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btree: Resurrected btree leaf caching

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This is an indulgence to simplify the upcoming auxiliary btree work.

Brings back the previously-reverted per-btree leaf caches, where each
lfs3_btree_t keeps track of two rbyds: The root and the most recently
accessed leaf.

At the surface level, this optimizes repeated access to the same btree
leaf. A common pattern for a number of littlefs's operations that has
proven tricky to manually optimize:

- Btree iteration
- Pokes for our crystalization heuristic
- Checksum collision resolution for dids and (FUTURE) ddkeys
- Related rattrs attached to a single bid

But the real motivation is to drop lfs3_btree_*lookupleaf and simplify
the internal APIs. If repeated lfs3_btree_lookup*s are already
efficient, there's no reason for extra leaf-level APIs, and in theory
any logic that interacts with btrees will be simpler.

---

This comes at a cost (humorously about the same amount as the
tag-returning refactor, if you ignore the extra 28 bytes of ctx).
Unsurprisingly, increasing the size of lfs3_btree_t has the biggest
impact on stack and ctx:

           code          stack          ctx
  before: 36084           2336          656
  after:  36784 (+1.9%)   2400 (+2.7%)  684 (+4.3%)

Also note from the previous commit messages: Btree leaf caching has
resulted in surprisingly little performance improvement for our current
benchmarks + implementation. It turns out if you're dominated by write
cost, optimizing btree lookups -- which already skip rbyd fetches, has
barely noticeable impact.

---

A note on reverting!

Eventually (after the auxiliary btree work) it will probably make sense
to revert this -- or at least provide a non-leaf-caching build for
code/RAM sensitive users.

I don't think this should be reverted as-is. Instead, I think we should
allow the option to just disable the leaf cache, while keeping the
simpler internal API. This would give us the best of all three worlds:

- A small code/RAM option
- Optimal btree iteration/nearby-lookup performance
- Simpler internal APIs

The only reason this isn't already implemented is because I want to
avoid fragmenting the codebase further while we're still in development
mode.
This commit is contained in:
Christopher Haster
2025-07-20 12:33:19 -05:00
parent ba9a45aa01
commit cd9f93d859
7 changed files with 687 additions and 697 deletions
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tests/test_trvs.toml
+45 -45
View File
@@ -3446,7 +3446,7 @@ code = '''
// create enough files for mroot to split
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "uloborus%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -3566,7 +3566,7 @@ code = '''
// create enough files for mroot to split
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "uloborus%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -3694,7 +3694,7 @@ code = '''
// create enough files for mroot to split
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "uloborus%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4222,7 +4222,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4232,8 +4232,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4258,8 +4258,8 @@ code = '''
// create enough files for mdir to split again
i = 0;
orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "vulsor%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4398,7 +4398,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4408,8 +4408,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4458,8 +4458,8 @@ code = '''
// create enough files for mdir to split again
i = 0;
orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "vulsor%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4581,7 +4581,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4591,8 +4591,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4646,8 +4646,8 @@ code = '''
// create enough files for mdir to split again
i = 0;
orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "vulsor%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4758,7 +4758,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4768,8 +4768,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4932,7 +4932,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -4942,8 +4942,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -5124,7 +5124,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -5134,8 +5134,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -5304,7 +5304,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "tarantula%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -5314,8 +5314,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "xnotata%03x", i);
lfs3_file_open(&lfs3, &file, name,
@@ -5841,7 +5841,7 @@ code = '''
lfs3_size_t i = 0;
while (true) {
// we should not have split yet
assert(lfs3.mtree.weight == 0);
assert(lfs3.mtree.r.weight == 0);
// we need internals to check this
lfs3_ssize_t estimate = lfs3_mdir_estimate__(&lfs3,
&file1.b.h.mdir, -1, -1,
@@ -6011,7 +6011,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "hydroid%03x", i);
lfs3_file_t file;
@@ -6022,8 +6022,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "medusa%03x", i);
lfs3_file_t file;
@@ -6246,7 +6246,7 @@ code = '''
// create enough files for mroot to split twice
lfs3_size_t i = 0;
while (lfs3.mtree.weight == 0) {
while (lfs3.mtree.r.weight == 0) {
char name[256];
sprintf(name, "hydroid%03x", i);
lfs3_file_t file;
@@ -6257,8 +6257,8 @@ code = '''
}
i = 0;
lfs3_size_t orig = lfs3.mtree.weight;
while (lfs3.mtree.weight == orig) {
lfs3_size_t orig = lfs3.mtree.r.weight;
while (lfs3.mtree.r.weight == orig) {
char name[256];
sprintf(name, "polyp%03x", i);
lfs3_file_t file;
@@ -6270,10 +6270,10 @@ code = '''
// create enough files to both compact and split
i = 0;
orig = lfs3.mtree.weight;
orig = lfs3.mtree.r.weight;
while (true) {
// we should not have split yet
assert(lfs3.mtree.weight == orig);
assert(lfs3.mtree.r.weight == orig);
// we need internals to check this
lfs3_ssize_t estimate = lfs3_mdir_estimate__(&lfs3,
&file2.b.h.mdir, -1, -1,
@@ -6535,7 +6535,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);
@@ -6844,7 +6844,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);
@@ -7008,7 +7008,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);
@@ -7181,7 +7181,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);
@@ -7354,7 +7354,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);
@@ -7523,7 +7523,7 @@ code = '''
assert(!(lfs3.flags & LFS3_I_MKCONSISTENT));
// which means there shouldn't be that many files left
assert(lfs3.mtree.weight <= (2 << lfs3.mbits));
assert(lfs3.mtree.r.weight <= (2 << lfs3.mbits));
assert(file1.b.h.mdir.r.weight <= 3);
assert(file2.b.h.mdir.r.weight <= 3);