[PATCH v3 0/7] use fsmonitor data in git diff eliminating O(num_files) calls to lstat

Credit to alexmv who made this commit back in Dec, 2017 when he was at dbx. I've rebased it and am submitting it now.

With fsmonitor enabled, git diff currently lstats every file in the repo This makes use of the fsmonitor extension to skip lstat() calls on files that fsmonitor judged as unmodified.

I was able to do some testing with/without this change in a large in-house repo (~ 400k files).

----------------------------------------- (1) With fsmonitor enabled - on master of git (2.29.0) ----------------------------------------- ../git/bin-wrappers/git checkout HEAD~200 strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.64    4.358994          10    446257         3 lstat
  0.12    0.005353           7       764       360 open

(A subsequent call) strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.84    4.380955          10    444904         3 lstat
  0.06    0.002564         135        19           munmap
...

----------------------------------------- (2) With fsmonitor enabled - with my patch ----------------------------------------- ../git/bin-wrappers/git checkout HEAD~200 strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 50.72    0.003090         163        19           munmap
 19.63    0.001196         598         2           futex
...
  0.00    0.000000           0         4         3 lstat

----------------------------------------- (3) With fsmonitor disabled entirely -----------------------------------------

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 98.52    0.277085       92362         3           futex
  0.27    0.000752           4       191        63 open
...
  0.14    0.000397           3       158         3 lstat

I was able to encode this into a perf test in one of the commits.

Changes since Patch Series V1

 * Add git diff -- <pathspec> to perf tests
 * improve readability of bitwise ops

Alex Vandiver (1):
  fsmonitor: use fsmonitor data in `git diff`

Nipunn Koorapati (6):
  t/perf/README: elaborate on output format
  t/perf/p7519-fsmonitor.sh: warm cache on first git status
  t/perf: add fsmonitor perf test for git diff
  perf lint: check test-lint-shell-syntax in perf tests
  p7519-fsmonitor: refactor to avoid code duplication
  p7519-fsmonitor: add a git add benchmark

 diff-lib.c                | 15 +++++-
 t/Makefile                |  3 +-
 t/perf/README             |  2 +
 t/perf/p3400-rebase.sh    |  6 +--
 t/perf/p7519-fsmonitor.sh | 96 ++++++++++++++++++++++-----------------
 5 files changed, 75 insertions(+), 47 deletions(-)

base-commit: d4a392452e292ff924e79ec8458611c0f679d6d4
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-756%2Fnipunn1313%2Fdiff_fsmon-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-756/nipunn1313/diff_fsmon-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/756

Range-diff vs v2:

 1:  cba03dd40b = 1:  cba03dd40b fsmonitor: use fsmonitor data in `git diff`
 2:  1c7876166f = 2:  1c7876166f t/perf/README: elaborate on output format
 3:  401f696c81 = 3:  401f696c81 t/perf/p7519-fsmonitor.sh: warm cache on first git status
 4:  f572e226bb ! 4:  b3ad8faac4 t/perf: add fsmonitor perf test for git diff
     @@ t/perf/p7519-fsmonitor.sh: test_expect_success "setup for fsmonitor" '
       	git config core.fsmonitor "$INTEGRATION_SCRIPT" &&
       	git update-index --fsmonitor &&
      +	mkdir 1_file 10_files 100_files 1000_files 10000_files &&
     -+	for i in `seq 1 10`; do touch 10_files/$i; done &&
     -+	for i in `seq 1 100`; do touch 100_files/$i; done &&
     -+	for i in `seq 1 1000`; do touch 1000_files/$i; done &&
     -+	for i in `seq 1 10000`; do touch 10000_files/$i; done &&
     ++	for i in $(test_seq 1 10); do touch 10_files/$i; done &&
     ++	for i in $(test_seq 1 100); do touch 100_files/$i; done &&
     ++	for i in $(test_seq 1 1000); do touch 1000_files/$i; done &&
     ++	for i in $(test_seq 1 10000); do touch 10000_files/$i; done &&
      +	git add 1_file 10_files 100_files 1000_files 10000_files &&
      +	git commit -m "Add files" &&
       	git status  # Warm caches
 -:  ---------- > 5:  28c1e488bf perf lint: check test-lint-shell-syntax in perf tests
 -:  ---------- > 6:  b38f2984f9 p7519-fsmonitor: refactor to avoid code duplication
 -:  ---------- > 7:  d392a523f2 p7519-fsmonitor: add a git add benchmark