Re: [PATCH v4 0/6] Fast git status via a file system watcher

On Thu, Jun 1, 2017 at 5:50 PM, Ben Peart <peartben@gmail.com> wrote:

I have watchman running finally, so aside from issues applying this I can finally test this. I set up a watch of linux.git:

$ watchman watch $PWD $ watchman watch-list|jq '.roots[]' "/home/avar/g/linux"

And first, for simplicity, I'll be turning core.fsmonitore on later:

$ for c in fsmonitor untrackedCache splitIndex; do git config core.$c false; done

On a hot fs cache running "git status" takes me 80ms, but if I flush caches:

# free && sync && echo 3 > /proc/sys/vm/drop_caches && free

Running "git status" takes me 4s. Now, right after flushing those caches:

$ date +%s 1496349235 $ time (echo '["query", "/home/avar/g/linux", {"since": 1496349235, "fields":["name"]}]' | watchman -j) | jq '.files[]'

real 0m0.664s user 0m0.000s sys 0m0.004s $ touch foo bar $ time (echo '["query", "/home/avar/g/linux", {"since": 1496349235, "fields":["name"]}]' | watchman -j) | jq '.files[]' "bar" "foo"

real 0m0.044s user 0m0.000s sys 0m0.000s

Without the monitor running "git status' takes me ~2.5s, i.e. from cold cache:

$ time GIT_TRACE=1 ~/g/git/git-status 20:34:49.154731 git.c:369 trace: built-in: git 'status' On branch master Your branch is up-to-date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        bar
        foo

nothing added to commit but untracked files present (use "git add" to track)

real 0m2.546s user 0m0.060s sys 0m0.228s

Now, I would expect the case where the working tree isn't in the fs cache to be lightning fast, since the watchman command is really fast (flushed the cache again, turned on the fsmonitor):

$ date +%s 1496349523 $ time (echo '["query", "/home/avar/g/linux", {"since": 1496349523, "fields":["name"]}]' | watchman -j) | jq '.files[]'

real 0m0.529s user 0m0.000s sys 0m0.004s $ touch foo bar $ time (echo '["query", "/home/avar/g/linux", {"since": 1496349523, "fields":["name"]}]' | watchman -j) | jq '.files[]' "bar" "foo"

real 0m0.312s user 0m0.000s sys 0m0.000s

But if I run "git status" (and I instrumented the hook to dump the changed files) it takes a long time (those 10s are just the disk being slow, but it should be ~0s, right?):

$ time GIT_TRACE=1 ~/g/git/git-status 20:39:11.250128 git.c:369 trace: built-in: git 'status' 20:39:14.586720 run-command.c:626 trace: run_command: '.git/hooks/query-fsmonitor' '1' '1496349494197821000' Watchman says these changed: bar foo hi there .git .git/index.lock .git/index On branch master Your branch is up-to-date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        bar
        foo

nothing added to commit but untracked files present (use "git add" to track)

real 0m10.791s user 0m0.108s sys 0m0.228s

If I re-run that it takes ~160-200ms with the hook, 80-120ms without it, so in this case once the data is in the fscache watchman is actually slower.

But manually running watchman shows that it's really fast, usually returning within 3ms, I flush the fscache in the middle of this, that's the 600ms time, that could be some global kernel lock or something due to the odd manual proc operation, I haven't actually tested this on a system under memory pressure:

$ for i in {1..10}; do (time (printf '["query", "/home/avar/g/linux", {"since": %s, "fields":["name"]}]' $(($(date +%s)-3)) | watchman -j | jq '.files[]')) 2>&1 | grep -e '"' -e ^real && touch $i && sleep 1; done real 0m0.004s "1" real 0m0.003s "2" "1" real 0m0.605s "3" real 0m0.003s "4" "3" real 0m0.003s "5" "4" real 0m0.003s "6" "5" real 0m0.003s "7" "6" real 0m0.003s "8" "7" real 0m0.003s "9" "8" real 0m0.003s

So something really odd is going on here. This should be speeding up "git status" a lot, even with a hot fs cache doing stat on all of linux.git takes a lot longer than 3ms, but for some reason it's slower.