May rerevert this in the future, but I'm on the fence.
It's true this only saves a small amount of code, but in theory it also
reduces stack consumption in name-related functions. Currently this
doesn't affect the stack hot-path, which is a bit surprising as this
includes lfs3_set, but it may in the future.
The arguments against this optimization are also a bit weak:
- Non-null-terminated strings - We probably shouldn't optimize for a
theoretical future feature. If anything, we want to optimize in the
opposite direction to best measure the theoretical code cost.
- Precomputing strlen early - While this is generally a good idea, our
rattrs benefit greatly from compact encodings, as rattrs sitting on
the stack are one of the bigger contributors to our stack hot-path.
So for now I'm unreverting to see how long this optimization makes
sense, but could see this being rereverted in the future.
At the very least we probably want to keep the test changes to make
future testing easier.
---
Saves a bit of code:
code stack ctx
before: 35188 2136 660
after: 35160 (-0.1%) 2136 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38048 2152 772
gbmap after: 38020 (-0.1%) 2152 (+0.0%) 772 (+0.0%)
As much as I don't want to admit it, our 3-word lfs3_data_t struct is
just too large to be treated as pass-by-value with today's compilers.
It's a real shame, because I don't think there's a great technical
reason, just that compiler's pass-by-value optimizations generally stop
after 2 words.
If we could expect 16-bit block sizes (off and size), we could fit in
2 words, but this is already challenged by today's NAND chips
(bs>=128KiB).
---
So, as a compromise, this stops treating lfs3_data_t as pass-by-value,
with the exception of the lfs3_data_from* functions that still return
lfs3_data_t directly.
So instead of:
lfs3_data_t data = lfs3_data_fromecksum(&ecksum, buffer);
data = lfs3_data_slice(data, 8, -1);
return lfs3_data_size(data);
Most operations take lfs3_data_t by pointer:
lfs3_data_t data = lfs3_data_fromecksum(&ecksum, buffer);
lfs3_data_slice(&data, 8, -1);
return lfs3_data_size(&data);
One of the main consequences is there are now several ways to slice data
(internally these all redirect to lfs3_data_slice), and LFS3_DATA_SLICE
will likely see more use since we need temporary allocations to pass the
data slice by address:
- lfs3_data_slice(data, a, b) - Slices the data in place
- lfs3_data_fromslice(data, a, b) - Returns a new data slice
- LFS3_DATA_SLICE(data, a, b) - Creates a new compound-literal slice
---
As a pragmatic compromise, this saves a nice chunk of both code and
stack:
code stack ctx
before: 35316 2176 660
after: 35188 (-0.4%) 2136 (-1.8%) 660 (+0.0%)
code stack ctx
gbmap before: 38172 2192 772
gbmap after: 38048 (-0.3%) 2152 (-1.8%) 772 (+0.0%)
This was originally dropped because it's not strictly necessary,
little-leb128s (28-bits) can always be encoded with the default leb128
encoder (31-bits). But it is useful if only for the assert.
Note this matches lfs3_data_readlleb128, which was never dropped, and is
useful for decreasing decoder DSIZEs.
Maybe it makes sense to drop both of these in the future, especially if
we start running into from-field pressure. But for now, this assert is
useful for ensuring disk compatibility with little 4-byte leb128s.
---
Surprisingly no code cost, at least by default (code alignment?). Though
it did add 4 bytes to the gbmap build (so yes, probably code alignment):
code stack ctx
before: 35316 2176 660
after: 35316 (+0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38168 2192 772
gbmap after: 38172 (+0.0%) 2192 (+0.0%) 772 (+0.0%)
See previous commit for why.
The merged commits surprisingly cost more than separate commit
functions. I guess because the compiler is smart enough to deduplicate
the two logic paths here:
code stack ctx
before: 35324 2176 660
after: 35316 (-0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38172 2192 772
gbmap after: 38168 (-0.0%) 2192 (+0.0%) 772 (+0.0%)
The good news is this is a win for readability, I think the separate
conditions are easier to understand than a merged commit muddied with a
bunch of lfs3->mtree.r.weight == 0 checks.
The idea here was to merge mtree split commits to try to minimize
redundant logic that only differs in whether or not we need to create
the initial mtree weight.
Surprisingly, this backfired, adding more code than it saved. I guess I
underestimated how effective the compiler is at deduplicating these two
paths of logic:
code stack ctx
before: 35316 2176 660
after: 35324 (+0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38168 2192 772
gbmap after: 38172 (+0.0%) 2192 (+0.0%) 772 (+0.0%)
I don't think there was anything inherently wrong with this idea, but:
- The code savings (28 bytes) was surprisingly small.
- Expecting lfs3_path_namelen may be a headache for future
non-null-terminated string support.
- Even if you don't care about non-null-terminated strings, precomputing
strlen as early as possible is a good idea to minimize repeated strlen
scans.
Reverting adds a bit of code:
code stack ctx
before: 35288 2176 660
after: 35316 (+0.1%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38140 2192 772
gbmap after: 38168 (+0.1%) 2192 (+0.0%) 772 (+0.0%)
I was poking around at possibly inlining small (<=255) name lens in
lfs3_rattr_t, but realized all LFS3_FROM_NAME rattrs in our system
already use the lfs3_path_namelen pattern (terminates in either
'\0' or '/').
Well, except for our tests, but who cares about those.
Adopting lfs3_path_namelen in LFS3_FROM_NAME saves a bit of code:
code stack ctx
before: 35316 2176 660
after: 35288 (-0.1%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38168 2192 772
gbmap after: 38140 (-0.1%) 2192 (+0.0%) 772 (+0.0%)
Here's one interesting use-case for the single-recurse LFS3_tag_RATTRS:
Avoiding a copy of the name creation rattrs in lfs3_file_sync_.
There is a concern with nesting LFS3_tag_RATTRS in that it risks
conflicts across layers, but currently this is ok as long as
high-level LFS3_tag_RATTRS stick to non-negative mids (the mtree split
commit in lfs3_mdir_commit_ only needs to recurse for mroot rattrs).
Saves a bit of code:
code stack ctx
before: 35320 2176 660
after: 35316 (-0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38172 2192 772
gbmap after: 38168 (-0.0%) 2192 (+0.0%) 772 (+0.0%)
This originally started as an attempt to drop LFS3_tag_TAIL entirely,
but that didn't really go anywhere. Any attempt to work around the
double rattr-lists during mtree splits results in more mess than this
magic rattr.
But I did notice we only need to support "simple" rattrs during mtree
splits, which means we can just call lfs3_rbyd_appendrattrs to handle
these.
Maybe this will be problematic if we ever want to deduplicate
lfs3_mdir_commit___ and lfs3_rbyd_appendrattrs, but I don't see that
happening because of the different concerns (mdir-specific rattrs):
function code stack ctx
lfs3_mdir_commit___ 1052 744 396
lfs3_rbyd_appendrattrs 142 584 388
The benefit of a single-recurse LFS3_tag_RATTRS:
- Simplifies lfs3_mdir_commit__, no more awkward loop recursion.
- May have other use cases for nesting simple rattrs?
Adds a bit of code:
code stack ctx
before: 35316 2176 660
after: 35320 (+0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38148 2192 772
gbmap after: 38172 (+0.1%) 2192 (+0.0%) 772 (+0.0%)
LFS3_tag_RATTRS, now LFS3_tag_TAIL, is a bit funny in that it only
supports tail-recursive rattrs. The whole point of littlefs is
bounded-RAM after all. And if we know LFS3_tag_TAIL will terminate an
rattr-list, why bother with an additional LFS3_tag_NULL?
Like LFS3_RATTR_NULL, LFS3_RATTR_TAIL sets length=0 to indicate the end
of the rattr-lists.
Saves a bit of code:
code stack ctx
before: 35324 2176 660
after: 35316 (-0.0%) 2176 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38156 2192 772
gbmap after: 38148 (-0.0%) 2192 (+0.0%) 772 (+0.0%)
It's funny to see what originally started as a simple list of rbyd attrs
slowly morph into a full isa. But it makes sense. What we really want is
an abstract description of operations that can be played and replayed as
necessary to atomically update the mtree.
Using a fixed lfs3_rattr_t struct to represent this in C is easy, and
avoids strict-aliasing issues, but ultimately limited when it comes to
the wide-range of data we want to attach to attributes.
Unlike a computer's isa, we want to be able to include full 12-24 byte
branch pointers directly in the instruction!
---
So here's a full variable-length isa organized by words (max(uintptr_t,
uint32_t)).
The first 32-bit word extends the 16-bit tag with an extra 16-bits of
control information:
wwll llff ffcc cccc tttt tttt tttt tttt
^'-.-''-.-''--.--' : :
'--|----|-----|----:-----------------:-- compressed weight
:: '----|-----|----:-----------------:-- total len
:: '-----|----:-----------------:-- from encoder
:: '----:-----------------:-- optional count
:: rgmm kkkk -kkk kkkk
11 => w=-1 ^^ ^ '-.' '---.---'
00 => w=0 '|-|---|------|------ rm bit
01 => w=+1 '-|---|------|------ grow bit
10 => w=attached '---|------|------ mask bits
'------|------ tag suptype
'------ tag subtype
The 4-bit length field always encodes the full length of the
instruction, including the instruction itself and optional weight. The
4-bit from + 6-bit count fields operate independently and tell
lfs3_rbyd_appendrattr_ how to actually encode the data related to the
instruction.
To work around strict-aliasing issues, complex structs are expected to
be broken down into words and reconstructed in lfs3_rbyd_appendrattr_.
Most of our structs are organized into words anyways. For example:
// new child
*r++ = LFS3_RATTR(5, LFS3_TAG_BRANCH, -2, LFS3_FROM_BRANCH);
*r++ = LFS3_RATTR_WEIGHT(+child_->weight);
*r++ = LFS3_RATTR_ARG(child_->blocks[0]);
*r++ = LFS3_RATTR_ARG(child_->trunk);
*r++ = LFS3_RATTR_ARG(child_->cksum);
This also changes rattr-lists to be null-terminated, which makes a bit
more sense in a variable-length isa:
*r++ = LFS3_RATTR_NULL; // all zeros, including length
One concern with null-terminated rattr-lists is how easy it is to
forget the null-terminator, but an assert that all non-null rattrs have
non-zero length seemed to catch the many many mistakes during adoption.
Alternatively, separate LFS3_FROM_NULL/LFS3_FROM_NIL from fields could
be used if encoding space gets tight.
I'm also quite happy with the 2-bit weight feild, which allows omitting
the optional weight word for -1,0,+1 weights. These should cover at
least all mdir operations.
Note the exact encoding of the rattr fields is less of a concern than
the tag fields, as it doesn't reside on-disk can be changed on whim.
---
Saves a nice chunk of code and stack:
code stack ctx
before: 35920 2280 660
after: 35324 (-1.7%) 2176 (-4.6%) 660 (+0.0%)
code stack ctx
gbmap before: 38812 2296 772
gbmap after: 38156 (-1.7%) 2192 (-4.5%) 772 (+0.0%)
The stack savings are obvious, but the code savings a bit less so. A
variable length isa _is_ more complicated, but by limiting most encoding
decisions to compile-time (2-bit weights vs 32-bit weights for example),
the savings from fewer word manipulations on the stack wins.
Now that LFS3_TAG_INTERNAL uses the 0x00tt prefix, this bit mask no
longer works.
Fortunately LFS3_TAG_INTERNAL is really only used in LFS3_ASSERTs, so a
less efficient test has no effect on code cost.
No code changes.
The idea is this is similar to ~ for the home directory, hopefully
simplifying littlefs path parsing in scripts:
- /hi -> hi in root dir
- ./hi -> hi in current dir
- ../hi -> hi in parent dir
- ~/hi -> hi in home dir
- %/hi -> hi in littlefs root dir
Note this only works when standalone:
- ~hi != ~/hi
- %hi != %/hi
And files named % can still be referenced with a ./ prefix:
- ./% -> % in current dir
- %/% -> % in littlefs root dir
---
This is probably overkill for dbglfs3.py, as the arg ordering was
already enough to disambiguate disk path vs mroot address vs littlefs
path, but eventually I think the idea will be useful for more powerful
scripts.
A hypothetical:
$ mklfs3 cp disk -b4096 -r image_files %/image_files
This is still a hack to make the seek _enum_ appear somewhat readable in
our dbg _flags_ script. But the previously internal SEEK_MODE was
causing all seek flags to be hidden from -l/--list confusingly.
Just a bit less typing than --o, and lowers risk of conflicts with
actual flags we may care about.
To be honest I was procrastinating because I thought this would be a lot
more work! I was prepared to write a hacky secondary parser, but argparse
already supports this natively with prefix_chars='-+'. Yay!
Test/bench filters have proven to be mostly non-optional, protecting
against bad configuration that doesn't make any sense.
It's still valid to want to override test filters sometimes, but using a
more, uh, forceful verb probably makes sense here.
The shortform would conflict with -f/--fail, so no shortform flag for
this, but some argue --force should never have a shortform flag anyways.
I'm trying to avoid the inevitable conflict with -w/--word, which will
probably become important when exploring non-32-bit filesystem
configurations.
Renaming this to -t/--wait still conflicts with -t/--tree and -t/--tiny,
but as a debug-only flag, I think these are less important.
Oh, and -t/--trace, but test.py/bench.py are already quite different in
their flag naming (see -d/--disk vs -d/--diff).
---
Renamed a few other flags while tweaking things:
- -t/--tiny -> --tiny (dropped shortform)
- -w/--word-bits -> -w/--word/--word-bits
- -t/--tree -> -R/--tree/--rbyd/--tree-rbyd
- -R/--tree-rbyd -> -Y/--rbyd-all/--tree-rbyd-all
- -B/--tree-btree -> -B/--btree/--tree-btree
After tinkering with it a bit, I think the -R/-Y/-B set of flags are a
decent way to organize the tree renderers. At least --tree-rbyd-all does
a better job of describing the difference between --tree-rbyd and
--tree-rbyd-all.
This changes -w/--wait to sleep _after_ -k/--keep-open, instead of
including the time spent waiting on inotifywait in the sleep time.
1. It's easier, no need to keep track of when we started waiting.
2. It's simpler to reason about.
3. It trivially avoids the multiple wakeup noise that plagued
watch.py + vim (vim likes to do a bunch of renaming and stuff when
saving files, including the file 4913 randomly?)
Avoiding this was previously impossible because -~/--sleep was
effectively a noop when combined with -k/--keep-open.
---
Also renamed from -~/--sleep -> -w/--wait, which is a bit more intuitive
and avoids possible shell issues with -~.
To make this work, dropped the -w/--block-cycles shortform flag in
dbgtrace.py. It's not like this flag is ever used anyways.
Though at the moment this is ignoring the possible conflict with
-w/--word-bits...
This was motivated by a discussion with a gh user, in which it was noted
that not having a reserved suptype for internal tags risks potential
issues with long-term future tag compatibility.
I think the risk is low, but, without a reserved suptype, it _is_
possible for a future tag to conflict with an internal tag in an older
driver version, potentially and unintentionally breaking compatibility.
Note this is especially concerning during mdir compactions, where we
copy tags we may not understand otherwise.
In littlefs2 we reserved suptype=0x100, though this was mostly an
accident due to saturating the 3-bit suptype space. With the larger tag
space in littlefs3, the reserved suptype=0x100 was dropped.
---
Long story short, this reserves suptype=0 for internal flags (well, and
null, which is _mostly_ internal only, but does get written to disk as
unreachable tags).
Unfortunately, adding a new suptype _did_ require moving a bunch of
stuff around:
LFS3_TAG_NULL 0x0000 v--- ---- +--- ----
LFS3_TAG_INTERNAL 0x00tt v--- ---- +ttt tttt
LFS3_TAG_CONFIG 0x01tt v--- ---1 +ttt tttt
LFS3_TAG_MAGIC 0x0131 v--- ---1 +-11 --rr
LFS3_TAG_VERSION 0x0134 v--- ---1 +-11 -1--
LFS3_TAG_RCOMPAT 0x0135 v--- ---1 +-11 -1-1
LFS3_TAG_WCOMPAT 0x0136 v--- ---1 +-11 -11-
LFS3_TAG_OCOMPAT 0x0137 v--- ---1 +-11 -111
LFS3_TAG_GEOMETRY 0x0138 v--- ---1 +-11 1---
LFS3_TAG_NAMELIMIT 0x0139 v--- ---1 +-11 1--1
LFS3_TAG_FILELIMIT 0x013a v--- ---1 +-11 1-1-
LFS3_TAG_ATTRLIMIT? 0x013b v--- ---1 +-11 1-11
LFS3_TAG_GDELTA 0x02tt v--- --1- +ttt tttt
LFS3_TAG_GRMDELTA 0x0230 v--- --1- +-11 ----
LFS3_TAG_GBMAPDELTA 0x0234 v--- --1- +-11 -1rr
LFS3_TAG_GDDTREEDELTA* 0x0238 v--- --1- +-11 1-rr
LFS3_TAG_GPTREEDELTA* 0x023c v--- --1- +-11 11rr
LFS3_TAG_NAME 0x03tt v--- --11 +ttt tttt
LFS3_TAG_BNAME 0x0300 v--- --11 +--- ----
LFS3_TAG_REG 0x0301 v--- --11 +--- ---1
LFS3_TAG_DIR 0x0302 v--- --11 +--- --1-
LFS3_TAG_STICKYNOTE 0x0303 v--- --11 +--- --11
LFS3_TAG_BOOKMARK 0x0304 v--- --11 +--- -1--
LFS3_TAG_SYMLINK? 0x0305 v--- --11 +--- -1-1
LFS3_TAG_SNAPSHOT? 0x0306 v--- --11 +--- -11-
LFS3_TAG_MNAME 0x0330 v--- --11 +-11 ----
LFS3_TAG_DDNAME* 0x0350 v--- --11 +1-1 ----
LFS3_TAG_DDTOMB* 0x0351 v--- --11 +1-1 ---1
LFS3_TAG_STRUCT 0x04tt v--- -1-- +ttt tttt
LFS3_TAG_BRANCH 0x040r v--- -1-- +--- --rr
LFS3_TAG_DATA 0x0404 v--- -1-- +--- -1--
LFS3_TAG_BLOCK 0x0408 v--- -1-- +--- 1err
LFS3_TAG_DDKEY* 0x0410 v--- -1-- +--1 ----
LFS3_TAG_DID 0x0420 v--- -1-- +-1- ----
LFS3_TAG_BSHRUB 0x0428 v--- -1-- +-1- 1---
LFS3_TAG_BTREE 0x042c v--- -1-- +-1- 11rr
LFS3_TAG_MROOT 0x0431 v--- -1-- +-11 --rr
LFS3_TAG_MDIR 0x0435 v--- -1-- +-11 -1rr
LFS3_TAG_MSHRUB+ 0x0438 v--- -1-- +-11 1---
LFS3_TAG_MTREE 0x043c v--- -1-- +-11 11rr
LFS3_TAG_BMRANGE 0x044u v--- -1-- +1-- ++uu
LFS3_TAG_BMFREE 0x0440 v--- -1-- +1-- ----
LFS3_TAG_BMINUSE 0x0441 v--- -1-- +1-- ---1
LFS3_TAG_BMERASED 0x0442 v--- -1-- +1-- --1-
LFS3_TAG_BMBAD 0x0443 v--- -1-- +1-- --11
LFS3_TAG_DDRC* 0x0450 v--- -1-- +1-1 ----
LFS3_TAG_DDPCOEFF* 0x0451 v--- -1-- +1-1 ---1
LFs3_TAG_PCOEFFMAP* 0x0460 v--- -1-- +11- ----
LFS3_TAG_ATTR 0x06aa v--- -11a +aaa aaaa
LFS3_TAG_UATTR 0x06aa v--- -11- +aaa aaaa
LFS3_TAG_SATTR 0x07aa v--- -111 +aaa aaaa
LFS3_TAG_SHRUB 0x1kkk v--1 kkkk +kkk kkkk
LFS3_TAG_ALT 0x4kkk v1cd kkkk +kkk kkkk
LFS3_TAG_CKSUM 0x300p v-11 ---- ++++ +pqq
LFS3_TAG_NOTE 0x3100 v-11 ---1 ++++ ++++
LFS3_TAG_ECKSUM 0x3200 v-11 --1- ++++ ++++
LFS3_TAG_GCKSUMDELTA 0x3300 v-11 --11 ++++ ++++
* Planned
+ Reserved
? Hypothetical
Some additional notes:
- I was on the fence on keeping the 0x30 prefix on config tags now that
it is not longer needed to differentiate from null, but ultimately
decided to keep it because: 1. it's fun, 2. it decreases the chance
of false positives, 3. it keeps the redund bits readable in hexdumps,
and 4. it reserves some tags < config, which is useful since order
matters.
Instead, I pushed the 0x30 prefix to _more_ tags, mainly gstate.
As a coincidence, meta related tags (MNAME, MROOT, MRTREE) all shifted
to also have the 0x30 prefix, which is a nice bit of unexpected
consistency.
- I also considered reserving the redund bits across the config tags
similarly to what we've done in struct/gstate tags, but decided
against it as 1. it significantly reduces the config tag space
available, and 2. makes alignment with VERSION + R/W/OCOMPAT a bit
awkward.
Instead I think would should relax the redund bit alignment in other
suptypes, though in practice the intermixing of non-redund and redund
tags makes this a bit difficult.
Maybe we should consider including redund bits as a hint for things
like DATA? DDKEY? BSHRUB? etc?
- I created a bit more space for file btree struct tags, allowing for
both the future planned DDKEY, and BLOCK with optional erased-bit. We
don't currently use this, but it may be useful for the future planned
gddtree, which in-theory can track erased-state in partially written
file blocks.
Currently tracking erased-state in file blocks is difficult due to
the potential of multiple references, and inability to prevent ecksum
conflicts in raw data blocks.
- UATTR/SATTR bumped up to 0x600/0x700 to keep the 1-bit alignment,
leaving the suptype 0x500 unused. Though this may be useful if we ever
run out of struct tags (suptype=0x400), which is likely where most new
tags will go.
---
Code changes were minimal, but with a bunch of noise:
code stack ctx
before: 35912 2280 660
after: 35920 (+0.0%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38800 2296 772
gbmap after: 38812 (+0.0%) 2296 (+0.0%) 772 (+0.0%)
Though note most high-level calls (lfs3_file_close, lfs3_dir_close,
etc), still include an assertion at a higher-level.
Why make the internal APIs harder to use than they need to be? As a
plus this drops the need for a separate bool-returning
lfs3_handle_close_.
Saves a bit of code:
code stack ctx
before: 35924 2280 660
after: 35912 (-0.0%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38812 2296 772
gbmap after: 38800 (-0.0%) 2296 (+0.0%) 772 (+0.0%)
Unintentionally arriving at the infamous "fsck" name is a bit funny.
But it's probably something we don't want to conflict with if we can
help it, on the off chance we want a sort of lfs3_fsck function in the
future. (This is all hypothetical, but lfs3_fsck may expect an unmounted
filesystem, and have a much larger scope than lfs3_fs_ck. Though typing
this out now I'm realizing how confusing that might be...)
Since lfs3_file_ck and lfs3_fs_ck share a subset of flags, it's not
_entirely_ unreasonable for lfs3_file_ck and lfs3_fs_ck to share the
same namespace.
There's a risk of confusing users around what flags lfs3_file_ck
accepts, but we have asserts, and said flags (LFS3_CK_MKCONSISTENT,
LFS3_CK_LOOKAHEAD, etc) just don't really make sense in lfs3_file_ck:
fs file
y LFS3_CK_MKCONSISTENT 0x00000800 Make the filesystem consistent
y LFS3_CK_LOOKAHEAD 0x00001000 Repopulate lookahead buffer
y LFS3_CK_LOOKGBMAP 0x00002000 Repopulate the gbmap
y LFS3_CK_PREERASE* 0x00004000 Pre-erase unused blocks
y LFS3_CK_COMPACTMETA 0x00008000 Compact metadata logs
y y LFS3_CK_CKMETA 0x00010000 Check metadata checksums
y y LFS3_CK_CKDATA 0x00020000 Check metadata + data checksums
y y LFS3_CK_REPAIRMETA* 0x00040000 Repair data blocks
y y LFS3_CK_REPAIRDATA* 0x00080000 Repair metadata + data blocks
* Planned
Another option would be to document that lfs3_fs_ck accepts both
LFS3_CK_* _and_ LFS3_GC_* flags, but I worry that would be more
confusing. It would also lock us into supporting all LFs3_GC_* flags in
lfs3_fs_ck, which may not always be the case.
Though this is an argument for doing away with the whole
LFS3_M/F/CK/GC/I_* duplication... (tbh another reason for this is to
reduce the number of namespaces by at least one).
No code changes.
TLDR: Replaced lfs3_file_ckmeta/ckdata and lfs3_fs_ckmeta/ckdata with
flag based ck functions:
- lfs3_file_ckmeta -> lfs3_file_ck + LFS3_CK_CKMETA
- lfs3_file_ckdata -> lfs3_file_ck + LFS3_CK_CKDATA
- lfs3_fs_ckmeta -> lfs3_fs_ck + LFS3_FSCK_CKMETA
- lfs3_fs_ckdata -> lfs3_fs_ck + LFS3_FSCK_CKDATA
Note lfs3_fs_ck is equivalent to lfs3_fs_gc, but:
1. Performs the work in one call (equivalent to littlefs2's lfs2_fs_gc)
2. Takes flags at call time (like lfs3_mount) instead of cfg time (like
lfs3_fs_gc)
3. Avoids the constant RAM necessary to track incremental GC state
---
Motivation:
I've been thinking: It's a bit weird that users are able to one-shot
janitorial work in lfs3_mount, but there's no equivalent function after
the filesystem is mounted.
Originally this is what lfs3_fs_gc was for, but after adding support for
incremental GC, it made sense to hide lfs3_fs_gc behind the opt-in
LFS3_GC ifdef due to the extra (ironically non-gc-able) state.
In theory lfs3_trv_t fills a bit of the gap, but, without the internal
i_flag handling and traversal restarts, it's a bit hard to use. And
basically requires duplicating said log, which we need anyways for
lfs3_mount!
So ideally we'd add an explicit one-shot GC function, but now lfs3_fs_gc
is taken.
While thinking about alternative names, I realized we can just call this
lfs3_fs_ck and completely replace lfs3_fs_ckmeta/ckdata.
This has some extra benefits:
- Avoids an explosion of ckmeta/ckdata/repairmeta/repairdata functions
- Discourages redundant traversals that could accomplish more work
- Makes it less confusing that ckdata implies ckmeta
---
I also tweaked lfs3_file_ck to match, but note that lfs3_file_ck is
internally very different from lfs3_fs_ck. For one, lfs3_file_ck only
supports "actual" check flags (LFS3_CK_*) vs all gc flags (LFS3_FSCK_*):
lfs3_file_ck:
LFS3_CK_CKMETA 0x00010000 Check metadata checksums
LFS3_CK_CKDATA 0x00020000 Check metadata + data checksums
LFS3_CK_REPAIRMETA* 0x00040000 Repair metadata blocks
LFS3_CK_REPAIRDATA* 0x00080000 Repair metadata + data blocks
* Planned
lfs3_fs_ck:
LFS3_FSCK_MKCONSISTENT 0x00000800 Make the filesystem consistent
LFS3_FSCK_LOOKAHEAD 0x00001000 Repopulate lookahead buffer
LFS3_FSCK_LOOKGBMAP 0x00002000 Repopulate the gbmap
LFS3_FSCK_PREERASE* 0x00004000 Pre-erase unused blocks
LFS3_FSCK_COMPACTMETA 0x00008000 Compact metadata logs
LFS3_FSCK_CKMETA 0x00010000 Check metadata checksums
LFS3_FSCK_CKDATA 0x00020000 Check metadata + data checksums
LFS3_FSCK_REPAIRMETA* 0x00040000 Repair metadata blocks
LFS3_FSCK_REPAIRDATA* 0x00080000 Repair metadata + data blocks
* Planned
As a plus, this also saves a bit of code:
code stack ctx
before: 35968 2280 660
after: 35924 (-0.1%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38828 2296 772
gbmap after: 38812 (-0.0%) 2296 (+0.0%) 772 (+0.0%)
This more closely matches behavior of functions like mkdir and remove,
even though mkgbmap/rmgbmap operate on a special object and not files.
Besides, returning an error is more useful as users are always free to
ignore said error.
Adds what appears to be one literal to mkgbmap (curiously not rmgbmap?
snuck into alignment?):
code stack ctx
before: 35968 2280 660
after: 35968 (+0.0%) 2280 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 38824 2296 772
gbmap after: 38828 (+0.0%) 2296 (+0.0%) 772 (+0.0%)
I can't think of a reason this should be uint8_t. Bumping it up to
uint32_t matches the type used for other flags (even though whence is
arguably not flags in a strict sense).
No code changes.
This includes the mask/rm/grow bits:
- LFS3_tag_RM
- LFS3_tag_GROW
- LFS3_tag_MASK0/2/8/12
Our in-device only handle types:
- LFS3_tag_ORPHAN
- LFS3_tag_TRV
- LFS3_tag_UNKNOWN
And in-device only tags with special behavior:
- LFS3_tag_INTERNAL
- LFS3_tag_RATTRS
- LFS3_tag_SHRUBCOMMIT
- LFS3_tag_GRMPUSH
- LFS3_tag_MOVE
- LFS3_tag_ATTRS
Usually I'm not a big fan of case-sensitive naming patterns, but this
has been useful for self-documenting what compat flags are in-device
only. Might as well extend the idea to our tag definitions.
Having on-disk definitions in one place is useful for referencing them
later, even if they aren't relevant for most API users.
.h files in C are already forced to expose a bunch of internal details
anyways, in order to provide struct size/alignment. Might as well
include on-disk information that would have even bigger consequences if
it changed.
Moved:
- Compat flag definitions
- Tag definitions
- DSIZEs and relevant encoding comments - Note some of these were
already required to define lfs3_t
Other than moving things around to make space for planned features, this
also adopts the idea of allowing compat flags to be ored into a single
32-bit integer, at least in the short-term.
Note though that these are still stored in separate wcompat/rcompat
tags, to make compat tests easier, and we may introduce conflicting
flags in the future if we run out of 32-bits. This is just an indulgence
to potentially make tooling/debugging easier until that happens.
Rcompat flags:
RCOMPAT_NONSTANDARD+
0x00000001 ---- ---- ---- ---- ---- ---- ---- ---1
RCOMPAT_WRONLY+ 0x00000004 ---- ---- ---- ---- ---- ---- ---- -1--
RCOMPAT_MMOSS 0x00000010 ---- ---- ---- ---- ---- ---- ---1 ----
RCOMPAT_MSPROUT+ 0x00000020 ---- ---- ---- ---- ---- ---- --1- ----
RCOMPAT_MSHRUB+ 0x00000040 ---- ---- ---- ---- ---- ---- -1-- ----
RCOMPAT_MTREE 0x00000080 ---- ---- ---- ---- ---- ---- 1--- ----
RCOMPAT_BMOSS+ 0x00000100 ---- ---- ---- ---- ---- ---1 ---- ----
RCOMPAT_BSPROUT+ 0x00000200 ---- ---- ---- ---- ---- --1- ---- ----
RCOMPAT_BSHRUB 0x00000400 ---- ---- ---- ---- ---- -1-- ---- ----
RCOMPAT_BTREE 0x00000800 ---- ---- ---- ---- ---- 1--- ---- ----
RCOMPAT_MDIRR1* 0x00001000 ---- ---- ---- ---- ---1 ---- ---- ----
RCOMPAT_MDIRR2* 0x00002000 ---- ---- ---- ---- --1- ---- ---- ----
RCOMPAT_MDIRR3* 0x00003000 ---- ---- ---- ---- --11 ---- ---- ----
RCOMPAT_BTREER1* 0x00004000 ---- ---- ---- ---- -1-- ---- ---- ----
RCOMPAT_BTREER2* 0x00008000 ---- ---- ---- ---- 1--- ---- ---- ----
RCOMPAT_BTREER3* 0x0000c000 ---- ---- ---- ---- 11-- ---- ---- ----
RCOMPAT_GRM 0x00010000 ---- ---- ---- ---1 ---- ---- ---- ----
RCOMPAT_GMV? 0x00020000 ---- ---- ---- --1- ---- ---- ---- ----
RCOMPAT_GDDTREE* 0x00100000 ---- ---- ---1 ---- ---- ---- ---- ----
RCOMPAT_GPTREE* 0x00200000 ---- ---- --1- ---- ---- ---- ---- ----
RCOMPAT_DATAR1* 0x00400000 ---- ---- -1-- ---- ---- ---- ---- ----
RCOMPAT_DATAR2* 0x00800000 ---- ---- 1--- ---- ---- ---- ---- ----
RCOMPAT_DATAR3* 0x00c00000 ---- ---- 11-- ---- ---- ---- ---- ----
rcompat_OVERFLOW+ 0x80000000 1--- ---- ---- ---- ---- ---- ---- ----
* Planned
+ Reserved
? Hypothetical
Wcompat flags:
WCOMPAT_NONSTANDARD+
0x00000001 ---- ---- ---- ---- ---- ---- ---- ---1
WCOMPAT_RDONLY+ 0x00000002 ---- ---- ---- ---- ---- ---- ---- --1-
WCOMPAT_GCKSUM 0x00040000 ---- ---- ---- -1-- ---- ---- ---- ----
WCOMPAT_GBMAP 0x00080000 ---- ---- ---- 1--- ---- ---- ---- ----
WCOMPAT_DIR 0x01000000 ---- ---1 ---- ---- ---- ---- ---- ----
WCOMPAT_SYMLINK? 0x02000000 ---- --1- ---- ---- ---- ---- ---- ----
WCOMPAT_SNAPSHOT? 0x04000000 ---- -1-- ---- ---- ---- ---- ---- ----
wcompat_OVERFLOW+ 0x80000000 1--- ---- ---- ---- ---- ---- ---- ----
+ Reserved
? Hypothetical
Ocompat flags:
OCOMPAT_NONSTANDARD+
0x00000001 ---- ---- ---- ---- ---- ---- ---- ---1
ocompat_OVERFLOW+ 0x80000000 1--- ---- ---- ---- ---- ---- ---- ----
+ Reserved
Other notes:
- M* and B* struct flags were reordered to match META -> DATA order
elsewhere. This no longer matches the tag ordering, but there's an
argument the B* tags apply more generally (all btrees) than the B*
compat flag (only file btrees).
- MDIR/BTREE/DATA redund flags were moved near relevant flags, rather
than sticking them in the higher-order bits as we are planning to do
in the M_*/F_* flags. The compat flags already won't match because of
the mdir/btree split (which is IMO too much detail to include in
M_*/F_* flags, but hard to argue against in the compat flags), and
this keeps the highest bit free for OVERFLOW, which is useful
internally.
- Moving DIR to the current-highest bit makes it easy to add 6 more file
types (7 if you ignore OVERFLOW), before things start getting cramped.
No code changes.
Yeah, after using these for a bit, the RE* names were not great.
Trying LOOK* now, as an alternative that hopefully still implies the
similar behavior without needing an additional prefix for LOOKAHEAD:
- LFS3_*_RELOOKAHEAD -> LFS3_*_LOOKAHEAD
- LFS3_*_REGBMAP -> LFS3_*_LOOKGBMAP
- cfg.regbmap_thresh -> cfg.lookgbmap_thresh
- cfg.gc_relookahead_thresh -> cfg.gc_lookahead_thresh
- cfg.gc_regbmap_thresh -> cfg.gc_lookgbmap_thresh
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%)
Now that we use LFS3_ERR_BUSY for traversals, we no longer have an
excuse for not returning LFS3_ERR_BUSY on root-related errors:
- lfs3_remove(&lfs3, "/") => LFS3_ERR_BUSY
- lfs3_rename(&lfs3, "/", *) => LFS3_ERR_BUSY
- lfs3_rename(&lfs3, *, "/") => LFS3_ERR_BUSY
This better aligns with POSIX. Arguably we should have defined
LFS3_ERR_BUSY for this case anyways, it's not like additional error
codes cost much.
No code changes.
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%)
Now that lfs3_mtree_traverse_ uses a sort of state matrix,
lfs3_rbyd_appendrattr_ is the only function still relying on a big
switch-case statement. Replacing it with a series of if-else statements
leaves the codebase switch-case free (ignoring test/bench runners, etc).
Switch-case statements are extremely error prone in C, with the shared
scope, implicit fallthrough, etc. And, with today's compilers, the
result still ends up the same, so switch-case statements offer no
benefit except maybe a more enjoyable syntax for masochists.
Avoiding switch-case statements in code where we care about correctness
is probably a good idea.
No code changes
Mostly for consistency with mtrv.b and gbmap.b, but also (1) this
hopefully reduces confusion around the fact that these can refer to both
bshrubs and btrees, and (2) saves a bit of typing with the messy struct
namespaces forced by C's strict aliasing.
Now that we no longer stage bshrubs in lfs3_trv_ts, bshrubs are limited
to LFS3_TYPE_REG handles. I'm not sure lfs3_o_isbshrub adds anything of
value in this case, so dropping.
I was considering dropping lfs3_bshrub_t completely, since we can always
expect these to be lfs3_file_ts, but decided against it for now as local
lfs3_bshrub_ts may be useful for bshrub commits during block eviction/
repair. Still need to see what that looks like.
Note the slight incongruity of lfs3_bshrub_t vs LFS3_TYPE_REG matches
the incongruity of lfs3_mgc_t and LFS3_type_TRV.
No code changes
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.
This is in preparation for some traversal simplification ideas, which
rely on all auxiliary/non-file btrees being visitable before file
btrees.
In theory the order of file vs auxiliary btrees doesn't really matter,
other than the number of different routes from mtree/mroot -> gbmap/file
btrees being a bit of a pain.
Note this is not true for the mtree, which must come first for
lfs3_mount to work.
---
Adds a bit of code when building with the gbmap:
code stack ctx
before: 36480 2304 660
after: 36476 (-0.0%) 2304 (+0.0%) 660 (+0.0%)
code stack ctx
gbmap before: 39464 2320 772
gbmap after: 39524 (+0.2%) 2320 (+0.0%) 772 (+0.0%)
code stack ctx
gc before: 36552 2304 804
gc after: 36548 (-0.0%) 2304 (+0.0%) 804 (+0.0%)
- 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...
This is motivated by the observation that the O(n log_b n) btree
iteration really just hasn't been a bottleneck in our benchmarks.
Our write performance is mostly dominated by compaction costs, and while
filesystem _traversals_ are a concern, it's easy to explicitly track
rbyds in lfs3_btrv_t.
Additionally:
- We track mdirs during mtree iteration, which are the true mtree
leaves.
- We already cache file leaves, i.e. bptrs and read-fragments.
On top of this, leaf caching adds complexity, both in terms of
code/stack costs, but also in terms of reliability. It introducing the
need for cache invalidation, which is infamously one of the two hard
problems in computer science!
This is the second(?) time btree leaf traversals have been reverted, so
see previous commit messages for even more arguments against.
---
Eventually, we should probably just delete the btree leaf cache logic to
avoid the maintenance headache (cache invalidation + opt+in/less
testing = ouch). But I want to do a bit more benchmarking comparing the
two modes, so just moving this behind an ifdef for now.
Saves code, and of course RAM:
code stack ctx
before btrv: 37160 2352 688
before: 37088 (-0.2%) 2384 (+1.4%) 688 (+0.0%)
after: 36480 (-1.8%) 2304 (-2.0%) 660 (-4.1%)
But note while this keeps the performance implications of btree leaf
caching, it does not keep the code/stack optimizations that internally
reuse the leaf cache for things (btrv, lookupnext_ rbyd side-channel,
etc).
In _theory_ these could have been kept with enough ifdefs, but it would
have made the codebase quite a bit of a hell to maintain:
code stack ctx
always-bleafcache: 37160 2352 688
no-bleafcache: 36480 (-1.8%) 2304 (-2.0%) 660 (-4.1%)
yes-bleafcache: 37044 (-0.3%) 2384 (+1.4%) 688 (+0.0%)
Gbmap mode has even more savings due to how many gbmap copies we have
flying around:
code stack ctx
gbmap + always-bleafcache: 40132 2368 856
gbmap + no-bleafcache: 39464 (-1.7%) 2320 (-2.0%) 772 (-9.8%)
gbmap + yes-bleafcache: 40052 (-0.2%) 2400 (+1.4%) 856 (+0.0%)
---
In the future, _maybe_ we can revisit this. But I think a better design
would be to cache btree leaves globally, in lfs3_t, similarly to the
theoretical mdir cache. This would allow a user-configurable number of
cached btree nodes, and may make cache invalidation easier.
Note, however, that btree nodes don't need to be fetched (even for
commits now!), so the benefits would be much smaller than for the
theoretical mdir cache.
But hey, it would defend the lack of low-level rbyd tracking during
iteration/rattr queries!
Brings back lfs3_btrv_t, but keeps some of the btree internal changes.
I think the biggest one is dropping the internal branch pointer, now
instead of internally pointing to the root rbyd, we just unconditionally
sync the rbyd state anytime the rbyd matches the root's weight. This is
necessary to avoid out-of-sync state when traversing bshrubs under
mutation.
Also after refactoring I think the current btree traversal logic is
easier to read.
---
This is in preparation for removing the leaf cache, or at least making
it opt-in.
It adds a chunk of stack, but in theory we can reclaim this by allowing
leaf caches to be disabled:
code stack ctx
before: 37160 2352 688
after: 37088 (-0.2%) 2384 (+1.4%) 688 (+0.0%)
I think these are good ideas to bring back when littlefs3 is more
mature, but at the moment the number of different builds is creating too
much friction.
LFS3_KVONLY and LFS3_2BONLY in particular _add_ significant chunks of
code (lfs3_file_readget_, lfs3_file_flushset_, and various extra logic
sprinkled throughout the codebase), and the current state of testing
means I have no idea if any of it still works.
These are also low-risk for introducing any disk related changes.
So, ripping out for now to keep the current experimental development
tractable. May reintroduce in the future (probably after littlefs3 is
stabilized) if there is sufficient user interest. But doing so will
probably also need to come with actual testing in CI.
This just fell out-of-sync a bit during the gbmap work. Note we _do_
support LFS3_RDONLY + LFS3_GBMAP, as fetching the gbmap is necessary for
CKMETA to check all metadata. Fortunately this is relatively cheap:
code stack ctx
rdonly: 10716 896 532
rdonly+gbmap: 10988 (+2.5%) 896 (+0.0%) 680 (+27.8%)
Though this does highlight that a sort of LFS3_NO_TRV mode could remove
quite a bit of code.
This walks back some of the attempt at strict object namespacing in
struct lfs3_cfg:
- cfg.file_cache_size -> cfg.fcache_size
- filecfg.cache_size -> filecfg.fcache_size
- filecfg.cache_buffer -> filecfg.fcache_buffer
- cfg.gbmap_re_thresh -> cfg.regbmap_thresh
Motivation:
- cfg.regbmap_thresh now matches cfg.gc_regbmap_thresh, instead of using
awkwardly different namespacing patterns.
- Giving fcache a more unique name is useful for discussion. Having
pcache, rcache, and then file_cache was a bit awkward.
Hopefully it's also more clear that cfg.fcache_size and
filecfg.fcache_size are related.
- Config in struct lfs3_cfg is named a bit more consistently, well, if
you ignore gc_*_* options.
- Less typing.
Though this gets into pretty subjective naming territory. May revert
this if the new terms are uncomfortable after use.
Christopher Haster