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Relaxed lookahead buffer alignment

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This drops the lookahead buffer from operating on 32-bit words to
operating on 8-bit bytes, and removes any alignment requirement. This
may have some minor performance impact, but it is unlikely to be
significant when you consider IO overhead.

The original motivation for 32-bit alignment was an attempt at
future-proofing in case we wanted some more complex on-disk data
structure. This never happened, and even if it did, it could have been
added via additional config options.

This has been a significant pain point for users, since providing
word-aligned byte-sized buffers in C can be a bit annoying.
This commit is contained in:
Christopher Haster
2023-12-20 00:32:17 -06:00
parent 1f9c3c04b1
commit b1b10c0e75
2 changed files with 11 additions and 13 deletions
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lfs.c
+7 -8
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@@ -600,7 +600,7 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
+ lfs->block_count) % lfs->block_count;
if (off < lfs->lookahead.size) {
lfs->lookahead.buffer[off / 32] |= 1U << (off % 32);
lfs->lookahead.buffer[off / 8] |= 1U << (off % 8);
}
return 0;
@@ -653,8 +653,8 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
while (true) {
// scan our lookahead buffer for free blocks
while (lfs->lookahead.next < lfs->lookahead.size) {
if (!(lfs->lookahead.buffer[lfs->lookahead.next / 32]
& (1U << (lfs->lookahead.next % 32)))) {
if (!(lfs->lookahead.buffer[lfs->lookahead.next / 8]
& (1U << (lfs->lookahead.next % 8)))) {
// found a free block
*block = (lfs->lookahead.start + lfs->lookahead.next)
% lfs->block_count;
@@ -666,8 +666,8 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
lfs->lookahead.ckpoint -= 1;
if (lfs->lookahead.next >= lfs->lookahead.size
|| !(lfs->lookahead.buffer[lfs->lookahead.next / 32]
& (1U << (lfs->lookahead.next % 32)))) {
|| !(lfs->lookahead.buffer[lfs->lookahead.next / 8]
& (1U << (lfs->lookahead.next % 8)))) {
return 0;
}
}
@@ -4199,10 +4199,9 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
lfs_cache_zero(lfs, &lfs->rcache);
lfs_cache_zero(lfs, &lfs->pcache);
// setup lookahead, must be multiple of 64-bits, 32-bit aligned
// setup lookahead buffer, note mount finishes initializing this after
// we establish a decent pseudo-random seed
LFS_ASSERT(lfs->cfg->lookahead_size > 0);
LFS_ASSERT(lfs->cfg->lookahead_size % 8 == 0 &&
(uintptr_t)lfs->cfg->lookahead_buffer % 4 == 0);
if (lfs->cfg->lookahead_buffer) {
lfs->lookahead.buffer = lfs->cfg->lookahead_buffer;
} else {