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@xxxxxxxx>
> ---
> 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
