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Modified robekras's optimization to avoid flush for all seeks in cache

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The basic idea is simple, if we seek to a position in the currently
loaded cache, don't flush the cache. Notably this ensures that seek is
always as fast or faster than just reading the data.

This is a bit tricky since we need to check that our new block and
offset match the cache, fortunately we can skip the block check by
reevaluating the block index for both the current and new positions.

Note this only works whene reading, for writing we need to always flush
the cache, or else we will lose the pending write data.
This commit is contained in:
Christopher Haster
2022-04-10 01:28:08 -05:00
parent a6f01b7d6e
commit 425dc810a5
1 changed files with 17 additions and 11 deletions
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28
lfs.c
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@@ -3091,17 +3091,23 @@ static lfs_soff_t lfs_file_rawseek(lfs_t *lfs, lfs_file_t *file,
return npos;
}
// Get the difference between new and current file position
// If new position is after the current file position
// If new position belongs to the currently cached data
// Set new file position,
// and update also the block related position
int offset = npos - file->pos;
if (offset > 0) {
if ((file->off + offset) < lfs->cfg->block_size) {
file->pos = npos;
file->off += offset;
// if we're only reading and our new offset is still in the file's cache
// we can avoid flushing and needing to reread the data
if (
#ifndef LFS_READONLY
!(file->flags & LFS_F_WRITING)
#else
true
#endif
) {
int oindex = lfs_ctz_index(lfs, &(lfs_off_t){file->pos});
lfs_off_t noff = npos;
int nindex = lfs_ctz_index(lfs, &noff);
if (oindex == nindex
&& noff >= file->cache.off
&& noff < file->cache.off + file->cache.size) {
file->pos = npos;
file->off = noff;
return npos;
}
}