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gbmap: Added gc_repoplookahead_thresh and gc_repopgbmap_thresh

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To allow relaxing when LFS3_I_REPOPLOOKAHEAD and LFS3_I_REPOPGBMAP will
be set, potentially reducing gc workload after allocating only a couple
blocks.

The relevant cfg comments have quite a bit more info.

Note -1 (not the default, 0, maybe we should explicitly flip this?)
restores the previous functionality of setting these flags on the first
block allocation.

---

Also tweaked gbmap repops during gc/traversals to _not_ try to repop
unless LFS3_I_REPOPGBMAP is set. We probably should have done this from
the beginning since repopulating the gbmap writes to disk and is
potentially destructive.

Adds code, though hopefully we can claw this back with future config
rework:

                 code          stack          ctx
  before:       37176           2352          684
  after:        37208 (+0.1%)   2352 (+0.0%)  688 (+0.6%)

                 code          stack          ctx
  gbmap before: 40024           2368          848
  gbmap after:  40120 (+0.2%)   2368 (+0.0%)  856 (+0.9%)
This commit is contained in:
Christopher Haster
2025-10-17 00:32:03 -05:00
parent 1dc1a26f11
commit 12874bff76
5 changed files with 394 additions and 88 deletions
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lfs3.h
+41 -10
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@@ -497,13 +497,43 @@ struct lfs3_cfg {
lfs3_soff_t gc_steps;
#endif
// Threshold for metadata compaction during gc in bytes. Metadata logs
// that exceed this threshold will be compacted during gc operations.
// Defaults to ~88% block_size when zero, though this default may change
// in the future.
// Threshold for repopulating the lookahead buffer during gc. This
// can be set lower than the lookahead size to delay gc work when
// only a few blocks have been allocated.
//
// Note this only affects explicit gc operations. Otherwise metadata is
// only compacted when full.
// Note this only affects explicit gc operations. During normal
// operations the lookahead buffer is only repopulated when empty.
//
// 0 only repopulates the lookahead buffer when empty, while -1 or
// any value >= 8*lookahead_size repopulates the lookahead buffer
// after any block allocation.
#ifndef LFS3_RDONLY
lfs3_block_t gc_repoplookahead_thresh;
#endif
// Threshold for repopulating the gbmap during gc. This can be set
// lower than the disk size to delay gc work when only a few blocks
// have been allocated.
//
// Note this only affects explicit gc operations. During normal
// operations gbmap repopulations are controlled by
// gbmap_repop_thresh.
//
// Any value <= gbmap_repop_thresh repopulates the gbmap when below
// gbmap_repop_thresh, while -1 or any value >= block_count
// repopulates the lookahead buffer after any block allocation.
#if !defined(LFS3_RDONLY) && defined(LFS3_GBMAP)
lfs3_block_t gc_repopgbmap_thresh;
#endif
// Threshold for metadata compaction during gc in bytes.
//
// Metadata logs that exceed this threshold will be compacted during
// gc operations. Defaults to ~88% block_size when zero, though this
// default may change in the future.
//
// Note this only affects explicit gc operations. During normal
// operations metadata is only compacted when full.
//
// Set to -1 to disable metadata compaction during gc.
#ifndef LFS3_RDONLY
@@ -578,15 +608,16 @@ struct lfs3_cfg {
lfs3_size_t crystal_thresh;
#endif
// Threshold for when to repopulate the global on-disk block-map
// (gbmap). When <= this many blocks have a known state, littlefs
// will traverse the filesystem and attempt to repopulate the gbmap.
// Threshold for repopulating the global on-disk block-map (gbmap).
//
// When <= this many blocks have a known state, littlefs will
// traverse the filesystem and attempt to repopulate the gbmap.
// Smaller values decrease repop frequency and improves overall
// allocator throughput, at the risk of needing to fallback to the
// slower lookahead allocator when empty.
//
// 0 only repopulates the gbmap when empty, minimizing gbmap
// repops but may introduce large latency spikes.
// repops at the risk of large latency spikes.
#ifdef LFS3_GBMAP
lfs3_block_t gbmap_repop_thresh;
#endif