This changes the cache to cache less things, yet retain the required
efficiency for our walk usecase. This uses less memory.
Specifically, instead of keeping result and child caches for each path
level, only keep a single cached child. In practice our operations are
depth-first, or almost depth-first, and then we retain the same hit
ratio for a smaller cache size.
I improved the benchmark so that it counts the Lstat and DirNames
operations performed, and they do not change significantly. The amount
of allocated memory is reduced by 20% and the walk itself is actually
slightly faster.
This also removes the clear based on number of cached names (as that is
not a thing any more) and the timer based clear (which was unused). This
means we'll retain the last cache state forever until it's cleared by a
write operation, but we did that before too and that state is now a lot
smaller...
The overhead compared to not using a casefs, for our typical "double
walk" (walk the tree then stat everything again) is 2x the dirnames we
would otherwise call, and no overhead on the stats (unchanged from old
implementation)
```
name old time/op new time/op delta
WalkCaseFakeFS100k/rawfs-8 306ms ± 1% 305ms ± 2% ~ (p=0.182 n=9+10)
WalkCaseFakeFS100k/casefs-8 579ms ± 5% 557ms ± 1% -3.77% (p=0.000 n=10+10)
name old B/entry new B/entry delta
WalkCaseFakeFS100k/rawfs-8 590 ± 0% 590 ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 1.09k ± 0% 0.87k ± 0% -19.98% (p=0.000 n=10+10)
name old DirNames/entry new DirNames/entry delta
WalkCaseFakeFS100k/rawfs-8 0.51 ± 0% 0.51 ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 1.02 ± 0% 1.02 ± 0% ~ (all equal)
name old DirNames/op new DirNames/op delta
WalkCaseFakeFS100k/rawfs-8 51.2k ± 0% 51.2k ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 102k ± 0% 102k ± 0% ~ (all equal)
name old Lstat/entry new Lstat/entry delta
WalkCaseFakeFS100k/rawfs-8 3.02 ± 0% 3.02 ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 3.02 ± 0% 3.02 ± 0% ~ (all equal)
name old Lstat/op new Lstat/op delta
WalkCaseFakeFS100k/rawfs-8 302k ± 0% 302k ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 302k ± 0% 302k ± 0% ~ (all equal)
name old allocs/entry new allocs/entry delta
WalkCaseFakeFS100k/rawfs-8 15.7 ± 0% 15.7 ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 27.5 ± 0% 26.1 ± 0% -5.09% (p=0.000 n=10+10)
name old ns/entry new ns/entry delta
WalkCaseFakeFS100k/rawfs-8 2.02k ± 1% 2.02k ± 2% ~ (p=0.163 n=9+10)
WalkCaseFakeFS100k/casefs-8 3.83k ± 5% 3.68k ± 1% -3.77% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
WalkCaseFakeFS100k/rawfs-8 89.2MB ± 0% 89.2MB ± 0% ~ (p=0.364 n=9+10)
WalkCaseFakeFS100k/casefs-8 164MB ± 0% 131MB ± 0% -19.97% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
WalkCaseFakeFS100k/rawfs-8 2.38M ± 0% 2.38M ± 0% ~ (all equal)
WalkCaseFakeFS100k/casefs-8 4.16M ± 0% 3.95M ± 0% -5.05% (p=0.000 n=10+10)
```
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