The one-entry cache lived inside schfs_file_read, so stat() never touched
it -- which is why `ls` barely moved. Moved into schfs_inode_get, where
every caller benefits, and widened to sixteen entries.
`ls` is the shape that matters: it stats every name in a directory, so a
single entry never hits and each file costs two descents, one to find the
name and one to read the inode. Sixteen keeps hitting through a directory
walk without a second reader evicting everything. Round-robin rather than
LRU: the pattern is a walk, not a working set, so the bookkeeping on every
hit would buy nothing.
Measured in the guest, TCG, against the original baseline:
ls /bin warm 0.70s -> 0.43s
ls /bin cold 2.80s -> 1.32s
read 834 KB 1.73s -> 0.35s
schfs-check passes including the round-trip; bootcheck 108 [ok].
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Every read descended the B-tree twice per block -- once for the inode and
once for the extent -- so an 834 KB file cost 418 descents, each a tree
walk plus a 4 KB copy out of the buffer cache per node visited. A
sequential read asks for the same inode every time and usually stays
inside one extent, so both answers were already known and thrown away.
One entry each, not a table: the pattern being optimised is a single file
read straight through, and one entry catches all of it without needing an
eviction policy. A second reader simply misses.
Correctness rests entirely on invalidation, so it is blunt: all five
write entry points in objw.c call schfs_cache_invalidate() before
touching anything. A cache that is merely usually right would hand back
another file's data. The volume pointer is part of the key because the
host tools can hold two images open at once.
Measured in the guest, bracketed inside one command line, TCG:
read 834 KB 1.73s -> 0.31s (5.6x, together with the BOUNCE fix)
ls /bin cold 2.80s -> 1.40s (2x)
ls /bin warm 0.70s -> 0.70s (unchanged)
Warm ls is untouched on purpose-by-consequence: it is 241 separate
inodes, so a one-entry cache never hits. That cost is the per-entry
lookup path -- dirlookup plus iload, each its own descent -- and it is
where the next order of magnitude is, not here.
schfs-check passes, including the round-trip; bootcheck 108 [ok].
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.
No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.
