Re: [PATCH] refs: fix interleaving hook calls with reference-transaction hook

[Jeff King <peff@peff.net>]

On Fri, Aug 07, 2020 at 11:04:12AM +0200, Patrick Steinhardt wrote:

> > It's perhaps more justifiable when there isn't a hook (we're still just
> > paying that one access(), but it's proportionally bigger). I kind of
> > doubt it's measurable, though, since a ref write requires a bunch of
> > syscalls anyway.
> 
> Yeah, this really was done to not have to pay a performance penalty if
> updating thousands of refs if no reference-transaction hook exists. E.g.
> if doing a non-atomic push of n reference, we'd have n calls to
> access(3P). See [1] for reference.
> 
> I've just did another quick benchmark without the cache, and it still
> consistently shows a non-zero performance hit without it:
> 
> Test                         pks-reftx-hook-interleaving   no-cache
> --------------------------------------------------------------------------------
> 1400.2: update-ref           2.82(2.13+0.81)               2.86(2.19+0.78) +1.4%
> 1400.3: update-ref --stdin   0.22(0.07+0.15)               0.22(0.07+0.15) +0.0%

I'm skeptical that those results are useful. In the first test, we're
running update-ref 1000 times, so:

  - the cache shouldn't be helping at all, since we only have one ref to
    update (well, I guess once for "prepare" and once for "commit", so
    really it's saving one syscall total per process).

  - I'd expect a lot of noise because we're spending most of our time in
    starting up the process

In the second test, we run 1000 ref operations per update-ref process.
So we should be cutting down on our hook-lookup overhead by a factor of
1000. Yet it shows no improvement.

That implies you're just seeing noise. And indeed, with the patch below
I get:

Test                         HEAD^             HEAD
--------------------------------------------------------------------
1400.2: update-ref           1.93(1.57+0.42)   1.91(1.55+0.42) -1.0%
1400.3: update-ref --stdin   0.07(0.02+0.05)   0.07(0.02+0.05) +0.0%

Running it a second time gets me +0.5%. :)

---
 refs.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/refs.c b/refs.c
index 2dd851fe81..427bf5843c 100644
--- a/refs.c
+++ b/refs.c
@@ -2031,24 +2031,16 @@ int ref_update_reject_duplicates(struct string_list *refnames,
 	return 0;
 }
 
-static const char hook_not_found;
-static const char *hook;
-
 static int run_transaction_hook(struct ref_transaction *transaction,
 				const char *state)
 {
 	struct child_process proc = CHILD_PROCESS_INIT;
 	struct strbuf buf = STRBUF_INIT;
+	const char *hook = find_hook("reference-transaction");
 	int ret = 0, i;
 
-	if (hook == &hook_not_found)
-		return ret;
 	if (!hook)
-		hook = find_hook("reference-transaction");
-	if (!hook) {
-		hook = &hook_not_found;
 		return ret;
-	}
 
 	argv_array_pushl(&proc.args, hook, state, NULL);
 	proc.in = -1;
-- 
2.28.0.520.g10e2ce7e11