I mean, why not? These redirect to the same internal lfs3_fs_gc_
function anyways. Might as well keep things consistent.
Added:
LFS3_F_MKCONSISTENT 0x00000800 Make the filesystem consistent
LFS3_F_RELOOKAHEAD 0x00001000 Repopulate lookahead buffer
LFS3_F_MKCONSISTENT is guaranteed to be a noop, but LFS3_F_RELOOKAHEAD
forces a filesystem traversal, which may have some niche use case.
No code changes.
These are unlikely to make much progress, but that doesn't seem like a
great reason to disallow these flags in lfs3_format:
LFS3_F_REGBMAP 0x00002000 Repopulate the gbmap
LFS3_F_COMPACTMETA 0x00008000 Compact metadata logs
These are actually guaranteed to do _no_ work when formatting _without_
the gbmap, but with the gbmap it's less clear. Looking forward to the
planned ckfactory feature, these may be useful for cleaning up any rbyd
commits created as a part of building the initial gbmap.
---
Also tweaked the formatting for LFS3_F_* flags a bit, including making
all ifdefs explicit (mainly ifdef LFS3_RDONLY). Mixed ifdefs are a real
pain to read.
No code changes.
LFS3_REVDBG introduced a lot of overhead for something I'm not sure
anyone will actually use (I have enough tooling that the state of an
rbyd is rarely a mystery, see dbgbmap.py). That, and we're running out
of flags!
So this reduces LFS3_REVDBG to just store one of "himb" in the first
(lowest) byte of the revision count; information that is easily
available:
vvvv---- -------- -------- --------
vvvvrrrr rrrrrr-- -------- --------
vvvvrrrr rrrrrrnn nnnnnnnn nnnnnnnn
vvvvrrrr rrrrrrnn nnnnnnnn dddddddd
'-.''----.----''----.- - - '---.--'
'------|----------|----------|---- 4-bit relocation revision
'----------|----------|---- recycle-bits recycle counter
'----------|---- pseudorandom noise (if revnoise)
'---- h, i, m, or b (if revdbg)
-11-1--- - h = mroot anchor
-11-1--1 - i = mroot
-11-11-1 - m = mdir
-11---1- - b = btree node
Some other notes:
- Enabled LFS3_REVDBG and LFS3_REVNOISE to work together, now that
LFS3_REVDBG doesn't consume all unused rev bits.
Note that LFS3_REVDBG has priority over LFS3_REVNOISE, but _not_
recycle-bits, etc. Otherwise problems would happen for recycle-bits
>2^20 (though do we care?).
- Fixed an issue where using the gcksum as a noise source results in
noise=0 when there is only an mroot. This is due to how we xor out
the current mdir cksum during an mdir commit.
Fixed by using gcksum_p instead of gcksum.
- Added missing LFS3_I_REVDBG/REVNOISE flags in the tests, so now you
can actually run the tests with LFS3_REVDBG/REVNOISE (this probably
just fell out-of-date at some point).
---
Curiously, despite LFS3_REVDBG/REVNOISE being disabled by default, this
did save some code. I'm guessing the non-tail-call mtree/gbmap commit
functions prevented some level of inlining?:
code stack ctx
before: 35964 2280 660
after: 35964 (+0.0%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38940 2296 772
gbmap after: 38828 (-0.3%) 2296 (+0.0%) 772 (+0.0%)
With the relaxation of traversal behavior under mutation, I think it
makes sense to bring back LFS3_T_EXCL. If only to allow traversals to
gaurantee termination under mutation. Now that traversals no longer
guarantee forward progress, it's possible to get stuck looping
indefinitely if the filesystem is constantly being mutated.
Non-excl traversals are probably still useful for GC work and debugging
threads, but LFS3_T_EXCL now allows traversals to terminate immediately
with LFS3_ERR_BUSY at the first sign of unrelated filesystem mutation:
LFS3_T_EXCL 0x00000008 Error if filesystem modified
Internally, we already track unrelated mutation to avoid corrupt state
(LFS3_t_DIRTY), so this is a very low-cost feature:
code stack ctx
before: 35944 2280 660
after: 35964 (+0.1%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38916 2296 772
gbmap after: 38940 (+0.1%) 2296 (+0.0%) 772 (+0.0%)
code stack ctx
gc before: 36016 2280 768
gc after: 36036 (+0.1%) 2280 (+0.0%) 768 (+0.0%)
This solves the previous gc-needs-block-queue-so-we-can-clobber-block-
queue issue by adding an additional LFS3_t_STALE flag to indicate when
any block queues would be invalid.
So instead of clearing block queues in lfs3_alloc_ckpoint, we just set
LFS3_t_STALE, and any lfs3_trv_ts can clear their block queues in
lfs3_trv_read. This allows lfs3_mgc_ts to be allocated without a block
queue when doing any LFS3_M_*/LFS3_F_*/LFS3_GC_* work.
LFS3_t_STALE is set at the same time as LFS3_t_CKPOINT and LFS3_t_DIRTY,
but we need a separate bit so lfs3_trv_read can clear the flag after
flushing without losing ckpoint/dirty information.
---
Unfortunately, none of the stack-allocated lfs3_mgc_ts are on the stack
hot-path, so we don't immediate savings. But note the 2-words saved in
ctx when compiling in LFS3_GC mode:
code stack ctx
before: 35940 2280 660
after: 35944 (+0.0%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38916 2296 772
gbmap after: 38916 (+0.0%) 2296 (+0.0%) 772 (+0.0%)
code stack ctx
gc before: 36012 2280 776
gc after: 36016 (+0.0%) 2280 (+0.0%) 768 (-1.0%)
The main idea here is to drop the flag-encoded tstate state machine, and
replace it with a matrix controlled by special mid + bid values:
-- mid ->
-5 -4 -3 -2 >=-1
bid -2 x x x --> mdir
v >=-1 x gbm gbm x --> bshrub/btree
'----|----|----|----|----> mroot anchor
'----|----|----|----> mroot chain + mtree
'----|----|----> gbmap (in-ram gbmap)
'----|----> gbmap_p (on-disk gbmap)
'----> file bshrubs/btrees
This was motivated by the observation that everything in our filesystem
can be modeled as mdir + bshrub/btree tuples, as long as some states are
noops. And we can cleanly encode these tuples in the unused negative
mid + bid ranges without needing an explicit state machine.
Well, that and the previous tstate state machine approach being an ugly
pile of switch cases and messy logic.
Note though that some mids may need to traverse multiple mdirs/bshrub/
btrees:
- The mroot chain + mtree (mid=-4) needs to traverse all mroots in the
mroot chain, and detect any cycles.
- File mdirs (mid>=-1) need to traverse both the on-disk bshrub/btree
and any opened file handles' bshrubs/btrees before moving onto the
next mid.
This grows O(n^2) because all file handles are in one big unsorted
linked-list, but as usual we don't care.
In addition to the greatly simplified traversal logic, the new state
matrix simplifies traversal clobbering: Setting bid=-2 always forces a
bshrub/btree refetch.
This comes at the cost of traversal _precision_, i.e. we can now revisit
previously visited bshrub/btree nodes. But I think this is well worth it
for more robust traversal clobbering. Traversal clobbering is delicate
and difficult to get right.
Besides, we can already revisit blocks due to CoW references, so what's
the harm in revisiting blocks when under mutation?
---
The simpler traversal logic leads to a nice amount of code savings
across the board:
code stack ctx
before: 36476 2304 660
after: 35940 (-1.5%) 2280 (-1.0%) 660 (+0.0%)
code stack ctx
gbmap before: 39524 2320 772
gbmap after: 38916 (-1.5%) 2296 (-1.0%) 772 (+0.0%)
code stack ctx
gc before: 36548 2304 804
gc after: 36012 (-1.5%) 2280 (-1.0%) 776 (-3.5%)
Note the ctx savings in LFS3_GC mode. Most of the stack/ctx savings
comes from the smaller lfs3_mtrv_t struct, which no longer needs to
stage bshrubs (we no longer care about bshrubs across mdir commit as a
part of the above clobbering simplifications):
before after
lfs3_mtrv_t: 128 100 (-21.9%)
lfs3_mgc_t: 128 100 (-21.9%)
lfs3_trv_t: 136 108 (-20.6%)
Unfortunately, the simpler clobbering means now any gc work needs the
block queue (i.e. lfs3_trv_t), solely so clobbering the block queue
doesn't clobber unallocated memory. Not great but hopefully fixable.
---
Some other notes:
- As a part of simplifying traversal clobbering, everything is triggered
by lfs3_alloc_ckpoint (via lfs3_trv_ckpoint_).
This may clobber traversals more than is strictly necessary, but
that's kinda the idea. Better safe than sorry.
And no more need to explicit lfs3_handle_clobber calls is nice.
- Opened file handle iteration is now tracked by the traversal handle's
position in the handle linked-list, instead of a separate handle
pointer. This means one less thing to disentangle and makes traversals
no longer a special case for things like lfs3_handle_close.
You may think this bumps traversals up to O(n^3) in-ram, but because
we only ever visit each unique handle + mid once, we can keep the
total O(n^2) if we're smart about linked-list updates!
- lfs3_mdir_commit needed to be tweaked to accept mids<=-1, instead of
just mid=-1 for the mroot. Unfortunately I don't know how much this
costs on its own.
- The reorganization of lfs3_mtrv_t means lfs3_mtortoise_t gets its own
struct again!
- No more tstate state machine also frees up a big chunk of the
traversal flag space, which was getting pretty cramped.
- lookupleaf -> lookupnext_
- namelookupleaf -> namelookup_
I want to move away from lookupleaf usage in general in the dbg scripts,
like we have in lfs3.c, but I also just really don't want to touch these
scripts again unless I need to. They've been useful, but also a big time
sink.
Maybe I should actually learn Python's new type system. That would
probably help here...
So:
- cfg.gc_repoplookahead_thresh -> cfg.gc_relookahead_thresh
- cfg.gc_repopgbmap_thresh -> cfg.gc_regbmap_thresh
- cfg.gbmap_repop_thresh -> cfg.gbmap_re_thresh
- LFS3_*_REPOPLOOKAHEAD -> LFS3_*_RELOOKAHEAD
- LFS3_*_REPOPGBMAP -> LFS3_*_REGBMAP
Mainly trying to reduce the mouthful that is REPOPLOOKAHEAD and
REPOPGBMAP.
As a plus this also avoids potential confusion of "repop" as a push/pop
related operation.
This should make tag editing less tedious/error-prone. We already used
self-parsing to generate -l/--list in dbgtag.py, but this extends the
idea to tagrepr (now Tag.repr), which is used in quite a few more
scripts.
To make this work the little tag encoding spec had to become a bit more
rigorous, fortunately the only real change was the addition of '+'
characters to mark reserved-but-expected-zero bits.
Example:
TAG_CKSUM = 0x3000 ## v-11 ---- ++++ +pqq
^--^----^----^--^-^-- valid bit, unmatched
'----|----|--|-|-- matches 1
'----|--|-|-- matches 0
'--|-|-- reserved 0, unmatched
'-|-- perturb bit, unmatched
'-- phase bits, unmatched
dbgtag.py 0x3000 => cksumq0
dbgtag.py 0x3007 => cksumq3p
dbgtag.py 0x3017 => cksumq3p 0x10
dbgtag.py 0x3417 => 0x3417
Though Tag.repr still does a bit of manual formatting for the
differences between shrub/normal/null/alt tags.
Still, this should reduce the number of things that need to be changed
from 2 -> 1 when adding/editing most new tags.
This required a bit of a hack: LFS3_seek_MODE, which is marked internal
to try to minimize confusion, but really doesn't exist in the code at
all.
But a hack is probably good enough for now.
This has just proven much easier to tweak in dbgtag.py, so adopting the
same self-parsing pattern in dbgflags.py/dbgerr.py. This makes editing
easier by (1) not needing to worry about parens/quotes/commas, and
(2) allowing for non-python expressions, such as the mode flags in
dbgflags.py.
The only concern is script startup may be slightly slower, but we really
don't care.
This drops LFS3_t_MUTATED in favor of just using LFS3_t_CKPOINTED
everywhere:
1. These meant roughly the same thing, with LFS3_t_MUTATED being a bit
tighter at the cost of needing to be explicitly set.
2. The implicit setting of LFS3_t_CKPOINTED by lfs3_alloc_ckpoint -- a
function that already needs to be called before mutation -- means we
have one less thing to worry about.
Implicit properties like LFS3_t_CKPOINTED are great for building a
reliable system. Manual flags like LFS3_t_MUTATED, not so much.
3. Why use two flags when we can get away with one?
The only downside is we may unnecessarily clobber gc/traversal work when
we don't actually mutate the filesystem. Failed file open calls are a
good example.
However this tradeoff seems well worth it for an overall simpler +
more reliable system.
---
Saves a bit of code:
code stack ctx
before: 37220 2352 688
after: 37160 (-0.2%) 2352 (+0.0%) 688 (+0.0%)
code stack ctx
gbmap before: 40184 2368 856
gbmap after: 40132 (-0.1%) 2368 (+0.0%) 856 (+0.0%)
Note: This affects the blocking lfs3_alloc_repopgbmap as well as
incremental gc/traversal repopulations. Now all repop attempts return
LFS3_ERR_NOSPC when we don't have space for the gbmap, motivation below.
This reverts the previous LFS3_t_NOSPC soft error, in which traversals
were allowed to continue some gc/traversal work when encountering
LFS3_ERR_NOSPC. This results in a simpler implementation and fewer error
cases to worry about.
Observation/motivation:
- The main motivation is noticing that when we're in low-space
conditions, we just start spamming gbmap repops even if they all fail.
That's really not great! We might as well just mark the flash as dead
if we're going to start spamming erases!
At least with an error the user can call rmgbmap to try to make
progress.
- If we're in a low-space condition, something else will probably return
LFS3_ERR_NOSPC anyways. Might as well report this early and simplify
our system.
- It's a simpler model, and littlefs3 is already much more complicated
than littlefs2. Maybe we should lean more towards a simpler system
at the cost of some niche optimizations.
---
This had the side-effect of causing more lfs3_alloc_ckpoints to return
errors during testing, which revealed a bug in our uz/uzd_fuzz tests:
- We weren't flushing after writes to the opened RDWR files, which could
cause delayed errors to occur during the later read checks in the
test.
Fortunately LFS3_O_FLUSH provides a quick and easy fix!
Note we _don't_ adopt this in all uz/uzd_fuzz tests, only those that
error. It's good to test both with and without LFS3_O_FLUSH to test
that read-flushing also works under stress.
Saves a bit of code:
code stack ctx
before: 37260 2352 688
after: 37220 (-0.1%) 2352 (+0.0%) 688 (+0.0%)
code stack ctx
gbmap before: 40220 2368 856
gbmap after: 40184 (-0.1%) 2368 (+0.0%) 856 (+0.0%)
This relaxes error encountered during lfs3_mtree_gc to _not_ propagate,
but instead just log a warning and prevent the relevant work from being
checked off during EOT.
The idea is this allows other work to make progress in low-space
conditions.
I originally meant to limit this to gbmap repopulations, to match the
behavior of lfs3_alloc_repopgbmap, but I think extending the idea to all
filesystem mutating operations makes sense (LFS3_T_MKCONSISTENT +
LFS3_T_REPOPGBMAP + LFS3_T_COMPACTMETA).
---
To avoid incorrectly marking traversal work as completed, we need to
track if we hit any ENOSPC errors, thus the new LFS3_t_NOSPC flag:
LFS3_t_NOSPC 0x00800000 Optional gc work ran out of space
Not the happiest just throwing flags at problems, but I can't think of a
better solution at the moment.
This doesn't differentiate between ENOSPC errors during the different
types of work, but in theory if we're hitting ENOSPC errors whatever
work returns the error is a toss-up anyways.
---
Adds a bit of code:
code stack ctx
before: 37208 2352 688
after: 37248 (+0.1%) 2352 (+0.0%) 688 (+0.0%)
code stack ctx
gbmap before: 40120 2368 856
gbmap after: 40204 (+0.2%) 2368 (+0.0%) 856 (+0.0%)
- LFS3_T_COMPACT -> LFS3_T_COMPACTMETA
- gc_compact_thresh -> gc_compactmeta_thresh
And friends:
LFS3_M_COMPACTMETA 0x00000800 Compact metadata logs
LFS3_GC_COMPACTMETA 0x00000800 Compact metadata logs
LFS3_I_COMPACTMETA 0x00000800 Filesystem may have uncompacted metadata
LFS3_T_COMPACTMETA 0x00000800 Compact metadata logs
---
This does two things:
1. Highlights that LFS3_T_COMPACTMETA only interacts with metadata logs,
and has no effect on data blocks.
2. Better matches the verb+noun names used for other gc/traversal flags
(REPOPGBMAP, CKMETA, etc).
It is a bit more of a mouthful, but I'm not sure that's entirely a bad
thing. These are pretty low-level flags.
And friends:
LFS3_M_REPOPLOOKAHEAD 0x00000200 Repopulate lookahead buffer
LFS3_GC_REPOPLOOKAHEAD 0x00000200 Repopulate lookahead buffer
LFS3_I_REPOPLOOKAHEAD 0x00000200 Lookahead buffer is not full
LFS3_T_REPOPLOOKAHEAD 0x00000200 Repopulate lookahead buffer
To match LFS3_T_REPOPGBMAP, which is more-or-less the same operation.
Though this does turn into quite the mouthful...
And tweaked a few related comments.
I'm still on the fence with this name, I don't think it's great, but it
at least betters describes the "repopulation" operation than
"rebuilding". The important distinction is that we don't throw away
information. Bad/erased block info (future) is still carried over into
the new gbmap snapshot, and persists unless you explicitly call
rmgbmap + mkgbmap.
So, adopting gbmap_repop_thresh for now to see if it's just a habit
thing, but may adopt a different name in the future.
As a plus, gbmap_repop_thresh is two characters shorter.
A bit less simplified than I hoped, we don't _strictly_ need both
LFS3_t_DIRTY + LFS3_t_MUTATED if we're ok with either (1) making
multiple passes to confirm fixorphans succeeded or (2) clear the COMPACT
flag after one pass (which may introduce new uncompacted metadata). But
both of these have downsides, and we're not _that_ stressed for flag
space yet...
So keeping all three of:
LFS3_t_DIRTY 0x04000000 Filesystem modified outside traversal
LFS3_t_MUTATED 0x02000000 Filesystem modified during traversal
LFS3_t_CKPOINTED 0x01000000 Filesystem ckpointed during traversal
But I did manage to get rid of the bit swapping by tweaking LFS3_t_DIRTY
to imply LFS3_t_MUTATED instead of being exclusive. This removes the
"failed" gotos in lfs3_mtree_gc and makes things a bit more readable.
---
I also split lfs3_fs/handle_clobber into separate lfs3_fs/handle_clobber
and lfs3_fs/handle_mutate functions. This added a bit of code, but I
think is worth it for a simpler internal API. A confusing internal API
is no good.
In total these simplifications saved a bit of code:
code stack ctx
before: 37208 2360 684
after: 37176 (-0.1%) 2360 (+0.0%) 684 (+0.0%)
code stack ctx
gbmap before: 40100 2432 848
gbmap after: 40060 (-0.1%) 2432 (+0.0%) 848 (+0.0%)
This adds LFS3_T_REBUILDGBMAP and friends, and enables incremental gbmap
rebuilds as a part of gc/traversal work:
LFS3_M_REBUILDGBMAP 0x00000400 Rebuild the gbmap
LFS3_GC_REBUILDGBMAP 0x00000400 Rebuild the gbmap
LFS3_I_REBUILDGBMAP 0x00000400 The gbmap is not full
LFS3_T_REBUILDGBMAP 0x00000400 Rebuild the gbmap
On paper, this is more or less identical to repopulating the lookahead
buffer -- traverse the filesystem, mark blocks as in-use, adopt the new
gbmap/lookahead buffer on success -- but a couple nuances make
rebuilding the gbmap a bit trickier:
- Unlike the lookahead buffer, which eagerly zeros in allocation, we
need an explicit zeroing pass before we start marking blocks as
in-use. This means multiple traversals can potentially conflict with
each other, risking the adoption of a clobbered gbmap.
- The gbmap, which stores information on disk, relies on block
allocation and the temporary "in-flight window" defined by allocator
ckpoints to avoid circular block states during gbmap rebuilds. This
makes gbmap rebuilds sensitive to allocator ckpoints, which we
consider more-or-less a noop in other parts of the system.
Though now that I'm writing this, it might have been possible to
instead include gbmap rebuild snapshots in fs traversals... but that
would probably have been much more complicated.
- Rebuilding the gbmap requires writing to disk and is generally much
more expensive/destructive. We want to avoid trying to rebuild the
gbmap when it's not possible to actually make progress.
On top of this, the current trv-clobber system is a delicate,
error-prone mess.
---
To simplify everything related to gbmap rebuilds, I added a new
internal traversal flag: LFS3_t_CKPOINTED:
LFS3_t_CKPOINTED 0x04000000 Filesystem ckpointed during traversal
LFS3_t_CKPOINTED is set, unconditionally, on all open traversals in
lfs3_alloc_ckpoint, and provides a simple, robust mechanism for checking
if _any_ allocator checkpoints have occured since a traversal was
started. Since lfs3_alloc_ckpoint is required before any block
allocation, this provides a strong guarantee that nothing funny happened
to any allocator state during a traversal.
This makes lfs3_alloc_ckpoint a bit less cheap, but the strong
guarantees that allocator state is unmodified during traversal are well
worth it.
This makes both lookahead and gbmap passes simpler, safer, and easier to
reason about.
I'd like to adopt something similar+stronger for LFs3_t_MUTATED, and
reduce this back to two flags, but that can be a future commit.
---
Unfortunately due to the potential for recursion, this ended up reusing
less logic between lfs3_alloc_rebuildgbmap and lfs3_mtree_gc than I had
hoped, but at like the main chunks (lfs3_alloc_remap,
lfs3_gbmap_setbptr, lfs3_alloc_adoptgbmap) could be split out into
common functions.
The result is a decent chunk of code and stack, but the value is high as
incremental gbmap rebuilds are the only option to reduce the latency
spikes introduced by the gbmap allocator (it's not significantly worse
than the lookahead buffer, but both do require traversing the entire
filesystem):
code stack ctx
before: 37164 2352 684
after: 37208 (+0.1%) 2360 (+0.3%) 684 (+0.0%)
code stack ctx
gbmap before: 39708 2376 848
gbmap after: 40100 (+1.0%) 2432 (+2.4%) 848 (+0.0%)
Note the gbmap build is now measured with LFS3_GBMAP=1, instead of
LFS3_YES_GBMAP=1 (maybe-gbmap) as before. This includes the cost of
mkgbmap, lfs3_f_isgbmap, etc.
The gbmap introduces quite a bit of complexity with how it interacts
with config: block_count => gbmap weight, and wcompat => gbmap enabled.
On one hand this means fewer sources of truth, on the other hand it
makes the gbmap logic cross subsystems and a bit messy.
To avoid trying to parse a bunch of disabled/garbage gstate, this adds
wcompat/rcompat checks to our Gstate class, exposed via __bool__.
This also means we actually need to parse wcompat/rcompat/ocompat flags,
but that wasn't to difficult (though currently only supports 32-bits).
---
I added conditional repr logic for the grm and gbmap, but didn't bother
with the gcksum. The gcksum is used too many other places in these
scripts to expect a nice rendering when disabled.
In lfs3_fs_grow, we need to update any gbmaps to match the new disk
size. The actual patch to the gbmap is easy, but it does get a bit
delicate since we need to feed the gbmap with an allocator in the new
disk size.
Fortunately, the opportunistism of the gbmap allocator avoids any
catch-22 issues, as long as we make sure to not trigger any gbmap
rebuilds.
Adds a bit of code, but not much:
code stack ctx
before: 37168 2352 684
after: 37168 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
code stack ctx
gbmap before: 39000 2456 800
gbmap after: 39116 (+0.3%) 2456 (+0.0%) 800 (+0.0%)
TLDR: This drops the idea of different bmap strategies/modes, and sorts
out most of the compile-time/runtime conditional bmap interactions.
---
Motivation: Benchmarking (at least up to the 32-bit word limit) has
shown the bmap will unlikely be a significant bottleneck, even on large
disks. The largest disks tend to be NAND, and NAND's ridiculous block
size limits pressure on block allocation.
There are still concerns for areas I haven't measured yet:
- SD/eMMC/FTL - Small blocks, so more pressure on block allocation. In
theory the logical block size can be artificially increased, but this
comes with a granularity tradeoff.
- I've only measured throughput, latency is a whole other story.
However, users have reported lfs3_fs_gc is useful for mitigating this,
so maybe latency is less of a concern now?
But while there may still be room for improvement via alternative bmap
strategies, the risk a concerning amount of complexity. Yes,
configuration gets more complicated, but the real issue is any bmap
strategies that try to track _deallocations_ (the original idea being
treediffing) risk falling leaking blocks if all cases aren't covered.
The current "bmap cache" strategy strikes a really nice balance where it
reduces _amortized_ block allocation -> ~O(log n) without RAM, while
retaining the safe, bug-resistant, single-source-of-truth properties
that come with lookahead-based allocation.
---
So, long story short, dropping other strategies, and now the presence of
the bmap is a boolean flag.
This is also the first format-specific flag:
- Define LFS3_BMAP to enable the bmap logic, but note by default the
bmap will still not be used.
- Define LFS3_YES_BMAP to force the bmap to be used.
- With LFS3_BMAP, passing LFS3_F_GBMAP to lfs3_format will include the
on-disk block-map.
- No flag is needed during mount, the presence of the bmap is determined
by the on-disk wcompat flags (LFS3_WCOMPAT_GBMAP). This also prevents
rw mounting if the bmap is not supported, but rdonly mounting is
allowed.
- Users can check if the bmap is in use via lfs3_fs_stat, which reports
LFS3_I_GBMAP in the flags field.
There's still some missing pieces, but these will be a bit more
involved:
- lfs3_fs_grow needs to be made bmap aware!
- We probably want something like lfs3_fs_mkgbmap and lfs3_fs_rmgbmap to
allow converting between bmap backed/not-backed filesystem images.
Code changes minimal:
code stack ctx
before: 37172 2352 684
after: 37172 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38844 2456 800
bmap after: 38852 (+0.0%) 2456 (+0.0%) 800 (+0.0%)
New bmap range tags:
LFS3_TAG_BMRANGE 0x033u v--- --11 --11 uuuu
LFS3_TAG_BMFREE 0x0330 v--- --11 --11 ----
LFS3_TAG_BMINUSE 0x0331 v--- --11 --11 ---1
LFS3_TAG_BMERASED 0x0332 v--- --11 --11 --1-
LFS3_TAG_BMBAD 0x0333 v--- --11 --11 --11
Note 0x334-0x33f are still reserved for future bmap tags, but the new
encoding fits in the surprisingly common 2-bit subfield that may
deduplicate some decoding code.
Fitting in 2-bits is the main reason for this, now that in-flight ranges
look like they won't be worth exploring further. Worst case we can
always add more bm tags in the future. And it may even make sense to use
an entire bit for in-flight tags, since in theory the concept can apply
to more than just in-use blocks.
---
Another benefit of this encoding: In-use vs free is a bit check, and I
like the implication that an in-use + erased block can only be a bad
block.
No code changes:
code stack ctx
before: 37172 2352 684
after: 37172 (+0.0%) 2352 (+0.0%) 684 (+0.0%)
code stack ctx
bmap before: 38844 2456 800
bmap after: 38844 (+0.0%) 2456 (+0.0%) 800 (+0.0%)
Too much room for confusion, and potential flag conflicts in the future.
Note it already conflicted with -e/--error-* flags.
--exec is a rather technical flag anyways, and will probably be wrapped
in other ci/script scaffolding most of the time.
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!
- 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.
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%)
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.
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.
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.
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.
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.
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
The --no-internal flag avoids building any internal tests/benches
(tests/benches with in="lfs3.c"), which can be useful for quickly
testing high-level things while refactoring. Refactors tend to break all
the internal tests, and it can be a real pain to update everything.
Note that --no-internal can be injected into the build with TESTCFLAGS:
TESTCFLAGS=--no-internal make test-runner -j \
&& ./scripts/test.py -j -b
For a curious data point, here's the current number of
internal/non-internal tests:
suites cases perms
total: 24 808 633968/776298
internal: 22 (91.7%) 532 (65.8%) 220316/310247 (34.8%)
non-internal: 2 ( 8.3%) 276 (34.2%) 413652/466051 (65.2%)
It's interesting to note that while internal tests have more test cases,
the non-internal tests generate a larger number of test permutations.
This is probably because internal tests tend to target specific corner
cases/known failure points, and don't invite much variants.
---
While --no-internal may be useful for high-level testing during a
refactor, I'm not sure it's a good idea to rely on it for _debugging_ a
refactor.
The whole point of internal testing is to catch low-level bugs early,
with as little unnecessary state as possible. Skipping these to debug
integration tests is a bit counterproductive!
Note this includes both the lfs3_config -> lfs3_cfg structs as well as
the LFS3_CONFIG -> LFS3_CFG include define:
- LFS3_CONFIG -> LFS3_CFG
- struct lfs3_config -> struct lfs3_cfg
- struct lfs3_file_config -> struct lfs3_file_cfg
- struct lfs3_*bd_config -> struct lfs3_*bd_cfg
- cfg -> cfg
We were already using cfg as the variable name everywhere. The fact that
these names were different was an inconsistency that should be fixed
since we're committing to an API break.
LFS3_CFG is already out-of-date from upstream, and there's plans for a
config rework, but I figured I'd go ahead and change it as well to lower
the chances it gets overlooked.
---
Note this does _not_ affect LFS3_TAG_CONFIG. Having the on-disk vs
driver-level config take slightly different names is not a bad thing.
Currently this just has one flag the replaces the previous `erase`
argument:
LFS3_ALLOC_ERASE 0x00000001 Please erase the block
Benefits include:
- Slightly better readability at lfs3_alloc call sites.
- Possibility of more allocator flags in the future:
- LFS3_ALLOC_EMERGENCY - Use reserved blocks
- Uh, that's all I can think of right now
No code changes.
LFS3_CKDATACKSUMREADS is just too much.
The downside is it may not be clear how LFS3_CKDATACKSUMREADS interacts
with the future planned LFS3_CKREADS (LFS3_CKREADS implies
LFS3_CKDATACKSUMS + LFS3_CKMETAREDUND), but on the flip side you may
actually be able to type LFS3_CKDATACKSUMS on the first try.
This goes ahead and makes all dbg scripts available in dbg.gdb.py, via
the magic of globbing __file__ relative, and dynamic python class
generation.
Probably one of the more evil scripts I've written, but this means we
don't need to worry about dbg.gdb.py falling out-of-date when adding new
dbg scripts.
Not all of the dbg scripts are useful inside gdb, but most of them are.
After all, what's cooler than this!
(gdb) dbgrbyd -b4096 "disk" -t \
file->b.shrub.blocks[0] \
--trunk lfs3_rbyd_trunk(&file->b.shrub)
rbyd 0x46.23a w2048, rev 00000000, size 629, cksum 8f5169e1
00000004: .-> 0-334 data w335 0
00000009: .-+-> 335 data w1 1 71
0000000e: | .-> 336 data w1 1 67
00000013: .-+-+-> 337 data w1 1 66
...
00000144: | | | | .-> 350 data w1 1 74
0000019a: | | | | .-+-> 351 data w1 1 78
000001f5: | | | | | .-> 352-739 data w388 1 76
00000258: +-+-+-+-+-+-> 740-2047 data w1308 1 6c
Note some tricks to help interact with bash and gdb:
- Flags are passed as is (-b4096, -t, --trunk)
- All non-flags are parsed as expressions (file->b.shrub.blocks[0])
- String expressions may be useful for paths and stuff ("./disk")
So instead of:
$ ./scripts/dbgflags.py o 0x10000003
The filter is now specified as a normal(ish) argparse flag:
$ ./scripts/dbgflags.py --o 0x10000003
This is a bit easier to interop with in dbg.gdb.py, and I think a bit
more readable.
Though -a and --a now do _very_ different things. I'm sure that won't
confuse anyone...
Like test.py --gdb-script, being able to specify multiple header files
seems useful and is easy enough to add.
---
Note that the default is only used if no other header files are
specified, so this _replaces_ the default header file:
$ ./scripts/test.py --include=my_header.h
If you don't want to replace the default header file, you currently need
to specify it explicitly:
$ ./scripts/test.py \
--include=runners/test_runner.h \
--include=my_header.h
Christopher Haster