index-pack and unpack-objects both read pack data from stdin through
a 4 KiB static buffer. In index-pack, each fill() flushes consumed
bytes to the pack file via write_or_die(), capping every write(2)
at 4 KiB. unpack-objects uses the same buffer pattern for reads.
On FUSE-backed filesystems every write(2) is a synchronous round
trip through the FUSE protocol (userspace -> kernel -> userspace ->
back), so the 4 KiB buffer turns a clone into many unnecessary tiny
writes with noticeable latency overhead.
Increase the buffer from 4 KiB to 128 KiB. Introduce a shared
DEFAULT_IO_BUFFER_SIZE constant in git-compat-util.h (next to
MAX_IO_SIZE) and use it in index-pack, unpack-objects, and the
hashfile layer in csum-file (which already used 128 KiB but
hardcoded the value).
Pack file writes to a FUSE filesystem with writeback caching
disabled during HTTPS clones of git/git (~293 MB pack):
74,958 -> 4,687 (94% fewer)
Wall-clock time of git clone over HTTPS onto a FUSE passthrough
filesystem with writeback caching disabled, 3 runs per variant:
vscode (~1.26 GB pack): 84.5s -> 75.7s avg (10% faster)
git/git (~306 MB pack): 22.6s -> 20.0s avg (11% faster)
Signed-off-by: Scott Bauersfeld <sbauersfeld@g.ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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