perf: gc might try to populate the lookahead buffer each time

[Ryan-CW-Code]

Sorry my English is very poor, so I use machine translation.
When the user incorrectly sets lookahead_size, if lookahead_size > (block_count / 8), gc might try to populate the lookahead buffer each time.
I'm not sure if this PR is the optimal solution. Perhaps it should help users set the correct value during the mount stage.
Because similar processing is also done in the lfs_format_ and lfs_alloc_scan functions, in fact, this processing can be completely optimized out.

[geky-bot]

Tests passed ✓, Code: 17112 B (+0.0%), Stack: 1448 B (+0.0%), Structs: 812 B (+0.0%)

	Code	Stack	Structs		Coverage
Default	17112 B (+0.0%)	1448 B (+0.0%)	812 B (+0.0%)	Lines	2434/2595 lines (+0.0%)
Readonly	6230 B (+0.0%)	448 B (+0.0%)	812 B (+0.0%)	Branches	1283/1616 branches (+0.0%)
Threadsafe	17964 B (+0.0%)	1448 B (+0.0%)	820 B (+0.0%)		Benchmarks
Multiversion	17184 B (+0.0%)	1448 B (+0.0%)	816 B (+0.0%)	Readed	29369693876 B (+0.0%)
Migrate	18776 B (+0.0%)	1752 B (+0.0%)	816 B (+0.0%)	Proged	1482874766 B (+0.0%)
Error-asserts	17924 B (+0.0%)	1440 B (+0.0%)	812 B (+0.0%)	Erased	1568888832 B (+0.0%)

[geky]

Interesting, it looks like you're right, this is needed to avoid redundant lookahead scans when 8*lookahead_size > block_count. Can bring this in (next patch?) and thanks for the PR!

I'm not sure if this PR is the optimal solution. Perhaps it should help users set the correct value during the mount stage.
Because similar processing is also done in the lfs_format_ and lfs_alloc_scan functions, in fact, this processing can be completely optimized out.

This gets tricky when block_count is not a multiple of 8. Unlikely, for large filesystems, but useful for carving out blocks for partition tables, reserved storage, XiP, etc.

But you're right this doesn't feel very optimal.

Eventually (the "Config API rework" in #1111), I would like to drop the idea that struct lfs_config needs to be statically allocated. If instead we store everything in lfs_t in RAM, we can apply lfs_min during the mount stage and simplify this logic. By default this would add RAM, but in theory in the "Config API rework" that RAM could be saved by defining LFS_CFG_LOOKAHEAD_SIZE at compile time.

[geky-bot]

Tests passed ✓, Code: 17112 B (+0.0%), Stack: 1448 B (+0.0%), Structs: 812 B (+0.0%)

	Code	Stack	Structs		Coverage
Default	17112 B (+0.0%)	1448 B (+0.0%)	812 B (+0.0%)	Lines	2435/2596 lines (+0.0%)
Readonly	6230 B (+0.0%)	448 B (+0.0%)	812 B (+0.0%)	Branches	1283/1616 branches (+0.0%)
Threadsafe	17964 B (+0.0%)	1448 B (+0.0%)	820 B (+0.0%)		Benchmarks
Multiversion	17184 B (+0.0%)	1448 B (+0.0%)	816 B (+0.0%)	Readed	29369693876 B (+0.0%)
Migrate	18776 B (+0.0%)	1752 B (+0.0%)	816 B (+0.0%)	Proged	1482874766 B (+0.0%)
Error-asserts	17924 B (+0.0%)	1440 B (+0.0%)	812 B (+0.0%)	Erased	1568888832 B (+0.0%)

[geky]

Looks great, thanks for this. Will bring this in on the next patch release.