFS_T_RDONLY 0x00000001 Open traversal as read only
In theory, these could also change internal allocations, but littlefs
doesn't really work that way.
Note we _don't_ add related LFS_GC_RDONLY, LFS_GC_RDWR, etc flags. These
are sort of implied by the relevant LFS_M_* flags.
Adds a bit more code, probably because of the slightly more complicated
internal constants for the internal traversals. But I think the
self-documentingness is worth it:
code stack ctx
before: 37200 2288 636
after: 37220 (+0.1%) 2288 (+0.0%) 636 (+0.0%)
This time to account for the new LFS_o_UNCRYST and LFS_o_UNGRAFT flags.
This required moving the T flags out of the way, which of course
conflicted with TSTATE, so that had to move...
One thing that helped was shoving LFS_O_DESYNC up with the internal
state flags. It's definitely more a state flag than the other public
flags, it just also happens to be user toggleable.
Here's the new jenga:
8 8 8 8
.----++----++----++----.
.-..----..-..-..-------.
o_flags: |t|| f ||o||t|| o |
|-||-.--':-:|-|'--.-.--'
|-||-|.----.|-'--------.
t_flags: |t||f||tstt|| t |
'-''-''----'|----.-----'
.----..-.:-:|----|:-:.-.
m_flags: | m ||c||o|| t ||o||m|
|----||-|'-'|-.--''-''-'
|----||-|---|-|.-------.
f_flags: | m ||c| |t|| f |
'----''-'---'-''-------'
This adds a bit of code, but that's not the end of the world:
code stack ctx
before: 37172 2288 636
after: 37200 (+0.1%) 2288 (+0.0%) 636 (+0.0%)
- LFS_CKPARITY -> LFS_CKMETAPARITY
- LFS_CKDATACKSUMS -> LFS_CKDATACKSUMREADS
The goal here is to provide hints for 1. what is being checked (META,
DATA, etc), and 2. on what operation (FETCHES, PROGS, READS, etc).
Note that LFS_CKDATACKSUMREADS is intended to eventually be a part of a
set of flags that can pull off closed fully-checked reads:
- LFS_CKMETAREDUNDREADS - Check data checksums on reads
- LFS_CKDATACKSUMREADS - Check metadata redund blocks on reads
- LFS_CKREADS - LFS_CKMETAREDUNDREADS + LFS_CKDATACKSUMREADS
Also it's probably not a bad idea for LFS_CKMETAPARITY to be harder to
use. It's really not worth enabling unless you understand its
limitations (<1 bit of error detection, yay).
No code changes.
Not sure why, but this just seems more intuitive/correct. Maybe because
LFSR_TAG_NAME is always the first tag in a file's attr set:
LFSR_TAG_NAMELIMIT 0x0039 v--- ---- --11 1--1
LFSR_TAG_FILELIMIT 0x003a v--- ---- --11 1-1-
Seeing as several parts of the codebase still use the previous order,
it seems reasonable to switch back to that.
No code changes.
Still on the fence about this, but in hindsight the code/stack
difference is not _that_ much:
code stack ctx
before: 36460 2280 636
after: 37092 (+1.7%) 2304 (+1.1%) 636 (+0.0%)
Especially with the potential to significantly speed up linear file
writes/rewrites, which are usually the most common file operation. You
ever just, you know, write a whole file at once?
Note we can still add the previous behavior as an opt-in write strategy
to save code/stack when preferred over linear write/rewrite speed.
This is actually the main reason I think we should prefer
lazy-crystallization by default. Of the theoretical/future write
strategies, lazy-crystallization was the only one trading performance
for code/stack and not vice versa (global-alignment, linear-only,
fully-fragmented, etc).
If we default to a small, but less performant filesystem, it risks users
thinking littlefs is slow when they just haven't turned on the right
flags.
That being said there's a balance here. Users will probably judge
littlefs based on its default code size for the same reason.
---
Note this includes the generalized lfsr_file_crystallize_ API, which
adds a bit of code:
code stack ctx
before gen-cryst: 37084 2304 636
after gen-cryst: 37092 (+0.0%) 2304 (+0.0%) 636 (+0.0%)
This reverts most of the lazy-grafting/crystallization logic, but keeps
the general crystallization algorithm rewrite and file->leaf for caching
read operations and erased-state.
Unfortunately lazy-grafting/crystallization is both a code and stack
heavy feature for a relatively specific write pattern. It doesn't even
help if we're forced to write fragments due to prog alignment.
Dropping lazy-grafting/crystallization trades off linear write/rewrite
performance for code and stack savings:
code stack ctx
before: 37084 2304 636
after: 36428 (-1.8%) 2248 (-2.4%) 636 (+0.0%)
But with file->leaf we still keep the improvements to linear read
performance!
Compared to pre-file->leaf:
code stack ctx
before file->leaf: 36016 2296 636
after lazy file->leaf: 37084 (+3.0%) 2304 (+0.3%) 636 (+0.0%)
after eager file->leaf: 36428 (+1.1%) 2248 (-2.1%) 636 (+0.0%)
I'm still on the fence about this, but lazy-grafting/crystallization is
just a lot of code... And the first 6 letters of littlefs don't spell
"speedy" last time I checked...
At the very least we can always add lazy-grafting/crystallization as an
opt-in write strategy later.
This adopts lazy crystallization in _addition_ to lazy grafting, managed
by separate LFS_o_UNCRYST and LFS_o_UNGRAFT flags:
LFS_o_UNCRYST 0x00400000 File's leaf not fully crystallized
LFS_o_UNGRAFT 0x00800000 File's leaf does not match bshrub/btree
This lets us graft not-fully-crystallized blocks into the tree without
needing to fully crystallize, avoiding repeated recrystallizations when
linearly rewriting a file.
Long story short, this gives file rewrites roughly the same performance
as linear file writes.
---
In theory you could also have fully crystallized but ungrafted blocks
(UNGRAFT + ~UNCRYST), but this doesn't happen with the current logic.
lfsr_file_crystallize eagerly grafts blocks once they're crystallized.
Internally, lfsr_file_crystallize replaces lfsr_file_graft for the
"don't care, gimme file->leaf" operation. This is analogous to
lfsr_file_flush for file->cache.
Note we do _not_ use LFS_o_UNCRYST to track erased-state! If we did,
erased-state wouldn't survive lfsr_file_flush!
---
Of course, this adds even more code. Fortunately not _that_ much
considering how many lines of code changed:
code stack ctx
before: 37012 2304 636
after 37084 (+0.2%) 2304 (+0.0%) 636 (+0.0%)
There is another downside however, and that's that our benchmarked disk
usage is slightly worse during random writes.
I haven't fully investigated this, but I think it's due to more
temporary fragments/blocks in the B-tree before flushing. This can cause
B-tree inner nodes to split earlier than when eagerly recrystallizing.
This also leads to higher disk usage pre-flush since we keep both the
old and new blocks around while uncrystallized, but since most rewrites
are probably going to be CoW on top of committed files, I don't think
this will be a big deal.
Note the disk usage ends up the same after lfsr_file_flush.
TLDR: Added file->leaf, which can track file fragments (read only) and
blocks independently from file->b.shrub. This speeds up linear
read/write performance at a heavy code/stack cost.
The jury is still out on if this ends up reverted.
---
This is another change motivated by benchmarking, specifically the
significant regression in linear reads.
The problem is that CTZ skip-lists are actually _really_ good at
appending blocks! (but only appending blocks) The entire state of the
file is contained in the last block, so file writes can resume without
any reads. With B-trees, we need at least 1 B-tree lookup to resume
appending, and this really adds up when writing extremely blocks.
To try to mitigate this, I added file->leaf, a single in-RAM bptr for
tracking the most recent leaf we've operated on. This avoids B-tree
lookups during linear reads, and allowing the leaf to fall out-of-sync
with the B-tree avoids both B-tree lookups and commits during writes.
Unfortunately this isn't a complete win for writes. If we write
fragments, i.e. cache_size < prog_size, we still need to incrementally
commit to the B-tree. Fragments are a bit annoying for caching as any
B-tree commit can discard the block they reside on.
For reading, however, this brings read performance back to roughly the
same as CTZ skip-lists.
---
This also turned into more-or-less a full rewrite of the lfsr_file_flush
-> lfsr_file_crystallize code path, which is probably a good thing. This
code needed some TLC.
file->leaf also replaces the previous eblock/eoff mechanism for
erased-state tracking via the new LFSR_BPTR_ISERASED flag. This should
be useful when exploring more erased-state tracking mechanisms (ddtree).
Unfortunately, all of this additional in-RAM state is very costly. I
think there's some cleanup that can be done (the current impl is a bit
of a mess/proof-of-concept), but this does add a significant chunk of
both code and stack:
code stack ctx
before: 36016 2296 636
after: 37228 (+3.4%) 2328 (+1.4%) 636 (+0.0%)
file->leaf also increases the size of lfsr_file_t, but this doesn't show
up in ctx because struct lfs_info dominates:
lfsr_file_t before: 116
lfsr_file_t after: 136 (+17.2%)
Hm... Maybe ctx measurements should use a lower LFS_NAME_MAX?
- codemapd3.py -> codemapsvg.py
- dbgbmapd3.py -> dbgbmapsvg.py
- treemapd3.py -> treemapsvg.py
Originally these were named this way to match plotmpl.py, but these
names were misleading. These scripts don't actually use the d3 library,
they're just piles of Python, SVG, and Javascript, modelled after the
excellent d3 treemap examples.
Keeping the *d3.py names around also felt a bit unfair to brendangregg's
flamegraph SVGs, which were the inspiration for the interactive
component. With d3 you would normally expect a rich HTML page, which is
how you even include the d3 library.
plotmpl.py is also an outlier in that it supports both .svg and .png
output. So having a different naming convention in this case makes
sense to me.
So, renaming *d3.py -> *svg.py. The inspiration from d3 is still
mentioned in the top-level comments in the relevant files.
This adds --xlim-stddev and --ylim-stddev as alternatives to -X/--xlim
and -Y/--ylim that define the plot limits in terms of standard
deviations from the mean, instead of in absolute values.
So want to only plot data within +-1 standard deviation? Use:
$ ./scripts/plot.py --ylim-stddev=-1,+1
Want to ignore outliers >3 standard deviations? Use:
$ ./scripts/plot.py --ylim-stddev=3
This is very useful for plotting the amortized/per-byte benchmarks,
which have a tendency to run off towards infinity near zero.
Before, we could truncate data explicitly with -Y/--ylim, but this was
getting very tedious and doesn't work well when you don't know what the
data is going to look like beforehand.
Whoops, looks like cumulative results were overlooked when multiple
bench measurements per bench were added. We were just adding all
cumulative results together!
This led to some very confusing bench results.
The solution here is to keep track of per-measurement cumulative results
via a Python dict. Which adds some memory usage, but definitely not
enough to be noticeable in the context of the bench-runner.
This should be floor (rounds towards -inf), not int (rounds towards
zero), otherwise sub-integer results get funky:
- floor si(0.00001) => 10u
- int si(0.00001) => 0.01m
- floor si(0.000001) => 1u
- int si(0.000001) => m (???)
This was a simple typo. Unfortunately went unnoticed because the
lingering dataset assigned in the above for loop made the results look
mostly correct. Yay.
Before this, the only option for ordering the legend was by specifying
explicit -L/--add-label labels. This works for the most part, but
doesn't cover the case where you don't know the parameterization of the
input data.
And we already have -s/-S flags in other csv scripts, so it makes sense
to adopt them in plot.py/plotmpl.py to allow sorting by one or more
explicit fields.
Note that -s/-S can be combined with explicit -L/--add-labels to order
datasets with the same sort field:
$ ./scripts/plot.py bench.csv \
-bBLOCK_SIZE \
-xn \
-ybench_readed \
-ybench_proged \
-ybench_erased \
--legend \
-sBLOCK_SIZE \
-L'*,bench_readed=bs=%(BLOCK_SIZE)s' \
-L'*,bench_proged=' \
-L'*,bench_erased='
---
Unfortunately this conflicted with -s/--sleep, which is a common flag in
the ascii-art scripts. This was bound to conflict with -s/--sort
eventually, so a came up with some alternatives:
- -s/--sleep -> -~/--sleep
- -S/--coalesce -> -+/--coalesce
But I'll admit I'm not the happiest about these...
Whoops! A missing splat repetition here meant we only ever accepted
floats with a single digit of precision and no e/E exponents.
Humorously this went unnoticed because our scripts were only
_outputting_ single digit floats, but now that that's fixed, float
parsing also needs a fix.
Fixed by allowing >1 digit of precision in our CsvFloat regex.
This adds __csv__ methods to all Csv* classes to indicate how to write
csv/json output, and adopts Python's default float repr. As a plus, this
also lets us use "inf" for infinity in csv/json files, avoiding
potential unicode issues.
Before this we were reusing __str__ for both table rendering and
csv/json writing, which rounded to a single decimal digit! This made
float output pretty much useless outside of trivial cases.
---
Note Python apparently does some of its own rounding (1/10 -> 0.1?), so
the result may still not be round-trippable, but this is probably fine
for our somewhat hack-infested csv scripts.
So now the hidden variants of field specifiers can be used to manipulate
by fields and field fields without implying a complete field set:
$ ./scripts/csv.py lfs.code.csv \
-Bsubsystem=lfsr_file -Dfunction='lfsr_file_*' \
-fcode_size
Is the same as:
$ ./scripts/csv.py lfs.code.csv \
-bfile -bsubsystem=lfsr_file -Dfunction='lfsr_file_*' \
-fcode_size
Attempting to use -b/--by here would delete/merge the file field, as
cvs.py assumes -b/-f specify all of the relevant field type.
Note that fields can also be explicitly deleted with -D/--define's new
glob support:
$ ./scripts/csv.py lfs.code.csv -Dfile='*' -fcode_size
---
This solves an annoying problem specific to csv.py, where manipulating
by fields and field fields would often force you to specify all relevant
-b/-f fields. With how benchmarks are parameterized, this list ends up
_looong_.
It's a bit of a hack/abuse of the hidden flags, but the alternative
would be field globbing, which 1. would be a real pain-in-the-ass to
implement, and 2. affect almost all of the scripts. Reusing the hidden
flags for this keeps the complexity limited to csv.py.
Globs in CLI attrs (-L'*=bs=%(bs)s' for example), have been remarkably
useful. It makes sense to extend this to the other flags that match
against CSV fields, though this does add complexity to a large number of
smaller scripts.
- -D/--define can now use globs when filtering:
$ ./scripts/code.py lfs.o -Dfunction='lfsr_file_*'
-D/--define already accepted a comma-separated list of options, so
extending this to globs makes sense.
Note this differs from test.py/bench.py's -D/--define. Globbing in
test.py/bench.py wouldn't really work since -D/--define is generative,
not matching. But there's already other differences such as integer
parsing, range, etc. It's not worth making these perfectly consistent
as they are really two different tools that just happen to look the
same.
- -c/--compare now matches with globs when finding the compare entry:
$ ./scripts/code.py lfs.o -c'lfs*_file_sync'
This is quite a bit less useful that -D/--define, but makes sense for
consistency.
Note -c/--compare just chooses the first match. It doesn't really make
sense to compare against multiple entries.
This raised the question of globs in the field specifiers themselves
(-f'bench_*' for example), but I'm rejecting this for now as I need to
draw the complexity/scope _somewhere_, and I'm worried it's already way
over on the too-complex side.
So, for now, field names must always be specified explicitly. Globbing
field names would add too much complexity. Especially considering how
many flags accept field names in these scripts.
I don't know how I completely missed that this doesn't actually work!
Using del _does_ work in Python's repl, but it makes sense the repl may
differ from actual function execution in this case.
The problem is Python still thinks the relevant builtin is a local
variables after deletion, raising an UnboundLocalError instead of
performing a global lookup. In theory this would work if the variable
could be made global, but since global/nonlocal statements are lifted,
Python complains with "SyntaxError: name 'list' is parameter and
global".
And that's A-Ok! Intentionally shadowing language builtins already puts
this code deep into ugly hacks territory.
This was broken:
$ ./scripts/plotmpl.py -L'*=bs=%(bs)s'
There may be a better way to organize this logic, but spamming if
statements works well enough.
This still forces the block_rows_ <= height invariant, but also prevents
ceiling errors from introducing blank rows.
I guess the simplest solution is the best one, eh?
This carves out two more bits in cksum tags to store the "phase" of the
rbyd block (maybe the name is too fancy, this is just the lowest 2 bits
of the block address):
LFSR_TAG_CKSUM 0x300p v-11 ---- ---- -pqq
^ ^
| '-- phase bits
'---- perturb bit
The intention here is to catch mrootanchors that are "out-of-phase",
i.e. they've been shifted by a small number of blocks.
This can happen if we find the wrong mrootanchor (after, say, a magic
scan), and risks filesystem corruption:
formatted
.-----------------'-----------------.
mounted
.-----------------'-----------------.
.--------+--------+--------+--------+ ...
|(erased)| mroot |
| | anchor | ...
| | |
'--------+--------+--------+--------+ ...
Including the lower 2 bits of the block address in cksum tags avoids
this, for up to a 3 block shift (the maximum number of redund
mrootanchors).
---
Note that cksum tags really are the only place we could put these bits.
Anywhere else and they would interfere with the canonical cksum, which
would break error correction. By definition these need to be different
per block.
We include these phase bits in every cksum tag (because it's easier),
but these don't really say much about mdirs that are not the
mrootanchor. Non-anchor mdirs can have arbitrary block addresses,
therefore arbitrary phase bits.
You _might_ be able to do something interesting if you sort the rbyd
addresses and use the index as the phase bits, but that would add quite
a bit of code for questionable benefit...
You could argue this adds noise to our cksums, but:
1. 2 bits seems like a really small amount of noise
2. our cksums are just crc32cs
3. the phase bits humorously never change when you rewrite a block
---
As with any feature this adds code, but only a small amount. I think
it's worth the extra protection:
code stack ctx
before: 35792 2368 636
after: 35824 (+0.1%) 2368 (+0.0%) 636 (+0.0%)
Also added test_mount_incompat_out_of_phase to test this.
The dbg scripts _don't_ error (block mismatch seems likely when
debugging), but dbgrbyd.py at least adds phase mismatch notes in
-l/--log mode.
This drops the leading count/mode byte, and instead uses mid=0 to
terminate grms. This shaves off 1 bytes from grmdeltas.
Previously, we needed the count/mode byte for a couple reasons:
- We needed to know the number of grm entries somehow, and there wasn't
always an obvious sentinel value. mid=-1, for example, is
unrepresentable with our unsigned leb128 encoding.
But now that development has settled, we can use mid=0.0 to figure out
the end-of-queue. mid=0.0 should always map to the root bookmark,
which doesn't make sense to delete, so it makes for a reasonable null
terminator here.
- It provided a route for future grm extensions, which could use the >2
count/mode encodings.
But I think we can use additional grm tag encodings for this.
There's only one gdelta tag so far, but the current plan for future
gdelta tags is to carve out the bottom 2 bits for redund like we do
with the struct tags:
LFSR_TAG_GDELTA 0x01tt v--- ---1 -ttt ttrr
LFSR_TAG_GRMDELTA 0x0100 v--- ---1 ---- ----
LFSR_TAG_GBMAPDELTA 0x0104 v--- ---1 ---- -1rr
LFSR_TAG_GDDTREEDELTA 0x0108 v--- ---1 ---- 1-rr
LFSR_TAG_GPTREEDELTA 0x010c v--- ---1 ---- 11rr
...
Decoding is a bit more complicated for gstate, since we will need to
xor those bits if mutable, but this avoids needing a full byte just
for redund in every auxiliary tree.
Long story short, we can leverage the lower 2 bits of the grm tag for
future extensions using the same mechanism.
This may seem like a lot of effort for only a handful of bytes, but keep
in mind each gdelta lives in more-or-less every mdir in the filesystem.
Also saves a bit of code/ctx:
code stack ctx
before: 35772 2368 640
after: 35768 (-0.0%) 2368 (+0.0%) 636 (-0.6%)
This prevents some pretty unintuitive behavior with dbgbmap.py -H2 (the
default) in the terminal.
Consider before:
bd 4096x256, 7.8% mdir, 0.4% btree, 0.0% data
mm--------b-----mm--mm--mm--mmmmmmm--mm--mmmm-----------------------
Vs after:
bd 4096x256, 7.8% mdir, 0.4% btree, 0.0% data
m-----------------------------------b-mmmmmmmm----------------------
Compared to the original bmap (-H5):
bd 4096x256, 7.8% mdir, 0.4% btree, 0.0% data
mm------------------------------------------------------------------
--------------------------------------------------------------------
----------b-----mm--mm--mm--mmmmmmm--mm--mmmm-----------------------
--------------------------------------------------------------------
What's happening is dbgbmap.py is prioritizing aspect ratio over pixel
boundaries, so it's happy drawing a 4-row bmap to a 1-row Canvas. But of
course we can't see subpixels, so the result is quite confusing.
Prioritizing rows while tiling avoids this.
I was toying with making this look more like the mtree API in lfs.c (so
no lookupleaf/namelookupleaf, only lookup/namelookup), but dropped the
idea:
- It would be tedious
- The Mtree class's lookupleaf/namelookupleaf are also helpful for
returning inner btree nodes when printing debug info
- Not embedding mids in the Mdir class would complicate things
It's ok for these classes to not match littlefs's internal API
_exactly_. The goal is easy access for debug info, not to port the
filesystem to Python.
At least dropped Mtree.lookupnext, because that function really makes no
sense.
Now that we don't have to worry about name tag conflicts as much, we
can add name tags for things that aren't files.
This adds LFSR_TAG_BNAME for branch names, and LFSR_TAG_MNAME for mtree
names. Note that the upper 4 bits of the subtype match LFSR_TAG_BRANCH
and LFSR_TAG_MDIR respectively:
LFSR_TAG_BNAME 0x0200 v--- --1- ---- ----
LFSR_TAG_MNAME 0x0220 v--- --1- --1- ----
LFSR_TAG_BRANCH 0x030r v--- --11 ---- --rr
LFSR_TAG_MDIR 0x0324 v--- --11 --1- -1rr
The encoding is somewhat arbitrary, but I figured reserving ~31 types
for files is probably going to be plenty for littlefs. POSIX seems to
do just fine with only ~7 all these years, and I think custom attributes
will be more enticing for "niche" file types (symlinks, compressed
files, etc), given the easy backwards compatibility.
---
In addition to the debugging benefits, the new name tags let us stop
btree lookups on the first non-bname/branch tag. Previously we always
had to fetch the first struct tag as well to check if it was a branch.
In theory this saves one rbyd lookup, but in practice it's a bit muddy.
The problem is that there's two ways to use named btrees:
1. As buckets: mtree -> mdir -> mid
2. As a table: ddtree -> ddid
The only named btree we _currently_ have is the mtree. And the mtree
operates in bucket mode, with each mdir acting more-or-less as an
extension to the btree. So we end up needing to do the second tag lookup
anyways, and all we've done is complicated up the code.
But we will _eventually_ need the table mode for the ddtree, where we
care if the ddname is an exact match.
And returning the first tag is arguably the more "correct" internal API,
vs arbitrarily the first struct tag.
But then again this change is pretty pricey...
code stack ctx
before: 35732 2440 640
after: 35888 (+0.4%) 2480 (+1.6%) 640 (+0.0%)
---
It's worth noting the new BNAME/MNAME tags don't _require_ the btree
lookup changes (which is why we can get away with not touching the dbg
scripts). The previous algorithm of always checking for branch tags
still works.
Maybe there's an argument for conditionally using the previous API when
compiling without the ddtree, but that sounds horrendously messy...
Block conflict detection was originally implemented with non-dags in
mind. But now that dags are allowed, we shouldn't treat them as errors!
Instead, we only report blocks as conflicts if multiple references have
mismatching types.
This should still be very useful for debugging the upcoming bmap work.
Mainly to make room for some future planned stuff:
- Moved the mroot's redund bits from LFSR_TAG_GEOMETRY to
LFSR_TAG_MAGIC:
LFSR_TAG_MAGIC 0x003r v--- ---- --11 --rr
This has the benefit of living in a fixed location (off=0x5), which
may make mounting/debugging easier. It also makes LFSR_TAG_GEOMETRY
less of a special case (LFSR_TAG_MAGIC is already a _very_ special
case).
Unfortunately, this does get in the way of our previous magic=0x3
encoding. To compensate (and to avoid conflicts with LFSR_TAG_NULL),
I've added the 0x3_ prefix. This has the funny side-effect of
rendering redunds 0-3 as ascii 0-3 (0x30-0x33), which is a complete
accident but may actually be useful when debugging.
Currently all config tags fit in the 0x3_ prefix, which is nice for
debugging but not a hard requirement.
- Flipped LFSR_TAG_FILELIMIT/NAMELIMIT:
LFSR_TAG_FILELIMIT 0x0039 v--- ---- --11 1--1
LFSR_TAG_NAMELIMIT 0x003a v--- ---- --11 1-1-
The file limit is a _bit_ more fundamental. It's effectively the
required integer size for the filesystem.
These may also be followed by LFSR_TAG_ATTRLIMIT based on how future
attr revisits go.
- Rearranged struct tags so that LFSR_TAG_BRANCH = 0x300:
LFSR_TAG_BRANCH 0x030r v--- --11 ---- --rr
LFSR_TAG_DATA 0x0304 v--- --11 ---- -1--
LFSR_TAG_BLOCK 0x0308 v--- --11 ---- 1err
LFSR_TAG_DDKEY* 0x0310 v--- --11 ---1 ----
LFSR_TAG_DID 0x0314 v--- --11 ---1 -1--
LFSR_TAG_BSHRUB 0x0318 v--- --11 ---1 1---
LFSR_TAG_BTREE 0x031c v--- --11 ---1 11rr
LFSR_TAG_MROOT 0x032r v--- --11 --1- --rr
LFSR_TAG_MDIR 0x0324 v--- --11 --1- -1rr
LFSR_TAG_MTREE 0x032c v--- --11 --1- 11rr
*Planned
LFSR_TAG_BRANCH is a very special tag when it comes to bshrub/btree
traversal, so I think it deserves the subtype=0 slot.
This also just makes everything fit together better, and makes room
for the future planned ddkey tag.
Code changes minimal:
code stack ctx
before: 35728 2440 640
after: 35732 (+0.0%) 2440 (+0.0%) 640 (+0.0%)
This adds LFSR_TAG_ORPHAN, which simplifies quite a bit of the internal
stickynote handling.
Now that we don't have to worry about conflicts with future unknown
types, we can add whatever types we want internally. One useful one
is LFSR_TAG_ORPHAN, which lets us determine stickynote's orphan status
early (in lfsr_mdir_lookupnext and lfsr_mdir_namelookup):
- non-orphan stickynotes -> LFSR_TAG_STICKYNOTE
- orphan stickynotes -> LFSR_TAG_ORPHAN
This simplifies all the places where we need to check if a stickynote
really exists, which is most of the high-level functions.
One downside is that this makes stickynote _manipulation_ a bit more
delicate. lfsr_mdir_lookup(LFSR_TAG_ORPHAN) no longer works as expected,
for example.
Fortunately we can sidestep this issue by dropping down to
lfsr_rbyd_lookup when we need to interact with stickynotes directly,
skipping the is-orphan checks.
---
Saves a nice bit of code:
code stack ctx
before: 35984 2440 640
after: 35832 (-0.4%) 2440 (+0.0%) 640 (+0.0%)
It got a little muddy since this now include the unknown-type changes,
but here's the code diff from before we exposed LFSR_TYPE_STICKYNOTE to
users:
code stack ctx
before: 35740 2440 640
after: 35832 (+0.3%) 2440 (+0.0%) 640 (+0.0%)
This drops the requirement that all file types are introduced with a
related wcompat flag. Instead, the wcompat flag is only required if
modification _would_ leak resources, and we treat unknown file types as
though they are regular files.
This allows modification of unknown file types without the risk of
breaking anything.
To compare with before the unknown-type rework:
Before:
> Unknown file types are allowed and may leak resources if modified,
> so attempted modification (rename/remove) will error with
> LFS_ERR_NOTSUP.
Now:
> Unknown file types are allowed but must not leak resources if
> modified. If an unknown file type would leak resources, it should set
> a related wcompat flag to only allow mounting RDONLY.
Note this includes directories, which can leak bookmarks if removed, so
filesystems using directories should set the LFSR_WCOMPAT_DIR flag.
But we no longer need the LFSR_WCOMPAT_REG/LFSR_WCOMPAT_STICKYNOTE
flags.
---
The real tricky part was getting lfsr_rename to work with unknown types,
as this broke the invariant that we only ever commit tags we know about.
Fixing this required:
- Fetching the non-unknown-mapped tag in lfsr_rename
- Mapping all name tags to LFSR_TAG_NAME in lfsr_rbyd_appendrattr_
- Adopting LFSR_RATTR_NAME for bookmark name tags
This was broken by the above lfsr_rbyd_appendrattr_ change, but it's
probably good to handle these the same as other name tags anyways.
This adds a bit of code, but not enough that I think this isn't worth
it (or worth a build-time option):
code stack ctx
before: 35924 2440 640
after: 35992 (+0.0%) 2440 (+0.0%) 640 (+0.0%)
This changes how we approach unknown file types.
Before:
> Unknown file types are allowed and may leak resources if modified,
> so attempted modification (rename/remove) will error with
> LFS_ERR_NOTSUP.
Now:
> Unknown file types are only allowed in RDONLY mode. This avoids the
> whole leaking resources headache.
Additionally, unknown types are now mapped to LFS_TYPE_UNKNOWN, instead
of just being forwarded to the user. This allows us to add internal
types/tags to the LFSR_TAG_NAME type space without worrying about
conflicts with future types:
- reg -> LFS_TYPE_REG
- dir -> LFS_TYPE_DIR
- stickynote -> LFS_TYPE_STICKYNOTE
- everything else -> LFS_TYPE_UNKNOWN
Thinking about potential future types, it seems most (symlinks,
compressed files, etc) can be better implemented via custom attributes.
Using custom attributes doesn't mean the filesystem _can't_ inject
special behavior, and custom attributes allow for perfect backwards
compatibility.
So with future types less likely, forwarding type info to users is less
important (and potentially error prone). Instead, allowing on-disk +
internal types to be represented densely is much more useful.
And it avoids setting an upper bound on future types prematurely.
---
This also includes a minor rcompat/wcompat rework. Since we're probably
going to end up with 32-bit rcompat flags anyways, might as well make
them more human-readable (nibble-aligned):
LFS_RCOMPAT_NONSTANDARD 0x00000001 Non-standard filesystem format
LFS_RCOMPAT_WRONLY 0x00000002 Reading is disallowed
LFS_RCOMPAT_BMOSS 0x00000010 Files may use inlined data
LFS_RCOMPAT_BSPROUT 0x00000020 Files may use block pointers
LFS_RCOMPAT_BSHRUB 0x00000040 Files may use inlined btrees
LFS_RCOMPAT_BTREE 0x00000080 Files may use btrees
LFS_RCOMPAT_MMOSS 0x00000100 May use an inlined mdir
LFS_RCOMPAT_MSPROUT 0x00000200 May use an mdir pointer
LFS_RCOMPAT_MSHRUB 0x00000400 May use an inlined mtree
LFS_RCOMPAT_MTREE 0x00000800 May use an mdir btree
LFS_RCOMPAT_GRM 0x00001000 Global-remove in use
LFS_WCOMPAT_NONSTANDARD 0x00000001 Non-standard filesystem format
LFS_WCOMPAT_RDONLY 0x00000002 Writing is disallowed
LFS_WCOMPAT_REG 0x00000010 Regular file types in use
LFS_WCOMPAT_DIR 0x00000020 Directory file types in use
LFS_WCOMPAT_STICKYNOTE 0x00000040 Stickynote file types in use
LFS_WCOMPAT_GCKSUM 0x00001000 Global-checksum in use
---
Code changes:
code stack ctx
before: 35928 2440 640
after: 35924 (-0.0%) 2440 (+0.0%) 640 (+0.0%)
Now that LFS_TYPE_STICKYNOTE is a real type users can interact with, it
makes sense to group it with REG/DIR. This also has the side-effect of
making these contiguous.
---
LFSR_TAG_BOOKMARKs, however, are still hidden from the user. This
unfortunately means there will be a bit of a jump if we ever add
LFS_TYPE_SYMLINK in the future, but I'm starting to wonder if that's the
best way to approach symlinks in littlefs...
If instead LFS_TYPE_SYMLINKS were implied via custom attribute, you
could avoid the headache that comes with adding a new tag encoding, and
allow perfect compatibility with non-symlink drivers. Win win.
This seems like a better approach for _all_ of the theoretical future
types (compressed files, device files, etc), and avoids the risk of
oversaturating the type space.
---
This had a surprising impact on code for just a minor encoding tweak. I
guess the contiguousness pushed the compiler to use tables/ranges for
more things? Or maybe 3 vs 5 is just an easier constant to encode?
code stack ctx
before: 35952 2440 640
after: 35928 (-0.1%) 2440 (+0.0%) 640 (+0.0%)
This tweaks a number of extended revision count things:
- Added LFS_REVDBG, which adds debug info to revision counts.
This initializes the bottom 12 bits of every revision count with a
hint based on rbyd type, which may be useful when debugging:
- 68 69 21 v0 (hi!.) => mroot anchor
- 6d 72 7e v0 (mr~.) => mroot
- 6d 64 7e v0 (md~.) => mdir
- 62 74 7e v0 (bt~.) => file btree node
- 62 6d 7e v0 (bm~.) => mtree node
This may be overwritten by the recycle counter if it overlaps, worst
case the recycle counter takes up the entire revision count, but these
have been chosen to at least keep some info if partially overwritten.
To make this work required the LFS_i_INMTREE hack (yay global state),
but a hack for debug info isn't the end of the world.
Note we don't have control over data blocks, so there's always a
chance they end up containing what looks like one of the above
revision counts.
- Renamed LFS_NOISY -> LFS_REVNOISE
- LFS_REVDBG and LFS_REVNOISE are incompatible, so using both asserts.
This also frees up the theoretical 0x00000030 state for an additional
rev mode in the future.
- Adopted LFS_REVNOISE (and LFS_REVDBG) in btree nodes as well.
If you need rev noise, you probably want it in all rbyds/metadata
blocks, not just mdirs.
---
This had no effect on the default code size, but did affect
LFS_REVNOISE:
code stack ctx
before: 35688 2440 640
after: 35688 (+0.0%) 2440 (+0.0%) 640 (+0.0%)
revnoise before: 35744 2440 640
revnoise after: 35880 (+0.4%) 2440 (+0.0%) 640 (+0.0%)
default: 35688 2440 640
revdbg: 35912 (+0.6%) 2448 (+0.3%) 640 (+0.0%)
revnoise: 35880 (+0.5%) 2440 (+0.0%) 640 (+0.0%)
This was caused by including the shrub bit in the tag comparison in
Rbyd.lookup.
Fixed by adding an extra key mask (0xfff). Note this is already how
lfsr_rbyd_lookup works in lfs.c.
- Fixed Mtree.lookupleaf accepting mbid=0, which caused dbglfs.py to
double print all files with mbid=-1
- Fixed grm mids not being mapped to mbid=-1 and related orphan false
positives
I've made this mistake before!
One would think that it would be more interesting to show progs over
erases when they overlap, since progs always subset erases and show more
detail. However, erases occur much more rarely and are usually followed
by progs, so when rendering is low resolution (ascii) it's easy for
progs to completely cover up all erase operations.
Prioritizing erases prevents this.
At least this nuance is better documented this time around.
This was a surprising side-effect the script rework: Realizing the
internal btree/rbyd lookup APIs were awkwardly inconsistent and could be
improved with a couple tweaks:
- Adopted lookupleaf name for functions that return leaf rbyds/mdirs.
There's an argument this should be called lookupnextleaf, since it
returns the next bid, unlike lookup, but I'm going to ignore that
argument because:
1. A non-next lookupleaf doesn't really make sense for trees where
you don't have to fetch the leaf (the mtree)
2. It would be a bit too verbose
- Adopted commitleaf name for functions that accept leaf rbyds.
This makes the lfsr_bshrub_commit -> lfsr_btree_commit__ mess a bit
more readable.
- Strictly limited lookup and lookupnext to return rattrs, even in
complex trees like the mtree.
Most use cases will probably stick to the lookupleaf variants, but at
least the behavior will be consistent.
- Strictly limited lookup to expect a known bid/rid.
This only really matters for lfsr_btree/bshrub_lookup, which as a
quirk of their implementation _can_ lookup both bid + rattr at the
same time. But I don't think we'll need this functionality, and
limited the behavior may allow for future optimizations.
Note there is no lfsr_file_lookup. File btrees currently only ever
have a single leaf rattr, so this API doesn't really make sense.
Internal API changes:
- lfsr_btree_lookupnext_ -> lfsr_btree_lookupleaf
- lfsr_btree_lookupnext -> lfsr_btree_lookupnext
- lfsr_btree_lookup -> lfsr_btree_lookup
- added lfsr_btree_namelookupleaf
- lfsr_btree_namelookup -> lfsr_btree_namelookup
- lfsr_btree_commit__ -> lfsr_btree_commit_
- lfsr_btree_commit_ -> lfsr_btree_commitleaf
- lfsr_btree_commit -> lfsr_btree_commit
- added lfsr_bshrub_lookupleaf
- lfsr_bshrub_lookupnext -> lfsr_bshrub_lookupnext
- lfsr_bshrub_lookup -> lfsr_bshrub_lookup
- lfsr_bshrub_commit_ -> lfsr_bshrub_commitleaf
- lfsr_bshrub_commit -> lfsr_bshrub_commit
- lfsr_mtree_lookup -> lfsr_mtree_lookupleaf
- added lfsr_mtree_lookupnext
- added lfsr_mtree_lookup
- added lfsr_mtree_namelookupleaf
- lfsr_mtree_namelookup -> lfsr_mtree_namelookup
- added lfsr_file_lookupleaf
- lfsr_file_lookupnext -> lfsr_file_lookupnext
- added lfsr_file_commitleaf
- lfsr_file_commit -> lfsr_file_commit
Also added lookupnext to Mdir/Mtree in the dbg scripts.
Unfortunately this did add both code and stack, but only because of the
optional mdir returns in the mtree lookups:
code stack ctx
before: 35520 2440 636
after: 35548 (+0.1%) 2472 (+1.3%) 636 (+0.0%)
