[v2,4/5] commit-reach: use heuristic in remove_redundant()

Commit Message

Derrick Stolee Feb. 1, 2021, 12:47 p.m. UTC

From: Derrick Stolee <[e-mail]>

Reachability algorithms in commit-reach.c frequently benefit from using
the first-parent history as a heuristic for satisfying reachability
queries. The most obvious example was implemented in 4fbcca4e
(commit-reach: make can_all_from_reach... linear, 2018-07-20).

Update the walk in remove_redundant() to use this same heuristic. Here,
we are walking starting at the parents of the input commits. Sort those
parents and walk from the highest generation to lower. Each time, use
the heuristic of searching the first parent history before continuing to
expand the walk.

The order in which we explore the commits matters, so update
compare_commits_by_gen to break generation number ties with commit date.
This has no effect when the commits are in a commit-graph file with
corrected commit dates computed, but it will assist when the commits are
in the region "above" the commit-graph with "infinite" generation
number. Note that we cannot shift to use
compare_commits_by_gen_then_commit_date as the method prototype is
different. We use compare_commits_by_gen for QSORT() as opposed to as a
priority function.

The important piece is to ensure we short-circuit the walk when we find
that there is a single non-redundant commit. This happens frequently
when looking for merge-bases or comparing several tags with 'git
merge-base --independent'. Use a new count 'count_still_independent' and
if that hits 1 we can stop walking.

To update 'count_still_independent' properly, we add use of the RESULT
flag on the input commits. Then we can detect when we reach one of these
commits and decrease the count. We need to remove the RESULT flag at
that moment because we might re-visit that commit when popping the
stack.

We use the STALE flag to mark parents that have been added to the new
walk_start list, but we need to clear that flag before we start walking
so those flags don't halt our depth-first-search walk.

On my copy of the Linux kernel repository, the performance of 'git
merge-base --independent <all-tags>' goes from 1.1 seconds to 0.11
seconds.

Signed-off-by: Derrick Stolee <[e-mail]>
---
 commit-reach.c | 72 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 56 insertions(+), 16 deletions(-)

Comments

Junio C Hamano Feb. 1, 2021, 8:05 p.m. UTC | #1

"Derrick Stolee via GitGitGadget" <[e-mail]> writes:

> The important piece is to ensure we short-circuit the walk when we find
> that there is a single non-redundant commit. This happens frequently
> when looking for merge-bases or comparing several tags with 'git
> merge-base --independent'. Use a new count 'count_still_independent' and
> if that hits 1 we can stop walking.

That is because when you are left one single thing, it may be able
to reach many other things, but the fact that it by itself won't be
reachable by remaining independent things will not change (because,
that sole remaining independent thing is itself)?

> To update 'count_still_independent' properly, we add use of the RESULT
> flag on the input commits. Then we can detect when we reach one of these
> commits and decrease the count. We need to remove the RESULT flag at
> that moment because we might re-visit that commit when popping the
> stack.
>
> We use the STALE flag to mark parents that have been added to the new
> walk_start list, but we need to clear that flag before we start walking
> so those flags don't halt our depth-first-search walk.
>
> On my copy of the Linux kernel repository, the performance of 'git
> merge-base --independent <all-tags>' goes from 1.1 seconds to 0.11
> seconds.

These two numbers are with commit-graph fully populated with usable
generation numbers, I presume, and it is quite impressive.

Derrick Stolee Feb. 1, 2021, 9:02 p.m. UTC | #2

On 2/1/2021 3:05 PM, Junio C Hamano wrote:
> "Derrick Stolee via GitGitGadget" <[e-mail]> writes:
> 
>> The important piece is to ensure we short-circuit the walk when we find
>> that there is a single non-redundant commit. This happens frequently
>> when looking for merge-bases or comparing several tags with 'git
>> merge-base --independent'. Use a new count 'count_still_independent' and
>> if that hits 1 we can stop walking.
> 
> That is because when you are left one single thing, it may be able
> to reach many other things, but the fact that it by itself won't be
> reachable by remaining independent things will not change (because,
> that sole remaining independent thing is itself)?

Right. If there is only one non-STALE input commit left, then it will
be the only returned result. We will never find that all commits are
redundant because the commit graph is acyclic. The performance
improvement comes from halting the DFS walk: there might be more
commits to walk but they won't change the result.

>> To update 'count_still_independent' properly, we add use of the RESULT
>> flag on the input commits. Then we can detect when we reach one of these
>> commits and decrease the count. We need to remove the RESULT flag at
>> that moment because we might re-visit that commit when popping the
>> stack.
>>
>> We use the STALE flag to mark parents that have been added to the new
>> walk_start list, but we need to clear that flag before we start walking
>> so those flags don't halt our depth-first-search walk.
>>
>> On my copy of the Linux kernel repository, the performance of 'git
>> merge-base --independent <all-tags>' goes from 1.1 seconds to 0.11
>> seconds.
> 
> These two numbers are with commit-graph fully populated with usable
> generation numbers, I presume, and it is quite impressive.
 
Yes, these numbers are with a full commit-graph.

Thanks,
-Stolee

Patch

diff --git a/commit-reach.c b/commit-reach.c
index 7bf52e94429..d3a6e2bdd04 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -29,6 +29,10 @@  static int compare_commits_by_gen(const void *_a, const void *_b)
 		return -1;
 	if (generation_a > generation_b)
 		return 1;
+	if (a->date < b->date)
+		return -1;
+	if (a->date > b->date)
+		return 1;
 	return 0;
 }
 
@@ -231,11 +235,10 @@  static int remove_redundant_no_gen(struct repository *r,
 static int remove_redundant_with_gen(struct repository *r,
 				     struct commit **array, int cnt)
 {
-	int i, count_non_stale = 0;
+	int i, count_non_stale = 0, count_still_independent = cnt;
 	timestamp_t min_generation = GENERATION_NUMBER_INFINITY;
 	struct commit **walk_start;
 	size_t walk_start_nr = 0, walk_start_alloc = cnt;
-	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
 
 	ALLOC_ARRAY(walk_start, walk_start_alloc);
 
@@ -245,6 +248,7 @@  static int remove_redundant_with_gen(struct repository *r,
 		timestamp_t generation;
 
 		repo_parse_commit(r, array[i]);
+		array[i]->object.flags |= RESULT;
 		parents = array[i]->parents;
 
 		while (parents) {
@@ -253,7 +257,6 @@  static int remove_redundant_with_gen(struct repository *r,
 				parents->item->object.flags |= STALE;
 				ALLOC_GROW(walk_start, walk_start_nr + 1, walk_start_alloc);
 				walk_start[walk_start_nr++] = parents->item;
-				prio_queue_put(&queue, parents->item);
 			}
 			parents = parents->next;
 		}
@@ -264,26 +267,63 @@  static int remove_redundant_with_gen(struct repository *r,
 			min_generation = generation;
 	}
 
-	/* push the STALE bits up to min generation */
-	while (queue.nr) {
-		struct commit_list *parents;
-		struct commit *c = prio_queue_get(&queue);
+	QSORT(walk_start, walk_start_nr, compare_commits_by_gen);
 
-		repo_parse_commit(r, c);
+	/* remove STALE bit for now to allow walking through parents */
+	for (i = 0; i < walk_start_nr; i++)
+		walk_start[i]->object.flags &= ~STALE;
 
-		if (commit_graph_generation(c) < min_generation)
-			continue;
+	/*
+	 * Start walking from the highest generation. Hopefully, it will
+	 * find all other items during the first-parent walk, and we can
+	 * terminate early. Otherwise, we will do the same amount of work
+	 * as before.
+	 */
+	for (i = walk_start_nr - 1; i >= 0 && count_still_independent > 1; i--) {
+		/* push the STALE bits up to min generation */
+		struct commit_list *stack = NULL;
 
-		parents = c->parents;
-		while (parents) {
-			if (!(parents->item->object.flags & STALE)) {
-				parents->item->object.flags |= STALE;
-				prio_queue_put(&queue, parents->item);
+		commit_list_insert(walk_start[i], &stack);
+		walk_start[i]->object.flags |= STALE;
+
+		while (stack) {
+			struct commit_list *parents;
+			struct commit *c = stack->item;
+
+			repo_parse_commit(r, c);
+
+			if (c->object.flags & RESULT) {
+				c->object.flags &= ~RESULT;
+				if (--count_still_independent <= 1)
+					break;
 			}
-			parents = parents->next;
+
+			if (commit_graph_generation(c) < min_generation) {
+				pop_commit(&stack);
+				continue;
+			}
+
+			parents = c->parents;
+			while (parents) {
+				if (!(parents->item->object.flags & STALE)) {
+					parents->item->object.flags |= STALE;
+					commit_list_insert(parents->item, &stack);
+					break;
+				}
+				parents = parents->next;
+			}
+
+			/* pop if all parents have been visited already */
+			if (!parents)
+				pop_commit(&stack);
 		}
+		free_commit_list(stack);
 	}
 
+	/* clear result */
+	for (i = 0; i < cnt; i++)
+		array[i]->object.flags &= ~RESULT;
+
 	/* rearrange array */
 	for (i = count_non_stale = 0; i < cnt; i++) {
 		if (!(array[i]->object.flags & STALE))