index-pack, unpack-objects: increase input buffer from 4 KiB to 128 KiB

Both index-pack and unpack-objects read pack data from stdin through
a 4 KiB static buffer (input_buffer[4096]). On each fill(), consumed
bytes are flushed to the output pack file via write_or_die(), so
every write(2) moves at most 4 KiB.

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, matching the default
already used by the hashfile layer in csum-file.c.

Testing with strace on HTTPS clones of git/git (~296 MB pack, 5 runs
per variant, isolated builds from the same v2.54.0 source) shows:

  index-pack pack file writes: 72,465 -> 24,943 avg (66% reduction)
  total write() syscalls:     310,192 -> 259,530 avg (17% reduction)
  writes of exactly 4096 bytes: ~40,077 -> 0 (eliminated)

All clones produce identical HEAD, file count, and pass fsck.

Signed-off-by: Scott Bauersfeld <sbauersfeld@g.ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sbauersfeld

builtin/index-pack.c

@@ -145,8 +145,8 @@ static int check_self_contained_and_connected;
 
 static struct progress *progress;
 
-/* We always read in 4kB chunks. */
-static unsigned char input_buffer[4096];
+#define INPUT_BUFFER_SIZE (128 * 1024)
+static unsigned char input_buffer[INPUT_BUFFER_SIZE];
 static unsigned int input_offset, input_len;
 static off_t consumed_bytes;
 static off_t max_input_size;

builtin/unpack-objects.c

@@ -23,8 +23,8 @@
 static int dry_run, quiet, recover, has_errors, strict;
 static const char unpack_usage[] = "git unpack-objects [-n] [-q] [-r] [--strict]";
 
-/* We always read in 4kB chunks. */
-static unsigned char buffer[4096];
+#define INPUT_BUFFER_SIZE (128 * 1024)
+static unsigned char buffer[INPUT_BUFFER_SIZE];
 static unsigned int offset, len;
 static off_t consumed_bytes;
 static off_t max_input_size;