On 6/7/2017 5:46 PM, Ævar Arnfjörð Bjarmason wrote:
On Wed, Jun 7, 2017 at 9:51 PM, Ben Peart <peartben@xxxxxxxxx> wrote:
On 6/2/2017 7:06 PM, Ævar Arnfjörð Bjarmason wrote:
I don't have time to update the perf test now or dig into it, but most
of what you're describing in this mail doesn't at all match with the
ad-hoc tests I ran in
https://public-inbox.org/git/CACBZZX5e58bWuf3NdDYTxu2KyZj29hHONzN=rp-7vXd8nURyWQ@xxxxxxxxxxxxxx/
There (at the very end of the E-Mail) I'm running watchman in a tight
loop while I flush the entire fs cache, its runtime is never longer
than 600ms, with 3ms being the norm.
I added a perf trace around the entire query-fsmonitor hook proc (patch
below) to measure the total actual impact of running the hook script +
querying watchman + parsing the output with perl + passing the result back
to git. On my machine, the total cost of the hook runs between 130 ms and
180 ms when there are zero changes to report (ie best case).
With short status times, the overhead of watchman simply outweighs any gains
in performance - especially when you have a warm file system cache as that
cancels out the biggest win of avoiding the IO associated with scanning the
working directory.
diff --git a/fsmonitor.c b/fsmonitor.c
index 763a8a3a3f..cb47f31863 100644
--- a/fsmonitor.c
+++ b/fsmonitor.c
@@ -210,9 +210,11 @@ void refresh_by_fsmonitor(struct index_state *istate)
* If we have a last update time, call query-monitor for the set of
* changes since that time.
*/
- if (istate->fsmonitor_last_update)
+ if (istate->fsmonitor_last_update) {
query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION,
istate->fsmonitor_last_update, &query_result);
+ trace_performance_since(last_update, "query-fsmonitor");
+ }
if (query_success) {
/* Mark all entries returned by the monitor as dirty */
I.e. flushing the cache doesn't slow things down much at all compared
to how long a "git status" takes from cold cache. Something else must
be going on, and the smoking gun is the gprof output I posted in the
follow-up E-Mail:
https://public-inbox.org/git/CACBZZX4eZ3G8LQ8O+_BkbkJ-ZXTOkUi9cW=QKYjfHKtmA3pgrA@xxxxxxxxxxxxxx/
There with the fsmonitor we end up calling blk_SHA1_Block ~100K times
during "status", but IIRC (I don't have the output in front of me,
this is from memory) something like twenty times without the
fsmonitor.
It can't be a coincidence that with the fscache:
$ pwd; git ls-files | wc -l
/home/avar/g/linux
59844
And you can see that in the fsmonitor "git status" we make exactly
that many calls to cache_entry_from_ondisk(), but those calls don't
show up at all in the non-fscache codepath.
I don't see how the gprof numbers for the non-fsmonitor case can be correct.
It appears they don't contain any calls related to loading the index while
the fsmonitor gprof numbers do. Here is a typical call stack:
git.exe!cache_entry_from_ondisk()
git.exe!create_from_disk()
git.exe!do_read_index()
git.exe!read_index_from()
git.exe!read_index()
During read_index(), cache_entry_from_ondisk() gets called for every item in
the index (which explains the 59K calls). How can the non-fsmonitor
codepath not be loading the index?
So, again, I haven't dug and really must step away from the computer
now, but this really looks like the fscache saves us the recursive
readdir() / lstat() etc, but in return we somehow fall though to a
codepath where we re-read the entire on-disk state back into the
index, which we don't do in the non-fscache codepath.
I've run multiple profiles and compared them with fsmonitor on and off and
have been unable to find any performance regression caused by fsmonitor
(other than flagging the index as dirty at times when it isn't required
which I have fixed for the next patch series).
I have done many performance runs and when I subtract the _actual_ time
spent in the hook from the overall command time, it comes in at slightly
less time than when status is run with fsmonitor off. This also leads me to
believe there is no regression with fsmonitor on.
All this leads me back to my original conclusion: the reason status is
slower in these specific cases is because the overhead of calling the hook
exceeds the savings gained. If your status calls are taking less than a
second, it just doesn't make sense to add the complexity and overhead of
calling a file system watcher.
I'm working on an updated perf test that will demonstrate the best case
scenario (warm watchman, cold file system cache) in addition to the worst
case (cold watchman, warm file system cache). The reality is that in normal
use cases, perf will be between the two. I'll add that to the next iteration
of the patch series.
I'll try to dig further once we have the next submission + that perf test.
On Linux the time spent calling the hook itself is minimal:
$ touch foo; time .git/hooks/query-fsmonitor 1 $(($(date +%s) *
1000000000 - 10))
Watchman says these changed: foo
foo
real 0m0.009s
user 0m0.004s
sys 0m0.000s
Wow, what a difference:
$ touch foo; time .git/hooks/query-fsmonitor 1 $(($(date +%s) *
> 1000000000 - 10))
.git/hooks/query-fsmonitor 1 1496885562999999990
.gitfoo
real 0m0.170s
user 0m0.045s
sys 0m0.091s
So I'm fairly sure something else entirely is going on, but anyway, we
can look at that later with the next submission.
Aside from that, I wonder on Windows how much speedier this could be
for you if the entire hook was written in perl instead of a
shellscript that calls perl. I could help with that if you'd like.
Anything we can do to speed this up and have fewer moving pieces is all
good. I'm not a perl expert so I'd very much appreciate your expertise.
I have a couple of notes from Wez (author of Watchman) on things we
could do to better optimize certain scenarios I'll pass along as well.
You can likely replace that call to uname with reading the $^O variable.
What you're doing shelling out to cygpath can probably be done by
loading the File::Spec library, but I'm not sure.
The echo / watchman / perl would need to be an IPC::Open3 invocation I think.
I think it being in shellscript is fine, just a suggestion in case
having it all in one process (aside from the watchman shell-out) would
help or Windows.