perf: add a performance test for core.fsmonitor

Add a performance test for the new core.fsmonitor facility using the
sample query-fsmonitor hook.

This is WIP code for the reasons explained in the setup comments,
unfortunately the perf code doesn't easily allow you to run different
setup code for different versions you're testing. This test will stop
working if the fsmonitor is merged into the master branch.

Output against linxu.git:

    $ GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8' ./run origin/master avar/fsmonitor ./p7519-fsmonitor.sh
    [...]
    Test                          origin/master     avar/fsmonitor
    -----------------------------------------------------------------------
    7519.2: status (first)        0.08(0.04+0.09)   0.12(0.07+0.10) +50.0%
    7519.3: status (subsequent)   0.08(0.04+0.09)   0.12(0.06+0.11) +50.0%
    7519.4: status -uno           0.02(0.02+0.05)   0.06(0.05+0.06) +200.0%
    7519.5: status -uall          0.08(0.06+0.07)   0.12(0.07+0.10) +50.0%

And against a larger in-house monorepo I have here, with the same
options (except the repo path):

    Test                          origin/master     avar/fsmonitor
    -----------------------------------------------------------------------
    7519.2: status (first)        0.20(0.11+0.18)   0.27(0.15+0.21) +35.0%
    7519.3: status (subsequent)   0.20(0.11+0.18)   0.27(0.15+0.21) +35.0%
    7519.4: status -uno           0.04(0.03+0.10)   0.22(0.08+0.12) +450.0%
    7519.5: status -uall          0.20(0.13+0.16)   0.27(0.18+0.19) +35.0%

Against linux.git with a hack to flush the FS cache (on Linux) before
running the first 'git status', only running one test so the result
isn't discarded as the slowest of N:

    $ GIT_PERF_REPEAT_COUNT=1 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_COMMAND='sudo sync && echo 3 | sudo tee /proc/sys/vm/drop_caches >/dev/null && make -j8' ./run origin/master avar/fsmonitor ./p7519-fsmonitor.sh
    [...]
    Test                          origin/master     avar/fsmonitor
    ------------------------------------------------------------------------
    7519.2: status (first)        0.30(0.18+0.10)   8.26(0.22+0.10) +2653.3%
    7519.3: status (subsequent)   0.08(0.04+0.08)   0.81(0.09+0.07) +912.5%
    7519.4: status -uno           0.02(0.01+0.06)   0.08(0.04+0.07) +300.0%
    7519.5: status -uall          0.08(0.06+0.07)   0.15(0.08+0.09) +87.5%

Now obviously due to 1 run that has a lot of noise, but I would expect
that first invocation to be blindingly fast since watchman has info on
what files were modified since the cache was flushed.

The same on the large monorepo noted above:

    Test                          origin/master     avar/fsmonitor
    -----------------------------------------------------------------------
    7519.2: status (first)        0.59(0.28+0.24)   0.93(0.35+0.19) +57.6%
    7519.3: status (subsequent)   0.20(0.10+0.19)   0.28(0.16+0.20) +40.0%
    7519.4: status -uno           0.04(0.04+0.09)   0.11(0.08+0.12) +175.0%
    7519.5: status -uall          0.29(0.11+0.18)   0.40(0.16+0.19) +37.9%

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/perf/p7519-fsmonitor.sh

@@ -0,0 +1,58 @@
+#!/bin/sh
+
+test_description="Test core.fsmonitor"
+
+. ./perf-lib.sh
+
+test_perf_large_repo
+test_checkout_worktree
+
+test_expect_success 'setup' '
+	# Maybe set untrackedCache & splitIndex depending on the
+	# environment, defaulting to false.
+	if test -n "$GIT_PERF_7519_UNTRACKED_CACHE"
+	then
+		git config core.untrackedCache true
+	else
+		git config core.untrackedCache false
+	fi &&
+	if test -n "$GIT_PERF_7519_SPLIT_INDEX"
+	then
+		git config core.splitIndex true
+	else
+		git config core.splitIndex false
+	fi &&
+
+	# Relies on core.fsmonitor not being merged into master. Needs
+	# better per-test ways to disable it if it gets merged.
+	git config core.fsmonitor true &&
+
+	# Hook scaffolding
+	mkdir .git/hooks &&
+	cp ../../../templates/hooks--query-fsmonitor.sample .git/hooks/query-fsmonitor &&
+
+	# Setup watchman & ensure it is actually watching
+	watchman watch-del "$PWD" >/dev/null 2>&1 &&
+	watchman watch "$PWD" >/dev/null 2>&1 &&
+	watchman watch-list | grep -q -F "$PWD"
+'
+
+# Setting:
+#
+#    GIT_PERF_REPEAT_COUNT=1 GIT_PERF_MAKE_COMMAND='sudo sync && echo 3 | sudo tee /proc/sys/vm/drop_caches && make -j8'
+#
+# Can be used as a hack to performance test 'git status' on a cold fs
+# cache with an existing watchman watching the directory, which should
+# be blindingly fast, compared to amazingly slow without watchman.
+test_perf 'status (first)'       'git status'
+
+
+# The same git-status once the fs cache has been warmed, if using the
+# GIT_PERF_MAKE_COMMAND above. Otherwise the same as above.
+test_perf 'status (subsequent)'  'git status'
+
+# Let's see if -uno & -uall make any difference
+test_perf 'status -uno'          'git status -uno'
+test_perf 'status -uall'         'git status -uall'
+
+test_done