I'm not sure if this was ever implemented, or broken during a refactor,
but we were ignoring -s/-S flags when writing .csv/.json output with
-o/-O.
Curious, because the functionality _was_ implemented in fold, just
unused. All this required was passing -s/-S to fold correctly.
Note we _don't_ sort diff_results, because these are never written to
.csv/.json output.
At some point this behavior may have been a bit more questionable, since
we use to allow mixing -o/-O and table rendering. But now that -o/-O is
considered an exclusive operation, ignoring -s/-S doesn't really make
sense.
---
Why did this come up? Well imagine my frustration when:
1. In tikz/pgfplots, \addplot table only really works with sorted data
2. csv.py has a -s/-S flag for sorting!
3. -s/-S doesn't work!
This was a nasty performance hole found while benchmarking.
Basically, any time crystallization is triggered, the crystallization
algorithm tries to pack as much data into as few blocks as possible.
When fruncating (the common, and performance sensitive, use case being
logging), this can lead to the algorithm rewriting fruncated blocks.
What the crystallization algorithm doesn't realize, however, is that
when fruncating/logging, we're probably going to fruncate again on the
next call, so rewriting the block is a waste of effort.
Worst case -- a 1 block file -- this can cause littlefs to rewrite the
entire file on every append.
---
The solution implemented here, which is a bit of a hack, is to use the
actual block start for block alignment instead of the logical
start-of-block referenced by our btree/bshrub.
This solves the fruncating/logging performance hole, with the tradeoff
of using more storage than is strictly necessary. This tradeoff is
probably expected with logging however.
Code changes minimal:
code stack ctx
before: 37024 2352 684
after: 37036 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
We're not currently using these (at the moment it's unclear if the
original intention behind the treediff algorithms is worth pursuing),
and they are showing up in our heap benchmarks.
The good news is that means our heap benchmarks are working.
Also saves a bit of code/ctx in bmap mode:
code stack ctx
before: 37024 2352 684
after: 37024 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38752 2456 812
bmap after: 38704 (-0.1%) 2456 (+0.0%) 800 (-1.5%)
Useful for emulating much larger disks in a file (or in RAM). kiwibd
doesn't have all the features of emubd, but this allows it to prioritize
disk size and speed for benchmarking.
kiwibd still keeps some features useful for benchmarking/emulation:
- Optional erase value emulation, including nor-masking
- Read/prog/erase trackers for measuring bd operations
- Read/prog/erase sleeps for slowing down the simulation to a human
viewable speed
- Added --small-total. Like --small-header, this omits the first column
which usually just has the informative text TOTAL.
- Tweaked -Q/--small-table so it renders with --small-total if
-Y/--summary is provided.
- Added --total as an alias for --summary + --no-header + --small-total,
i.e. printing only the totals (which may be multiple columns) and no
other decoration.
This is useful for scripting, now it's possible to extract just, say,
the sum of some csv and embed with $():
echo $(./scripts/code.py lfs3.o --total)
- Tweaked total to always output a number (0) instead of a dash (-),
even if we have no results.
This relies on Result() with no args, which risks breaking scripts
where the Result type expects an argument. To hopefully catch this
early, the table renderer currently creates a Result() before trying
to fold the total result.
- If first column is empty (--small-total + --small-header, --no-header,
etc) collapse width to zero. This avoids a bunch of extra whitespace,
but still includes the two spaces normal used to separate names from
fields.
But I think those spaces are a good thing. It makes it hard to miss
the implicit padding in the table renderer that risks breaking
dependent scripts.
Found when trying to measure ctx of yaffs2, which relies on incomplete
structs to hide some internal state (yaffs_summary_tags, yaffs_DIR).
This is less common in microcontroller filesystems since almost all
structs end up statically/stack allocated, and you can't statically
allocate incomplete structs.
It's not too surprising, but incomplete structs have no associated
DW_AT_byte_size in the relevant dwarf info, which broke ctx.py and
structs.py...
As a workaround, I'm now defaulting to size=0 if DW_AT_byte_size is
missing.
---
With this fix, at least structs.py is able to pick up the later internal
definition of yaffs_summary_tags. ctx.py doesn't because it only looks
at the unique dwarf offset referenced by the function definition, but
I'm hesitant to try anything more clever here.
yaffs_DIR is noteworthy in that there is simply no complete definition.
Internally, yaffs_DIR pointers alias yaffsfs_DirSearchContext structs.
In this case I think returning size=0 is the only reasonable option.
I've been struggling to keep plots readable with --x/ylim-stddev, it may
have been the wrong tool for the job.
This adds --x/ylim-ratio as an alternative, which just sets the limit to
include x-percent of the data (I avoided "percen"t in the name because
it should be --x/ylim-ratio=0.98, not 98, though I'm not sure "ratio" is
great either...).
Like --x/ylim-stddev, this can be used in both one and two argument
forms:
$ ./scripts/plot.py --ylim-ratio=0.98
$ ./scripts/plot.py --ylim-=-0.98,+0.98
So far, --x/ylim-ratio has proven much easier to use, maybe because our
amortized results don't follow a normal distribution? --x/ylim-ratio
seems to do a good job of clipping runaway amortized results without too
much information loss.
This adds NOR-style masking emulation to emubd when erase_value is set
to -2:
erase => 0xff
prog 0xf0 => 0xf0
prog 0xcc => 0xc0
We do _not_ rely on this property in littlefs, and so this feature will
probably go unused in our tests, but it's useful for running other
filesystems (SPIFFS) on top of emubd.
It may be a bit of a scope violation to merge this into littlefs's core
repo, but it's useful to centralize emubd's features somewhere...
Counterintuitively, LFS3_RDONLY + LFS3_BMAP _does_ make sense for cases
where you want to include the bmap in things like ckmeta/ckdata scans.
Though this is another argument for a LFS3_RDONLY + LFS3_NO_TRV build.
Traversals add quite a bit of code to the rdonly build that is probably
not always needed.
---
This just required another bunch of ifdefs.
Current bmap rdonly code size:
code stack ctx
rdonly: 10616 896 532
rdonly+bmap: 10892 (+2.6%) 896 (+0.0%) 636 (+19.5%)
Just a few alloc/eoff references slipped through in the bmap work.
Current rdonly code size:
code stack ctx
default: 37024 2352 684
rdonly: 10616 (-71.3%) 896 (-61.9%) 532 (-22.2%)
This biggest change was tweaking our mtortoise again to use the unused
trunk field for the power-of-two bound. The original intention of using
eoff was an extra precaution to avoid the mtortoise looking like a valid
shrub at any point, but eoff is not available in LFS3_RDONLY.
And we definitely want our mtortoise in LFS3_RDONLY!
---
Note I haven't actually tested LFS3_RDONLY + LFS3_BMAP. Does this config
even make sense? I guess ckmeta/ckdata will need to traverse the bmap,
so, counterintuitively, yes?
Whoops, this was an oversight when readopting lazy grafting.
It turns out the crystallization refactor that led to
lfs3_file_crystallize_ operating directly on file->leaf.bptr was a bit
incompatible with lazy grafting.
If we encounter an error and need to relocate, we need to rewrite any
data in our crystal, _including data in ungrafted leaves_.
By pure luck, the previous lazy grafting implementation side-stepped
this issue by including ungrafted leaves in lfs3_file_lookupnext calls.
This implicitly included the ungrafted leaf in any recrystallizations,
as long as it wasn't modified on error.
---
The fix required two tweaks:
- Recrystallize into a copy in case we hit an error.
Instead of a full lfs3_bptr_t, I just copied the relevant
block_/off_/pos_ pieces we need.
- Include file leaves in the crystallization logic.
Fortunately the multi-data-prioritization loop we already have for
any cached data was relatively easy to adapt for this.
As a plus lfs3_file_crystallize_ can also now short-circuit a
bshrub/btree lookup if the data we're crystallizing happens to be in the
file leaf.
This adds a bit more code, but doesn't break if we hit an error. In
theory this would add stack for the recrystallization copy, but
lfs3_file_crystallize_ is just off the stack hot-path:
code stack ctx
before: 36972 2352 684
after: 37024 (+0.1%) 2352 (+0.0%) 684 (+0.0%)
Another fix I considered -- calling lfs3_file_graft on error -- may have
been a bit less code, but would have moved the stack hot-path under
lfs3_file_crystallize_. An error triggering _more_ progs/commits also
doesn't really sound like the greatest of ideas.
Found by test_ck_spam_fwrite_fuzz.
Currently LFS3_BMAP implies LFS3_YES_BMAP, which is an ugly hack because
I don't want to figure out the BMAP flag logic right now.
As a side-effect, this makes it impossible to test LFS3_BIGGEST without
LFS3_BMAP, which breaks a number of tests that have not been updated to
support >2 format blocks.
Saves a bit of code, at the cost of making this logic a bit more
difficult to read:
code stack ctx
before: 36992 2352 684
after: 36972 (-0.1%) 2352 (+0.0%) 684 (+0.0%)
Now that ungrafted leaves are much more limited in scope, I'm not sure
lfs3_file_weight_ still makes sense as a separate function.
The only call was in lfs3_file_read, where we decide if we bother
flushing things before actually flushing things. Given that we should
now generally graft before expecting lfs3_file_lookupnext to make sense,
relying on lfs3_file_weight_ too much probably hints at a logic mistake.
This logic ends up inlined anyways, so no code changes:
code stack ctx
before: 36992 2352 684
after: 36992 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
We no longer need to discard the leaf, since we can just leave ungrafted
leaves around as long as LFS3_o_UNGRAFT is set.
This let's us flatten lfs3_file_crystallize_, saving a bit of code and
cleaning up the logic a bit:
code stack ctx
before: 37032 2352 684
after: 36992 (-0.1%) 2352 (+0.0%) 684 (+0.0%)
Note that lfs3_file_crystallize_ is still NOINLINE to force it off the
stack hot-path in lfs3_file_flush_, etc.
Continued benchmarking efforts are indicating this isn't really an
optional optimization.
This brings back lazy grafting, where the file leaf is allowed to fall
out-of-date to minimize bshrub/btree updates. This is controlled by
LFS3_o_UNGRAFT, which is similar, but independent from LFS3_o_UNCRYST:
- LFS3_o_UNCRYST - File's leaf not fully crystallized
- LFS3_o_UNGRAFT - File's leaf does not match disk
Note it makes sense for files to be UNGRAFT only, in the case where the
current crystal terminates at the end-of-file but future appends are
likely. And it makes sense for files to be UNCRYST only, in cases where
we graft uncrystallized blocks so the bshrub/btree makes sense.
Which brings us to the main change from the previous lazy-grafting
implementation: lfs3_file_lookupnext no longer includes ungrafted
leaves.
Instead, functions should call lfs3_file_graft if they need
lfs3_file_lookupnext to make sense.
This significantly reduces the code cost of lazy grafting, at the risk
of needing to graft more frequently. Fortunately we don't actually need
to call lfs3_file_graft all that often:
- lfs3_file_read already flushes caches/leaves before attempting any
bshrub/btree reads for simplicity (heavy are not currently considered
a priority, if you need this consider opening two file handles).
- lfs3_file_flush_ _does_ need to call lfs3_file_graft before the
crystallization heuristic pokes, but if we can't resume
crystallization, we would probably need to graft the crystal to
satisfy the flush anyways.
---
Lazy grafting, i.e. procrastinating on bshrub/btree updates during block
appends, is an optimization previously dropped due to perceived
nicheness:
- We can only lazily graft blocks, inlined data fragments always require
bshrub/btree updates since they live in the bshrub/btree.
- Sync forces bshrub/btree updates anyways, so lazy grafting has no
benefit for most logging applications.
- This performance penalty of eagerly grafting goes away if your caches
are large enough.
Note that the last argument is a non-argument in littlefs's case. They
whole point of littlefs is that you _don't_ need RAM to fix things.
However these arguments are all moot when you consider that the "niche
use case" -- linear file writes -- is the default bottleneck for most
applications. Any file operation becomes a linear write bottleneck when
the arguments are large enough. And this becomes a noticeable issue when
benchmarking.
So... This brings back lazy grafting. But with a more limited scope
w.r.t. internal file operations (the above lfs3_file_lookupnext/
lfs3_file_graft changes).
---
Long story short, lazy grafting is back again, reverting the ~3x
performance regression for linear file writes.
But now with quite a bit less code/stack cost:
code stack ctx
before: 36820 2368 684
after: 37032 (+0.6%) 2352 (-0.7%) 684 (+0.0%)
I was wrong! New bmaps containing the old bmap is _not_ sufficient for
lookahead scans, in the case where we rebuild the bmap multiple times
before an mdir commit! The previous bug with lfs3_trv_read +
lfs3_alloc_ckpoint _was_ a bug because the traversal was rdonly, but in
theory the bmap shouldn't have been corrupted.
Since this case is possible, we also need to traverse the on-disk bmap.
Fortunately only when the on-disk bmap does not match the active bmap.
This is probably safer behavior anyways, and means ckmeta/ckdata will
traverse both the in-RAM and on-disk bmaps, which is probably a good
thing in case we need to revert to the on-disk bmap due to power-loss/
error.
---
The implementation got a bit messy, since we only track the on-disk
encoding of the on-disk bmap (we need the encoding for gdeltas to work).
This ended up adding code, and an annoying (avoidable? TODO?) stack
cost, but correct behavior is better than incorrect behavior:
code stack ctx
before: 36856 2368 684
after: 36820 (-0.1%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38452 2400 812
bmap after: 38504 (+0.1%) 2464 (+2.7%) 812 (+0.0%)
Once again the inaccuracy of our stack frame calculations strike
again...
This adds %i and %I as punescape modifiers for limited printing of
integers with SI prefixes:
- %(field)i - base-10 SI prefixes
- 100 => 100
- 10000 => 10K
- 0.01 => 10m
- %(field)I - base-2SI prefixes
- 128 => 128
- 10240 => 10Ki
- 0.125 => 128mi
These can also easily include units as a part of the punescape string:
- %(field)iops/s => 10Kops/s
- %(field)IB => 10KiB
This is particularly useful in plotmpl.py for adding explicit
x/yticklabels without sacrificing the automatic SI-prefixes.
So now these lex correctly:
- 1e9 => 1000000000
- 1e+9 => 1000000000
- 1e-9 => -1000000000
A bit tricky when you think about how these could be confused for binary
addition/subtraction. To fix we just eagerly grab any signs after the e.
These are particularly useful for manipulating simulated benchmarks,
where we need to convert things to/from nanoseconds.
This was causing a problem where the bmap was being rebuilt on every
lfs3_trv_read, even though the traversal was opened LFS3_T_RDONLY!
Also added a note on why we don't need to traverse both the active and
on-disk bmaps. Counterintuitively, we don't need to because the new bmap
always contains the entirety of the on-disk bmap.
Code changes minimal:
code stack ctx
before: 36840 2368 684
after: 36856 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38440 2400 812
bmap after: 38452 (+0.0%) 2400 (+0.0%) 812 (+0.0%)
Found while benchmarking, our shrub estimates were being recalculated
much more frequently than they should be (every shrub commit). The
problem is that we never staged shrub estimates!
In theory this is fine, shrub estimates don't necessarily need staging.
We update shrub.r.eoff/estimate directly in lfs3_bshrub_commitroot_
after the mdir commit succeeds. But then we _redundantly_ sync
shrub_ -> shrub.r in lfs3_bshrub_commit. Since we never staged the
shrub estimate, we end up with garbage.
---
It's not clear to me this (staging the shrub estimate) is the best fix
for this, but the reason for the redundant shrub sync is the shared
bshrub/btree post-commit path. Added a TODO comment and should look at
this again when not in a time crunch.
Code changes minimal:
code stack ctx
before: 36836 2368 684
after: 36840 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
Now that lfs3_alloc_ckpoint is more complicated, and can error, it makes
sense for lfs3_alloc_ckpoint to be implied by lfs3_mdir_commit.
Most lfs3_mdir_commit calls represent an atomic transaction from one
state -> another, so this saves a bit of code:
code stack ctx
before: 36912 2368 684
after: 36836 (-0.2%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38512 2400 812
bmap after: 38436 (-0.2%) 2400 (+0.0%) 812 (+0.0%)
The notable exception being bshrub-related commits in
lfs3_bshrub_commitroot_. Bshrub commits are trying to resolve an
in-flight btree, so the relevant blocks are very much _not_ at rest.
---
I've been hesitant to adopt this mostly just because it makes the
lfs3_mdir_commit* names even more of a mess:
- lfs3_mdir_commit__ -> lfs3_mdir_commit___
- lfs3_mdir_commit_ -> lfs3_mdir_commit__
- lfs3_mdir_commit -> lfs3_mdir_commit_
- added lfs3_mdir_commit
- lfs3_mdir_compact -> lfs3_mdir_compact_
- add lfs3_mdir_compact
- lfs3_mdir_alloc__ -> lfs3_mdir_alloc___
- lfs3_mdir_estimate__ -> lfs3_mdir_estimate___
- lfs3_mdir_swap__ -> lfs3_mdir_swap___
The main change is error propagation in lfs3_alloc_ckpoint. Since
lfs3_alloc_ckpoint writes to disk during bmap rebuilds, it can now fail
in all sorts of ways. Fortunately lfs3_alloc_ckpoint should only ever be
called by write operations, where these errors are be expected.
With bmap rebuild errors now reported correctly, this unblocks most of
the remaining test TODOs:
- Passing test_badblocks
- Passing test_ck
- Passing test_trvs
With this, LFS3_YES_BMAP is now passing all but two tests, which are
still ifndef-disabled as a temporary measure:
- test_btree - We make some low-level assumptions about the lookahead
allocator when testing btrees. It's probably not worth trying to get
this passing with the bmap allocator.
- test_grow - This one does need fixing! We currently don't update
on-disk bmaps correctly when growing the filesystem.
Code changes minimal:
code stack ctx
before: 36912 2368 684
after: 36912 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38456 2400 812
bmap after: 38512 (+0.1%) 2400 (+0.0%) 812 (+0.0%)
We need to adjust mids to ignore orphans during dir traversal, but we
shouldn't also adjust the dir position. In theory it shouldn't matter if
we use adjusted/non-adjusted dir positions, but it becomes a problem if
intermediate writes cause those orphans to be cleaned up. Now all your
dir positions are wrong.
Not entirely sure why this only started to fail with the bmap. I'm
guessing it's just due to the additional gstate causing the mdirs to
split differently.
Code changes minimal:
code stack ctx
before: 36920 2368 684
after: 36912 (-0.0%) 2368 (+0.0%) 684 (+0.0%)
Tangential, but toss this on the pile of problems with dir positions.
I'm increasingly convinced we should just remove the concept if we can
get away with it.
There's really no reason to immediately commit the bmap to disk, at
least no until the first mdir commit, when we need to at least discard
the previous bmap state.
We already do all the gstate handling in lfs3_mdir_commit anyways, and
piggybacking on mdir commit lets us get rid of the annoying extra mdir
param in lfs3_alloc_ckpoint.
This does mean a slightly higher risk of needing to re-rebuild the bmap
after a powerloss, but in theory only if the user does something weird
like writing to a file and never calling sync. Most on-disk operations
terminate in an mdir commit as that's how any state change becomes
atomically visibile in littlefs.
Saves a nice bit of stack:
code stack ctx
before: 36920 2368 684
after: 36920 (+0.0%) 2368 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38552 2472 812
bmap after: 38464 (-0.2%) 2400 (-2.9%) 812 (+0.0%)
At least at a proof-of-concept level, there's still a lot of cleanup
needed.
To make things work, lfs3_alloc_ckpoint now takes an mdir, which
provides the target for gbmap gstate updates.
When the bmap is close to empty (configurable via bmap_scan_thresh), we
opportunistically rebuild it during lfs3_alloc_ckpoints. The nice thing
about lfs3_alloc_ckpoint is we know the state of all in-flight blocks,
so rebuilding the bmap just requires traversing the filesystem + in-RAM
state.
We might still fall back to the lookahead buffer, but in theory a well
tuned bmap_scan_thresh can prevent this from becoming a bottleneck (at
the cost of more frequent bmap rebuilds).
---
This is also probably a good time to resume measuring code/ram costs,
though it's worth repeating the above note about the bmap work still
needing cleanup:
code stack ctx
before: 36840 2368 684
after: 36920 (+0.2%) 2368 (+0.0%) 684 (+0.0%)
Haha, no, the bmap isn't basically free, it's just an opt-in features.
With -DLFS3_YES_BMAP=1:
code stack ctx
no bmap: 36920 2368 684
yes bmap: 38552 (+4.4%) 2472 (+4.4%) 812 (+18.7%)
The gbmap's weight is defined by the block count stored in the geometry
config field, which should always be present in valid littlefs3 images.
But our scripts routinely try to parse _invalid_ littlefs3 images when
running in parallel with benchmarks/tests (littlefs3 does _not_ support
multiple read/writers), so this was causing exceptions to be thrown.
The fix is to just assume weight=0 when the geometry field is missing.
The image isn't valid, and the gbmap is optional anyways.
This doesn't fully replace the lookahead buffer, but at least augments
it with known bmap state when available.
To be honest, this is a minimal effort hack to try to get something
benchmarkable without dealing with all the catch-22 issues that a
self-support bmap allocator would encounter (allocating blocks for the
bmap requires a bmap, oh no).
Though now that I'm writing this, maybe this is a reasonable long-term
solution? Having the lookahead buffer to fall back on solves a lot of
problems, and, realistically, it's unlikely to be a performance
bottleneck unless the user has extreme write requests (>available
storage?).
---
Also tweaked field naming to be consistent between the bmap and
lookahead buffer.
This avoids issues with the different traversal paths with an mtree vs
inline-mtree. Previously this was broken when the mtree was inlined.
This order also makes more sense if we want to check mdirs before we
consider the gstate to be trustworthy enough for gbmap traversal.
The idea behind separate ctrled+unctrled airspaces was to try to avoid
multiple interpretations of the on-disk bmap, but I'm starting to think
this adds more complexity than it solves.
The main conflict is the meaning of "in-flight" blocks. When using the
"uncontrolled" bmap algorithm, in-flight blocks need to be
double-checked by traversing the filesystem. But in the "controlled"
bmap algorithm, blocks are only marked as "in-flight" while they are
truly in-flight (in-use in RAM, but not yet in use on disk).
Representing these both with the same "in-flight" state risks
incompatible algorithms misinterpreting the bmap across different
mounts.
In theory the separate airspaces solve this, but now all the algorithms
need to know how to convert the bmap from different modes, adding
complexity and code cost.
Well, in theory at least. I'm unsure separate airspaces actually solves
this due to subtleties between what "in-flight" means in the different
algorithms (note both in-use and free blocks are "in-flight" in the
unknown airspace!). It really depends on how the "controlled" algorithm
actually works, which isn't implemented/fully designed yet.
---
Long story short, due to a time crunch, I'm ripping this out for now and
just storing the current algorithm in the wcompat flags:
LFS3_WCOMPAT_GBMAP 0x00006000 Global block-map in use
LFS3_WCOMPAT_GBMAPNONE 0x00000000 Gbmap not in use
LFS3_WCOMPAT_GBMAPCACHE 0x00002000 Gbmap in cache mode
LFS3_WCOMPAT_GBMAPVFR 0x00004000 Gbmap in VFR mode
LFS3_WCOMPAT_GBMAPIFR 0x00006000 Gbmap in IFR mode
Note GBMAPVFR/IFR != BMAPSLOW/FAST! At least BMAPSLOW/FAST can share
bmap representations:
- GBMAPVFR => Uncontrolled airspace, i.e. in-flight blocks may or may
not be in use, need to traverse open files.
- GBMAPIFR => Controlled airspace, i.e. in-flight blocks are in use,
at least until powerloss, no traversal needed, but requires more bmap
writes.
- BMAPSLOW => Treediff by checking what blocks are in B but not in A,
and what blocks are in A but not in B, O(n^2), but minimizes bmap
updates.
Can be optimized with a bloom filter.
- BMAPFAST => Treediff by clearing all blocks in A, and then setting all
blocks in B, O(n), but also writes all blocks to the bmap twice even
on small changes.
Can be optimized with a sliding bitmap window (or a block hashtable,
though a bitmap converges to the same thing in both algorithms when
>=disk_size).
It will probably be worth unifying the bmap representation later (the
more algorithm-specific flags there are, the harder interop becomes for
users, but for now this opens a path to implementing/experimenting with
bmap algorithms without dealing with this headache.
The neat thing about the on-disk bmap is that it's a range tree. We can
leverage order-statistic properties to compactly represent ranges of
similar blocks.
However, this does make updating the bmap slightly more complicated...
This only works immediately after format, and only for one pass of the
disk, but it's a good way to test bmap lookups/allocation without
worrying about more complicated filesystem-wide interactions.
Except for niche file snapshotting, most btree updates until this point
are probably linear, i.e. a successful commit replaces any internal
rbyd state that has been claimed. In this model it makes sense to mark
claimed rbyd as "invalid", since failure to replace the claimed state
indicates something went wrong during the commit.
But this isn't necessarily true when snapshotting, since we don't
replace the state of claimed snapshots.
But wait, shouldn't snapshotted rbyds become readonly? Not necessarily!
Rbyds can have multiple trunks with unrelated (or in this case, shared)
histories, so there's nothing wrong with refetching an rbyd and
continuing to commit after another snapshot commits to tbe block.
Eventually both snapshots will need to compact and diverge into two
blocks, but until then sharing an rbyd makes the most of available
erased state.
At least in theory, experience will show us how well this works.
---
Also note this is not true for mdirs. We view mdirs as atomic and always
up-to-date, so snapshotting doesn't really make sense.
Fortunately the btree traversal logic is pretty reusable, so this just
required an additional tstate (LFS3_TSTATE_BMAP).
This raises an interesting question: _when_ do we traverse the bmap? We
need to wait until at least mtree traversal completes for gstate to be
reconstructed during lfs3_mount, but I think traversing before file
btrees makes sense.
This abandons the data-backed cache idea due to concerns around
readability and maintainability. Mixing const/mutable buffers in
lfs3_data_t was not great.
Instead, we now just allocate an indirect lfs3_data_t on the stack in
lfs3_file_sync_ to avoid the previous undefined behavior.
This actually results in less stack usage total, due to lfs3_file_t
allocations in lfs3_set/read, and avoid the more long-term memory cost
in lfs3_file_t:
code stack ctx
before: 36832 2376 684
after: 36840 (+0.0%) 2368 (-0.3%) 684 (+0.0%)
Oh. And lfs3_file_sync_ isn't even on the stack hot-path, so this is a
net benefit over the previous cache -> data cast:
code stack ctx
before sa: 36844 2368 684
after sa: 36840 (-0.0%) 2368 (+0.0%) 684 (+0.0%)
Still less cool though.
This fixes a strict aliasing violation in lfs3_file_sync_, where we cast
the file cache -> lfs3_data_t to avoid an extra stack allocation, by
modifying the file's cache struct to use an lfs3_data_t directly.
- file.cache.pos -> file.cache.pos
- file.cache.buffer -> file.cache.d.u.buffer_
- file.cache.size -> file.cache.d.size
- (const lfs3_data_t*)&file->cache -> &file->cache.d
Note the underscore_ in file.cache.d.u.buffer_. This did not fit
together as well as I had hoped, due to different const expectation
between the file cache and lfs3_data_t.
Up until this point lfs3_data_t has only been used to refer to const
data (ignoring side-band pointer casting in lfs3_mtree_traverse*), while
the file cache very much contains mutable data. To work around this I
added data.u.buffer_ as a mutable variant, which works, but risks an
accidental const violation in the future.
---
Unfortunately this does come with a minor RAM cost, since we no longer
hide file.cache.pos in lfs3_data_t's buffer padding:
code stack ctx
before: 36844 2368 684
after: 36832 (-0.0%) 2376 (+0.3%) 684 (+0.0%)
lfs3_file_t before: 164
lfs3_file_t after: 168 (+2.4%)
I think it's pretty fair to call C's strict aliasing rules a real wet
blanket. It would be interesting to create a -fno-strict-aliasing
variant of littlefs in the future, to see how much code/RAM could be
saved if we were given free reign to abuse the available memory.
Probably not enough to justify the extra work, but it would be an
interesting experiment.
I think modern C simply doesn't let us do what we want to do here, so
I'm giving up, discarding the lfs3_mtortoise_t type, and just abusing
various unrelated shrub fields to implement the tortoise. This
sacrifices readability, but at least avoids undefined behavior without
a RAM penalty:
- shrub.blocks => tortoise blocks
- shrub.weight => cycle distance
- shrub.eoff => power-of-two bound
Note this keeps trunk=0, which is a nice safety net in case some code
ever tries to read from the shrub in the future.
Fortunately the mtortoise logic is fairly self-contained in
lfs3_mtree_traverse_, so with enough comments hopefully the code is not
too confusing.
---
Apparently shaves off a couple more bytes of code. I'm guessing this is
just because of the slightly different struct offsets (we're reusing the
root's rbyd instead of the leaf's rbyd now):
code stack ctx
before: 36852 2368 684
after: 36844 (-0.0%) 2368 (+0.0%) 684 (+0.0%)
This forces our cycle detection tortoise (previously trv.u.mtortoise),
into the unused shrub leaf via pointer shenanigans.
This reclaims the remaining stack (and apparently code) we theoretically
gained from the btree traversal rework, up until the compiler got in the
way:
code stack ctx
before: 36876 2384 684
after: 36852 (-0.1%) 2368 (-0.7%) 684 (+0.0%)
And it only required some _questionably_ defined behavior.
---
It's probably not well-defined behavior, but trying to understand what
the standard actually means on this is giving me a headache. I think I
have to agree C99+strict-aliasing lost the plot on this one. Note
mtortoise is only ever written/read through the same type.
What I want:
lfs3_trv_t: lfs3_bshrub_t: lfs3_handle_t:
.---+---+---+---. .. .---+---+---+---. .. .---+---+---+---.
| handle | | handle | | handle |
| | | | | |
+---+---+---+---+ +---+---+---+---+ .. '---+---+---+---'
| root rbyd | | root rbyd |
| | | | lfs3_mtortoise_t:
+---+---+---+---+ +---+---+---+---+ .. .---+---+---+---.
| leaf rbyd | | leaf rbyd | | mtortoise |
| | | | | |
+---+---+---+---+ +---+---+---+---+ .. '---+---+---+---'
| staging rbyd | | staging rbyd |
| | | |
+---+---+---+---+ .. '---+---+---+---'
| |
: :
But I'm starting to think this is simply not possible in modern C.
At least this shows what is theoretically possible if we didn't have to
fight the compiler.
Looks like these traversal states were missed in the omdir -> handle
rename. I think HANDLES and HBTREE states make sense:
- LFS3_TSTATE_OMDIRS -> LFS3_TSTATE_HANDLES
- LFS3_TSTATE_OBTREE -> LFS3_TSTATE_HBTREE
This matches other internal rbyds: btree.r, mdir.r, etc.
The intention of the single-char names is to reduce clutter around these
severely nested structs, both btrees and mdirs _are_ rbyds, so the name
doesn't really besides C-level type info.
I was hesitant on btree.leaf.rbyd, but decided consistency probably wins
here.
This comes from an observation that we never actually use the leaf cache
during traversals, and there is surprisingly little risk of a lookup
creating a conflict in the future.
Btree traversal fall into two categories:
1. Full traversals, where we traverse a full btree all at once. These
are unlikely to have lookup conflicts because everything is
usually self-contained in one chunk of logic.
2. Incremental traversals. These _are_ at risk, but in our current
design limited to lfs3_trv_t, which already creates a fully
bshrub/btree copy for tracking purposes.
This copy unintentionally, but conveniently, protects against lookup
conflicts.
So, why not reuse the btree leaf cache to hold the rbyd state during
traversals? In theory this makes lfs3_btree_traverse the same cost and
lfs3_btree_lookupnext, drops the need for lfs3_btrv_t, and simplifies
the internal API.
The only extra bit of state we need is the current target bid, which is
now expected as a caller-incremented argument similar to
lfs3_btree_lookupnext iteration.
There was a bit of futzing around with bid=-1 being necessary to
initialize traversal (to avoid conflicts with bid=-1 => 0 caused by
empty btrees). But the end result is a btree traversal that only needs
one extra word of state.
---
Unfortunately, in practice, the savings were not as great as expected:
code stack ctx
before: 36792 2400 684
after: 36876 (+0.2%) 2384 (-0.7%) 684 (+0.0%)
This does claw back some stack, but less than a full rbyd due to the
union with the mtortoise in lfs3_trv_t. The mtortoise now dominates. It
might be possible to union the mtortoise and the bshrub/btree state
better (both are not needed at the same time), but strict aliasing rules
in C make this tricky.
The new lfs3_btree_traverse is also a bit more complicated in terms of
code cost. In theory this would be offset by the simpler traversal setup
logic, but we only actually call lfs3_btree_traverse twice:
1. In lfs3_mtree_traverse
2. In lfs3_file_ck
Still, some stack savings + a simpler internal API makes this worthwhile
for now. lfs3_trv_t is also due for a revisit, and hopefully it's
possible to better union things with btree leaf caches somehow.
I was confused, but this commit->bid update is used to limit the
commit->bid to the btree weight. Note we limit the bid after storing it
as the initial rid.
