On Mon, Oct 19, 2020 at 09:35:15PM +0000, Nipunn Koorapati via GitGitGadget wrote: > From: Nipunn Koorapati <nipunn@xxxxxxxxxxx> > > Results for the git-diff fsmonitor optimization > in patch in the parent-rev (using a 400k file repo to test) > > As you can see here - git diff with fsmonitor running is > significantly better with this patch series (80% faster on my > workload)! These t/perf numbers are very helpful, at least to me. > GIT_PERF_LARGE_REPO=~/src/server ./run v2.29.0-rc1 . -- p7519-fsmonitor.sh > > Test v2.29.0-rc1 this tree > ----------------------------------------------------------------------------------------------------------------- > 7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman) 1.46(0.82+0.64) 1.47(0.83+0.62) +0.7% > 7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman) 0.16(0.12+0.04) 0.17(0.12+0.05) +6.3% > 7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman) 1.36(0.73+0.62) 1.37(0.76+0.60) +0.7% Looks like about 0.01sec of overhead, which seems like an acceptable trade-off for when the user has at least 10,000 files. This reminds me; did you look at the 'git add' performance change? I recall Junio mentioning that 'git add' takes the same paths in the code. > 7519.5: diff (fsmonitor=.git/hooks/fsmonitor-watchman) 0.85(0.22+0.63) 0.14(0.10+0.05) -83.5% > 7519.6: diff -- 0_files (fsmonitor=.git/hooks/fsmonitor-watchman) 0.12(0.08+0.05) 0.13(0.11+0.02) +8.3% > 7519.7: diff -- 10_files (fsmonitor=.git/hooks/fsmonitor-watchman) 0.12(0.08+0.04) 0.13(0.09+0.04) +8.3% > 7519.8: diff -- 100_files (fsmonitor=.git/hooks/fsmonitor-watchman) 0.12(0.07+0.05) 0.13(0.07+0.06) +8.3% > 7519.9: diff -- 1000_files (fsmonitor=.git/hooks/fsmonitor-watchman) 0.12(0.09+0.04) 0.13(0.08+0.05) +8.3% > 7519.10: diff -- 10000_files (fsmonitor=.git/hooks/fsmonitor-watchman) 0.14(0.09+0.05) 0.13(0.10+0.03) -7.1% OK... so having fsmonitor turned on adds an imperceptible amount of slow-down to cases where there are [0, 10000) files. But, in exchange, you get much-improved whole-tree performance, as well as single-tree performance when that tree contains at least 10,000 files. I was going to say that this has little downside, because turning on fsmonitor is probably a good indicator that you don't have any fewer than 10,000 files in your repository, but I think that's missing the point. Likely true, but that doesn't exclude the possibility of having sub-10,000 file directories, which users may very well still be diff-ing. So, there's a slow-down, but it's hard to complain when you consider what we get in exchange. > 7519.12: status (fsmonitor=) 1.67(0.93+1.49) 1.67(0.99+1.42) +0.0% > 7519.13: status -uno (fsmonitor=) 0.37(0.30+0.82) 0.37(0.33+0.79) +0.0% > 7519.14: status -uall (fsmonitor=) 1.58(0.97+1.35) 1.57(0.86+1.45) -0.6% > 7519.15: diff (fsmonitor=) 0.34(0.28+0.83) 0.34(0.27+0.83) +0.0% > 7519.16: diff -- 0_files (fsmonitor=) 0.09(0.06+0.04) 0.09(0.08+0.02) +0.0% > 7519.17: diff -- 10_files (fsmonitor=) 0.09(0.07+0.03) 0.09(0.06+0.05) +0.0% > 7519.18: diff -- 100_files (fsmonitor=) 0.09(0.06+0.04) 0.09(0.06+0.04) +0.0% > 7519.19: diff -- 1000_files (fsmonitor=) 0.09(0.06+0.04) 0.09(0.05+0.05) +0.0% > 7519.20: diff -- 10000_files (fsmonitor=) 0.10(0.08+0.04) 0.10(0.06+0.05) +0.0% Great! No slow-down without fsmonitor enabled, as expected. Fantastic. > I also added a benchmark for a tiny git diff workload w/ a pathspec. > I see an approximately .02 second overhead added w/ and w/o fsmonitor > > From looking at these results, I suspected that refresh_fsmonitor > is already happening during git diff - independent of this patch > series' optimization. Confirmed that suspicion by breaking on > refresh_fsmonitor. So, the overhead that we're paying is purely the pipe+fork+exec? I.e., that watchman has already computed an answer in the earlier call, and we just have to read it again (or find out that the last results were unchanged)? > (gdb) bt [simplified] > 0 refresh_fsmonitor at fsmonitor.c:176 > 1 ie_match_stat at read-cache.c:375 > 2 match_stat_with_submodule at diff-lib.c:237 > 4 builtin_diff_files at builtin/diff.c:260 > 5 cmd_diff at builtin/diff.c:541 > 6 run_builtin at git.c:450 > 7 handle_builtin at git.c:700 > 8 run_argv at git.c:767 > 9 cmd_main at git.c:898 > 10 main at common-main.c:52 :-). > Signed-off-by: Nipunn Koorapati <nipunn@xxxxxxxxxxx> > --- > t/perf/p7519-fsmonitor.sh | 71 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 71 insertions(+) > > diff --git a/t/perf/p7519-fsmonitor.sh b/t/perf/p7519-fsmonitor.sh > index 9313d4a51d..2b4803707f 100755 > --- a/t/perf/p7519-fsmonitor.sh > +++ b/t/perf/p7519-fsmonitor.sh > @@ -115,6 +115,13 @@ test_expect_success "setup for fsmonitor" ' Everything in here looks very reasonable to me, except for the seq vs. test_seq() issue that I pointed out in another email in this thread. It's too bad that we have to write these twice, but that's not the fault of your patch. Thanks, Taylor
