On Wed, Sep 11, 2019 at 08:18:46PM -0400, Jeff King wrote:
> > That creates an interesting problem for commits that have _already_ been
> > parsed using the commit graph. Their commit->object.parsed flag is set,
> > their commit->graph_pos is set, but their commit->maybe_tree may still
> > be NULL. When somebody later calls repo_get_commit_tree(), we see that
> > we haven't loaded the tree oid yet and try to get it from the commit
> > graph. But since it has been freed, we segfault!
>
> I was surprised we ever called repo_get_commit_tree() at all, since
> we're literally just traversing commits here. It looks like
> list-objects.c is very happy to queue pending trees for each commit,
> even if we're just going to throw them away when we get to
> process_tree()! I wonder if could be checking revs->tree_objects here
> and saving ourselves some work.
Indeed, this seems to help quite a bit in the commit-graph case. I think
it's worth doing (and is independent of the other patch).
-- >8 --
Subject: list-objects: don't queue root trees unless revs->tree_objects is set
When traverse_commit_list() processes each commit, it queues the
commit's root tree in the pending array. Then, after all commits are
processed, it calls traverse_trees_and_blobs() to walk over the pending
list, calling process_tree() on each. But if revs->tree_objects is not
set, process_tree() just exists immediately!
We can save ourselves some work by not even bothering to queue these
trees in the first place. There are a few subtle points to make:
- we also detect commits with a NULL tree pointer here. But this isn't
an interesting check for broken commits, since the lookup_tree()
we'd have done during commit parsing doesn't actually check that we
have the tree on disk. So we're not losing any robustness.
- besides queueing, we also set the NOT_USER_GIVEN flag on the tree
object. This is used by the traverse_commit_list_filtered() variant.
But if we're not exploring trees, then we won't actually care about
this flag, which is used only inside process_tree() code-paths.
- queueing trees eventually leads to us queueing blobs, too. But we
don't need to check revs->blob_objects here. Even in the current
code, we still wouldn't find those blobs, because we'd never open up
the tree objects to list their contents.
- the user-visible impact to the caller is minimal. The pending trees
are all cleared by the time the function returns anyway, by
traverse_trees_and_blobs(). We do call a show_commit() callback,
which technically could be looking at revs->pending during the
callback. But it seems like a rather unlikely thing to do (if you
want the tree of the current commit, then accessing the tree struct
member is a lot simpler).
So this should be safe to do. Let's look at the benefits:
[before]
Benchmark #1: git -C linux rev-list HEAD >/dev/null
Time (mean ± σ): 7.651 s ± 0.021 s [User: 7.399 s, System: 0.252 s]
Range (min … max): 7.607 s … 7.683 s 10 runs
[after]
Benchmark #1: git -C linux rev-list HEAD >/dev/null
Time (mean ± σ): 7.593 s ± 0.023 s [User: 7.329 s, System: 0.264 s]
Range (min … max): 7.565 s … 7.634 s 10 runs
Not too impressive, but then we're really just avoiding sticking a
pointer into a growable array. But still, I'll take a free 0.75%
speedup.
Let's try it after running "git commit-graph write":
[before]
Benchmark #1: git -C linux rev-list HEAD >/dev/null
Time (mean ± σ): 1.458 s ± 0.011 s [User: 1.199 s, System: 0.259 s]
Range (min … max): 1.447 s … 1.481 s 10 runs
[after]
Benchmark #1: git -C linux rev-list HEAD >/dev/null
Time (mean ± σ): 1.126 s ± 0.023 s [User: 896.5 ms, System: 229.0 ms]
Range (min … max): 1.106 s … 1.181 s 10 runs
Now that's more like it. We saved over 22% of the total time. Part of
that is because the runtime is shorter overall, but the absolute
improvement is also much larger. What's going on?
When we fill in a commit struct using the commit graph, we don't bother
to set the tree pointer, and instead lazy-load it when somebody calls
get_commit_tree(). So we're not only skipping the pointer write to the
pending queue, but we're skipping the lazy-load of the tree entirely.
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
list-objects.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/list-objects.c b/list-objects.c
index b5651ddd5b..c837bcaca8 100644
--- a/list-objects.c
+++ b/list-objects.c
@@ -370,7 +370,9 @@ static void do_traverse(struct traversal_context *ctx)
* an uninteresting boundary commit may not have its tree
* parsed yet, but we are not going to show them anyway
*/
- if (get_commit_tree(commit)) {
+ if (!ctx->revs->tree_objects)
+ ; /* do not bother loading tree */
+ else if (get_commit_tree(commit)) {
struct tree *tree = get_commit_tree(commit);
tree->object.flags |= NOT_USER_GIVEN;
add_pending_tree(ctx->revs, tree);
--
2.23.0.663.gbe3d117559
