Re: [PATCH v2 1/2] commit-graph: bump DIE_ON_LOAD check to actual load-time

[Derrick Stolee <stolee@gmail.com>]

On 9/12/2019 10:44 AM, Jeff King wrote:
> Commit 43d3561805 (commit-graph write: don't die if the existing graph
> is corrupt, 2019-03-25) added an environment variable we use only in the
> test suite, $GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD. But it put the check for
> this variable at the very top of prepare_commit_graph(), which is called
> every time we want to use the commit graph. Most importantly, it comes
> _before_ we check the fast-path "did we already try to load?", meaning
> we end up calling getenv() for every single use of the commit graph,
> rather than just when we load.
> 
> getenv() is allowed to have unexpected side effects, but that shouldn't
> be a problem here; we're lazy-loading the graph so it's clear that at
> least _one_ invocation of this function is going to call it.
> 
> But it is inefficient. getenv() typically has to do a linear search
> through the environment space.
> 
> We could memoize the call, but it's simpler still to just bump the check
> down to the actual loading step. That's fine for our sole user in t5318,
> and produces this minor real-world speedup:
> 
>   [before]
>   Benchmark #1: git -C linux rev-list HEAD >/dev/null
>     Time (mean ± σ):      1.460 s ±  0.017 s    [User: 1.174 s, System: 0.285 s]
>     Range (min … max):    1.440 s …  1.491 s    10 runs
> 
>   [after]
>   Benchmark #1: git -C linux rev-list HEAD >/dev/null
>     Time (mean ± σ):      1.391 s ±  0.005 s    [User: 1.118 s, System: 0.273 s]
>     Range (min … max):    1.385 s …  1.399 s    10 runs

This looks like an important improvement on its own.

> Of course that actual speedup depends on how big your environment is. We
> can game it like this:
> 
>   for i in $(seq 10000); do
>     export dummy$i=$i
>   done
> 
> in which case I get:
> 
>   [before]
>   Benchmark #1: git -C linux rev-list HEAD >/dev/null
>     Time (mean ± σ):      6.257 s ±  0.061 s    [User: 6.005 s, System: 0.250 s]
>     Range (min … max):    6.174 s …  6.337 s    10 runs
> 
>   [after]
>   Benchmark #1: git -C linux rev-list HEAD >/dev/null
>   Time (mean ± σ):      1.403 s ±  0.005 s    [User: 1.146 s, System: 0.256 s]
>   Range (min … max):    1.396 s …  1.412 s    10 runs
> 
> So this is really more about avoiding the pathological case than
> providing a big real-world speedup.

This change is stunning. I'm _sure_ someone is hurting with this.
 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  commit-graph.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/commit-graph.c b/commit-graph.c
> index 9b02d2c426..baeaf0d1bf 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -468,14 +468,14 @@ static int prepare_commit_graph(struct repository *r)
>  {
>  	struct object_directory *odb;
>  
> -	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD, 0))
> -		die("dying as requested by the '%s' variable on commit-graph load!",
> -		    GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD);
> -
>  	if (r->objects->commit_graph_attempted)
>  		return !!r->objects->commit_graph;
>  	r->objects->commit_graph_attempted = 1;
>  
> +	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD, 0))
> +		die("dying as requested by the '%s' variable on commit-graph load!",
> +		    GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD);
> +
>  	prepare_repo_settings(r);
>  
>  	if (!git_env_bool(GIT_TEST_COMMIT_GRAPH, 0) &&

LGTM, thanks!

-Stolee