This sort of abuses the bptr/data type overlap again, taking an explicit
delta along with a list of datas where:
- data_count=-1 => single bptr
- data_count>=0 => list of concatenated fragments
It's a bit of a hack, but the previous rattr argument it replaces was
an arguably worse hack. I figured if we're going to interrogate the
rattr to figure out what type it is, we might as well just make the type
explicit.
Saved a surprising amount of stack! So that's nice:
code stack ctx
before: 37192 2360 636
after: 37080 (-0.3%) 2304 (-2.4%) 636 (+0.0%)
Except for the unknown flag checks. I don't know why but they really
mess with readability there for me. Maybe because the logic matches
english grammar ("is not any of these" vs "is any not of these")?
No code changes.
This is just a bit simpler/more flexible of an API. Taking flags
directly has worked well for similar functions.
This also drops lfsr_*_mkdirty. I think we should keep the mk* names
reserved for heavy-weight filesystem operations.
That being said, this does add a surprising bit of code. I because the
flags end up in literal pools? Doesn't thumb have a bunch of fancy
single-bit immediate encodings?
code stack ctx
before: 37180 2360 636
after: 37192 (+0.0%) 2360 (+0.0%) 636 (+0.0%)
Mostly adding convenience functions to deduplicate code:
- Adopted lfsr_bptr_claim
- Renamed lfsr_file_graft -> lfsr_file_graft_
- Adopted lfsr_file_graft
- Didn't bother with lfsr_file_discardleaf
This saves a bit of code, though not that much in the context of the
file->leaf code cost:
code stack ctx
before cleanup: 37228 2328 636
after: 37180 (-0.1%) 2360 (+1.4%) 636 (+0.0%)
code stack ctx
before file->leaf: 36016 2296 636
after: 37180 (+3.2%) 2360 (+2.8%) 636 (+0.0%)
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?
Maybe it's just habit, but the trailing underscores_ felt far more
useful serving only as a out-pointer/new/biproduct hint. Having trailing
underscores_ serve dual purposes as both a new/biproduct hint and
optional hint just muddies things and makes the hint much less useful.
No code changes.
This adds LFS_NOINLINE, and forces lfsr_file_sync_ (the commit logic in
lfsr_file_sync) off the stack hot-path.
This adds a bit of code, function calls are surprisingly expensive, but
saves a nice big chunk of stack:
code stack ctx
before: 35992 2408 636
after: 36016 (+0.1%) 2296 (-4.7%) 636 (+0.0%)
Well, maybe not _real_ stack. The fact that this worked suggests the
real stack usage is less than our measured value.
The reason is because our stack.py script is relatively simple. It just
adds together stack frames based on the callgraph at compile time, which
misses shrinkwrapping and similar optimizations. Unfortunately that sort
of information is simply not available via GCC short of parsing the
disassembly.
But this is the number that will be used for statically allocated stacks,
and of course the number that will probably end up associated with
littlefs, so it still seems like a worthwhile number to "optimize" for.
Maybe in the future this will be different as tooling around stack
measurements improves.
---
The other benefit of moving lfsr_file_sync_ off the hot-path is that we
now no longer incorrectly include the sync commit context in the
hot-path. This tells a much different story for the cost of 1-commit
shrubs:
code stack ctx
before 1c-shrubs: 35848 2296 636
after 1c-shrubs: 36016 (+0.5%) 2296 (+0.0%) 636 (+0.0%)
This adds an alternative sync path for small in-cache files, where we
combine the shrub commit with the file sync commit, potentially writing
everything out in a single prog.
This is reminiscent of bmoss (old inlined) files, but notably avoids the
additional on-disk data-structure and extra code necessary to manage it.
---
The motivation for this comes from ongoing benchmarking, where we're
seeing a fairly significant regression in small-file performance on NAND
flash. Especially curious since the whole goal of this work was to make
NAND flash tractable.
But it makes sense: 2 commits are more than 1.
While the separate shrub + sync commits are barely noticeable on NOR
flash, on NAND flash, with its huge >512B prog sizes, the extra commit
is hard to miss.
In theory, the most performant solution would be to merge all bshrub
commits with sync commits whenever possible. This is technically doable,
and may make sense for a more performance-focused littlefs driver, but
it would 1. require an invasive code rewrite, 2. entangle lfsr_file_sync
-> lfsr_file_flush -> lfsr_file_carve, and 3. add even more code.
If we only merge shrub + sync commits when the file fits in the cache,
we can skip lfsr_file_flush, craft a simple shrubcommit by hand, and
avoid all of this mess. While still speeding up the most common write
path for small files.
And sure enough, our bench-many benchmark, which creates ~1000 4 byte
files, shows a ~2x speed improvement on bs=128KiB NAND (basically just
because we compact/split ~5 times instead of ~10 times).
---
Unfortunately the shrub commit requires quite a bit of state to set up,
and in the middle of lfsr_file_sync, one of the more critical functions
on our stack hot-path. So this does have a big cost:
code stack ctx
before: 35836 2368 636
after: 35992 (+0.4%) 2408 (+1.7%) 636 (+0.0%)
Though this is also a perfect contender to be compile-time ifdefed. It
may be worth adding something like LFS_NO_MERGESHRUBCOMMITS (better
name?) to claw back some of the cost if you don't care about
performances as much.
This could also probably be a bit cheaper if our file write configs were
organized differently... At the moment we need to check inline_size,
fragment_size, _and_ crystal_thresh since these can sometimes overlap.
But this is waiting on the future config rework.
---
Actually... Looking at this closer, I'm not sure the added commit logic
should really be included in the hot-path cost...
lfsr_file_flush is the hot path, and flush -> sync are sequential
operations that don't really share stack (with the shrub commit we
humorously _never_ call flush). The commit logic is only being dragged
in because our stack measurements are pessimistic about shrinkwrapping,
which is a bit frustrating.
I've explored shrinkwrapping in stack.py before, but the idea pretty
much failed. Unfortunately GCC simply doesn't make this info available
short of parsing the per-arch disassembly.
- 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.
So:
$(filter-out %.t.c %.b.c %.a.c,$(wildcard bd/*.c))
Instead of:
$(filter-out $(wildcard bd/*.t.* bd/*.b.*),$(wildcard bd/*.c))
The main benefit is we no longer need to explicitly specify all
subdirectories, though the single wildcard is a bit less flexible if
test.py/bench.py ever end up with other non-C artifacts.
Unfortunately only a single wildcard is supported in filter-out.
This adds mattr_estimate, which is basically the same as rattr_estimate,
but assumes weight <= 1:
rattr tag:
.---+---+---+- -+- -+- -+- -+---+- -+- -+- -. worst case: <=11 bytes
| tag | weight | size | rattr est: <=3t + 4
'---+---+---+- -+- -+- -+- -+---+- -+- -+- -' <=37 bytes
mattr tag:
.---+---+---+---+- -+- -+- -. worst case: <=7 bytes
| tag | w | size | mattr est: <=3t + 4
'---+---+---+---+- -+- -+- -' <=25 bytes
This may seem like only a minor improvement, but with 3 tags for every
attr, this really adds up. And with our compaction estimate overheads we
need every byte of shaving we can get.
---
This ended up necessary to get littlefs running with 512 byte blocks
again. Now that our compaction overheads are so high, littlefs is having
a hard time fitting even just the filesystem config in a single block:
mroot estimate 512B before: 246/256
mroot estimate 512B after: 162/256 (-34.1%)
Whether or not it makes sense to run littlefs with 512 byte blocks is
still an open question, even after this tweak.
Note that even if 512 byte blocks ends up intractable, this doesn't mean
littlefs won't be able to run on SD/eMMC! The configured block_size can
always be a multiple, >=, of the physical block_size, and choosing a
larger block_size completely side-steps this problem.
The new design of littlefs is primarily focused on devices with very
large block sizes, so you may want to use larger block sizes on SD/eMMC
for performance reasons anyways.
---
Code changes were pretty minimal. This does add an additional field to
lfs_t, but it's just a byte and fits into padding with the other small
precomputed constants:
code stack ctx
before: 35824 2368 636
after: 35836 (+0.0%) 2368 (+0.0%) 636 (+0.0%)
This prevents runaway O(n^2) behavior on devices with extremely large
block sizes (NAND, bs=~128KiB - ~1MiB).
The whole point of shrubs is to avoid this O(n^2) runaway when inline
files become necessarily large. Setting FRAGMENT_SIZE to a factor of the
BLOCK_SIZE humorously defeats this.
The 512 byte cutoff is somewhat arbitrary, it's the natural BLOCK_SIZE/8
FRAGMENT_SIZE on most NOR flash (bs=4096), but it's probably worth
tuning based on actual device performance.
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.
Mainly formatting/comment things, but also a couple tweaks:
- Changed BUILDDIR mkdir hack to infer directories from SRC, TESTS,
TEST_SRC, etc
Avoids a hardcoded list of build directories.
- Added $(BUILDDIR)/%.c -> $(BUILDDIR)/%.{o,ci,s} rules
Without these, make doesn't know how to build .o files that depend on
generated .c files (.t.c, .b.c, .a.c, etc) when using an external
BUILDDIR.
This better matches how other filesystems refer to the number of in-use
blocks.
Which makes sense when you consider that "size" could also refer to the
configured block_count. The term "usage" avoids this ambiguity.
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.
Mainly the grm and ptail subsystems. This matches the internal mtree
API.
Unfortunately this _did_ add a little bit of code, I guess due to the
larger struct offsets. But since this simplifies the internal API I'm
going to chalk it up to compiler noise:
code stack ctx
before: 35768 2368 636
after: 35792 (+0.1%) 2368 (+0.0%) 636 (+0.0%)
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%)
I think this was left over from when we handled LFSR_TAG_SHRUBTRUNK in
lfsr_mdir_commit__, which needed to forward mode bits to the generated
rattr.
Now that lfsr_mdir_commit__ only handles high-level in-device tags, we
can drop the lfsr_tag_key masks and save a bit of code:
code stack ctx
before: 35796 2368 640
after: 35772 (-0.1%) 2368 (+0.0%) 640 (+0.0%)
So instead of special behavior for only bookmark tags, LFSR_TAG_GRMPUSH
allows pushing any mid to the grm queue.
The benefit of LFSR_TAG_GRMPUSH, vs just calling lfsr_grm_push before
lfsr_mdir_commit, is that you can push mids that don't exist yet. This
lets you to create self-grming mids that effectively don't exist until
some other work has completed.
We currently use this to atomically create directory + bookmark entries,
but it may have some other uses in the future.
---
The extra rattr does add a bit of code, but fortunately no stack, since
lfsr_mkdir is not on the stack hot-path:
code stack ctx
before: 35768 2368 640
after: 35796 (+0.1%) 2368 (+0.0%) 640 (+0.0%)
A bit of a hack, but this saves some stack:
code stack ctx
before: 35764 2392 640
after: 35768 (+0.0%) 2368 (-1.0%) 640 (+0.0%)
It's not like the rbyd is doing anything else until we fetch the mdir.
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.
Why?
- lfsr_mtree_lookupleaf vs lfsr_mtree_commit is inconsistent. Should
lfsr_mdir_commit be called lfsr_mtree_commitleaf? That'd be weird.
It's reasonable to call mdirs entries of the mtree, but it'd be weird
to call rbyds entries of btrees, so the inconsistency there is
expected.
- lfsr_mtree_lookup/lfsr_mtree_lookupnext (going mtree -> mdir) aren't
actually useful.
- The lfsr_mtree_namelookup/lfsr_mtree_namelookupleaf split is just more
of a headache than it's worth.
Saves a tiny bit of code:
code stack ctx
before: 35768 2392 640
after: 35764 (-0.0%) 2392 (+0.0%) 640 (+0.0%)
This matches the behavior of rbyd_/mdir_ out-pointers.
I mostly just wanted to see the separate affects on code size. Saves a
bit more code/stack:
code stack ctx
before: 35780 2408 640
after: 35768 (-0.0%) 2392 (-0.7%) 640 (+0.0%)
At least this simplifies lfsr_mtree_traverse_ quite a bit.
This makes all rbyd_/mdir_ out-pointers required, dropping all of the
internal copies needed to make lookup/namelookup/pathlookup/etc work.
Previously, the -- rough -- rule was to make out-pointers generally
optional (lfsr_data_read and other struct initers being notable
exceptions), the idea being you can opt-out of stack allocations where
possible.
In practice this kind of backfired, with many internal functions needing
redundant stack allocations in case the relevant parameter is NULL
(lfsr_btree_lookupleaf being an excellent example).
---
As an alternative rule, I think we should only expect optional
out-pointers for things you would pass-by-value (lfsr_rid_t, lfsr_tag_t,
lfsr_data_t, etc).
I've also developed a habit of naming optional out-pointers with a
trailing underscore_, to hopefully make this subtlety a bit less subtle.
This claws back all of the stack cost of BNAMEs/MNAMEs, and most of the
code cost:
code stack ctx
before: 35888 2480 640
after: 35780 (-0.3%) 2408 (-2.9%) 640 (+0.0%)
Though we still have more function calls than we started with
(lfsr_mtree_*lookup mtree -> mdir lookups).
This is the _nth_ time I've tried to force arbitrary btree name inserts
to work, so _clearly_ I need a bigger comment.
Hopefully this will prevent me from trying to delete the LFSR_RATTR_NOOP
in test_btree_find_general_fuzz _again_.
---
The gist is that insert-before-bid+1 is fundamentally different from
insert-after-bid when named btrees are involved:
.-----f-----. insert-after-d .-------f-----.
.-b--. .--j-. => .-b---. .--j-.
| .-. .-. | | .---. .-. |
a c d h i k a c d e h i k
^
insert-before-h
=> .-----f-------.
.-b--. .---j-.
| .-. .---. |
a c d g h i k
^
The problem is that lfsr_btree_commit_ needs to find the same leaf
rbyd as lfsr_btree_namelookup, and potentially insert-before the
first rid or insert-after the last rid.
Instead of separate insert-before/after flags, we make the first tag
in a commit insert-before, and all following non-grow tags
insert-after (splits).
This info is now captured in the above mentioned comment.
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.
Found another ckparity flaw! Only detected now due to the reworked tag
encoding, but it's just luck this wasn't detected earlier.
Consider the following bit flip:
03 04 80 04 80 04 6b ... data w512 512
43 04 80 04 80 04 6b ... altble 0x304 w512 -512
^
flip
Not only are leb128s problem for ckparity, but even the difference in
alt vs normal tag encoding presents a vulnerability.
So limiting the ckparity tests further, to just 47 of the first 48 bits.
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%)
Ok so, funny story, looks like we won't actually need pure-tree
bshrubs/btrees.
It _is_ true that the single-parent constraint imposed by pure-trees can
enable a wider range of algorithms. But looking forward into the planned
design, we just happen to not need this constraint at all. I made a
mistake here:
1. Block allocation - On paper block allocation benefits the most from
the single-parent constraint. But we have another daggish problem,
how do we efficiently account for in-flight/open btrees?
Naively, you might think we can just traverse all open btrees during
allocation, since we shouldn't have _that_ many. But this scales
O(n^2) when writing a large file. The key observation being that open
files reference on-disk btrees and are _not_ RAM constrained.
The current solution involves tree-diffing in order to figure out
bmap updates. Which, humorously, works perfectly fine even if the
trees are dags.
2. Error correction - I just completely forgot that the current plans
for block redundancy require the ddtree.
Each block gets mapped into the dense ddtree, with subranges of the
ddtree grouped into parity groups backed by the ptree. Instead of
bptrs, file btrees store indirect ddkeys into the ddtree. No bptrs?
No dag problem!
This is still a problem if we ever support naive data redund (redund
blocks in a bptrs), but that's out of scope for other reasons
(basically just a lot more code).
So reverting. Allowing dags allows for much faster random writes, at
least in theory.
---
For now I'm still keeping the dag-avoidance in lfsr_file_flush_ around
under the LFS_NONDAG ifdef. This will likely be dropped at some point,
but I'm curious how it affects benchmarks.
Ugh, and of course the unused label makes GCC unhappy. Added
-Wno-unused-label to CFLAGS because labels have other uses besides just
being goto targets (debug targets, code organization, etc).
We probably use labels more that other libraries because to littlefs's
no-recursion requirement.
Code changes minimal, still not sure where that stack difference comes
from:
code stack ctx
before: 35740 2424 640
after: 35736 (-0.0%) 2440 (+0.7%) 640 (+0.0%)
This tears out most of the implied lfsr_file_sync calls, and restricts
LFS_O_SYNC to only imply lfsr_file_sync on _write_ operations. So only
lfsr_file_write, and maybe pwrite/writev/etc in the future.
This mainly affects lfsr_file_truncate/fruncate (and punchhole/
insertrange/collapserange in the future), while reverting the LFS_O_SYNC
related changes in lfsr_file_open:
- lfsr_file_open + LFS_O_SYNC => does _not_ sync
- lfsr_file_close + LFS_O_SYNC => syncs (unless desynced)
- lfsr_file_write + LFS_O_SYNC => syncs
- lfsr_file_sync + LFS_O_SYNC => syncs
- lfsr_file_truncate + LFS_O_SYNC => does _not_ sync
- lfsr_file_fruncate + LFS_O_SYNC => does _not_ sync
Note LFS_O_FLUSH is unaffected, it was always limited to
lfsr_file_write since that's the only function that touches file
buffers.
Also note I want this rule to apply to the future lfsr_file_punchhole/
insertrange/collapserange functions as well. Even though you can argue
these effectuate writes, they're at a level of sophistication that we
can just expect users to just call lfsr_file_sync if they want to.
---
Ok, so a number of reasons:
- This matches behavior of LFS_O_APPEND, which is intentionally
restricted to only write operations.
In that case I think the explicit limitation is easier to understand
than trying to define an abstract model.
This makes LFS_O_SYNC, LFS_O_FLUSH, and LFS_O_APPEND consistent in
when the relevant behavior takes effect.
- This avoids the zero-sized files after powerloss. Which are just as
likely, if not more, to trip up users vs missing syncs.
- Most truncate/fruncate operations are immediately followed by a write
operation anyways. Which just makes the truncate/fruncate syncs wasted
prog/erase cycles.
Even in some of the more complicated truncate/function use cases, you
just don't care about when fruncates/truncates hit the disk.
Take logging via lfsr_file_fruncate for example. Yes the fruncate will
usually happen _after_ the write operation, but this just means the
log file will usually be one entry larger than expected. Which is a
state you can end up with anyways after powerloss.
- This avoids confusing/conflicting LFS_O_SYNC + LFS_O_DESYNC behavior.
Again, this simple rule is easier to reason about than a model.
You would think this would be well defined in POSIX, but it's really
not. POSIX limits O_SYNC to "write I/O operations", but doesn't really
define a "write" (it is a retroactive standard after all). ftruncate is
a bit funny in that it states "the extended area shall appear as if it
were zero-filled", but the term "write" doesn't appear in ftruncate's
documentation at all.
Searching through LKML, stack overflow, etc, it doesn't seem like anyone
else knows exactly what to do either. There was a bug report[1] in 2005
for ext3 + O_SYNC + ftruncate that was rejected, but a later bug
report[2] in 2012 for xfs + O_SYNC + fallocate that was fixed (but was
broken in almost every Linux fs?).
1: https://lore.kernel.org/lkml/1111610558.1998.193.camel@sisko.sctweedie.blueyonder.co.uk
2: https://lore.kernel.org/linux-ext4/20111116084256.GA22963@infradead.org
So, this may end up a bit controversial, but I'm going to go with the
simpler truncate/fruncate-do-not-imply-sync rule for the above reasons.
I think this is a bit more important for littlefs than other
filesystems, as it also defines the behavior of lfsr_file_open, and with
a rigorous powerloss model being core to the design.
---
This is also cheaper code/stack-wise, but if this was going to be a
deciding factor we should just put LFS_O_SYNC/LFS_O_FLUSH behind ifdefs:
code stack ctx
before: 35816 2480 640
after: 35740 (-0.2%) 2424 (-2.3%) 640 (+0.0%)
Compared to before the LFS_O_SYNC tweaks:
code stack ctx
before-tweaks: 35780 2440 640
before: 35816 (+0.1%) 2480 (+1.6%) 640 (+0.0%)
after: 35740 (-0.1%) 2424 (-0.7%) 640 (+0.0%)
Christopher Haster