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bmap: The initial bmapcache algorithm seems to be working

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At least at a proof-of-concept level, there's still a lot of cleanup
needed.

To make things work, lfs3_alloc_ckpoint now takes an mdir, which
provides the target for gbmap gstate updates.

When the bmap is close to empty (configurable via bmap_scan_thresh), we
opportunistically rebuild it during lfs3_alloc_ckpoints. The nice thing
about lfs3_alloc_ckpoint is we know the state of all in-flight blocks,
so rebuilding the bmap just requires traversing the filesystem + in-RAM
state.

We might still fall back to the lookahead buffer, but in theory a well
tuned bmap_scan_thresh can prevent this from becoming a bottleneck (at
the cost of more frequent bmap rebuilds).

---

This is also probably a good time to resume measuring code/ram costs,
though it's worth repeating the above note about the bmap work still
needing cleanup:

             code          stack          ctx
  before:   36840           2368          684
  after:    36920 (+0.2%)   2368 (+0.0%)  684 (+0.0%)

Haha, no, the bmap isn't basically free, it's just an opt-in features.
With -DLFS3_YES_BMAP=1:

             code          stack          ctx
  no bmap:  36920           2368          684
  yes bmap: 38552 (+4.4%)   2472 (+4.4%)  812 (+18.7%)
This commit is contained in:
Christopher Haster
2025-07-28 23:27:02 -05:00
parent 8666830515
commit 316ca1cc05
17 changed files with 329 additions and 135 deletions
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tests/test_bmap.toml
+6 -6
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@@ -9,7 +9,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;
@@ -70,7 +70,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;
@@ -135,7 +135,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;
@@ -201,7 +201,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;
@@ -268,7 +268,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;
@@ -318,7 +318,7 @@ code = '''
lfs3_t lfs3;
lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
lfs3_alloc_ckpoint(&lfs3);
lfs3_alloc_ckpoint(&lfs3, NULL);
// create an initial bmap
lfs3_btree_t bmap;