Re: [PATCH v2] refs: implement reference transaction hook

From: Patrick Steinhardt <ps@pks.im>
Date: Jun 4, 2020, 07:36 UTC
Message-ID: <20200604073632.GA498923@tanuki.pks.im>
In-Reply-To: <20200603165142.GA24049@syl.local>

On Wed, Jun 03, 2020 at 10:51:42AM -0600, Taylor Blau wrote:

Show 30 quoted lines

> Hi Patrick,
> 
> On Wed, Jun 03, 2020 at 02:27:50PM +0200, Patrick Steinhardt wrote:
> > In order to test the impact on the case where we don't have any
> > "reference-transaction" hook installed in the repository, this commit
> > introduces a new performance test for git-update-refs(1). Run against an
> > empty repository, it produces the following results:
> >
> >   Test                                   HEAD~             HEAD
> >   ------------------------------------------------------------------------------
> >   1400.2: update existing reference      2.05(1.58+0.54)   2.08(1.58+0.57) +1.5%
> >   1400.3: create and destroy reference   1.79(1.38+0.49)   1.82(1.39+0.51) +1.7%
> >
> > So the overhead is around ~1.5%. Given that git-update-refs(1) is a
> > near-direct wrapper around reference transactions, there likely is no
> > other command that is impacted much worse than this.
> 
> This is a serious performance regression that is worth considering. For
> example, a 1.5% slow-down on average in reference transactions would
> cause be enough for me to bisect the regression down to see what
> changed.
> 
> What are ways that this could be avoided? I bet that this would work
> quite well with Emily's work on hooks, where we could check in the
> configuration first whether a hook is even configured.
> 
> Could this be integrated with that? If not, could you cache the result
> of whether or not the hook exists, and/or implement some mechanism to
> say something like, for eg., "only run reference transaction hooks
> core.blah = 1" is true?

I wasn't aware of her work until now, so I'll take a look.

Meanwhile, I toyed around with performance and tried out two different caching mechanisms:

    - map-cache: `find_hook()` uses a map of hook names mapped to their
      resolved hook path (or `NULL` if none was found). This is a
      generic mechanism working across all hooks, but has some overhead
      in maintaining the map.

    - reftx-cache: `run_transaction_hook()` caches the path returned by
      `find_hook()`. It's got less overhead as it only caches the path,
      but obviously only applies to the reference-transaction hook.

In theory, we could go even further and cache the hook's file descriptor, executing it via fexecve(3P) whenever it's required, but I didn't go down that route yet. This would also solve the atomicity issue, though, if the administrator replaces the reference-transactions hook halfway through the transaction.

Benchmarking results are mixed, mostly in the sense that I can choose which run of the benchmarks I take in order to have my own work look better or worse, as timings vary quite a lot between runs. Which probably hints at the fact that the benchmarks themselves aren't really reliable. The issue is that a single git-update-ref(1) run finishes so quick that it's hard to measure with our benchmarks, but spawning thousands of them to get something different than 0.0s very much depends on the operating system and thus fluctuates. On the other hand, if we only spawn a single git-update-refs and have it perform a few thousand ref updates, overhead of the hook will not show at all as it is only executed once per prepare/commit of the transaction.

The following timings are for the case where we execute

    git update-ref refs/heads/update-branch PRE POST &&
    git update-ref refs/heads/update-branch POST PRE

respectively

    git update-ref refs/heads/new POST &&
    git update-ref -d refs/heads/new

a thousand times:

Test master ref-hook-no-cache ref-hook-map-cache ref-hook-reftx-cache ------------------------------------------------------------------------------------------------------------------------------ 1400.2: update existing reference 1.96(1.50+0.53) 2.00(1.54+0.53) +2.0% 2.02(1.54+0.55) +3.1% 1.98(1.52+0.52) +1.0% 1400.3: create and destroy reference 1.74(1.33+0.49) 1.77(1.38+0.47) +1.7% 1.77(1.36+0.48) +1.7% 1.76(1.35+0.49) +1.1%

For such a short-lived program like git-update-refs(1), one can see that the overhead of using a map negatively impacts performance compared to the no-cache case. But using the reftx-cache roughly cuts the overhead in half as expected, as we only need to look up the hook once instead of twice.

And here's the results if we use a single `git update-ref --stdin` with a thousand reference updates at once:

Test master ref-hook-no-cache ref-hook-map-cache ref-hook-reftx-cache ------------------------------------------------------------------------------------------------------------------------ 1400.2: git update-ref --stdin 0.21(0.09+0.12) 0.21(0.07+0.14) +0.0% 0.21(0.07+0.13) +0.0% 0.21(0.07+0.13) +0.0%

As expected, there's nothing much to be seen here because there's only a single transaction for all ref updates and, as a result, at most two calls to `access(refhook_path, X_OK)`.

Patrick