[PATCH 4/5] revision: implement sparse algorithm

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Nov 28, 2018, 21:52 UTC
Message-ID: <0725aac4bbf3d6881af483bc64e3499a59bcf1a9.1543441960.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.89.git.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

When enumerating objects to place in a pack-file during 'git pack-objects --revs', we discover the "frontier" of commits that we care about and the boundary with commit we find uninteresting. From that point, we walk trees to discover which trees and blobs are uninteresting. Finally, we walk trees to find the interesting trees.

This commit introduces a new, "sparse" way to discover the uninteresting trees. We use the perspective of a single user trying to push their topic to a large repository. That user likely changed a very small fraction of the paths in their working directory, but we spend a lot of time walking all reachable trees.

The way to switch the logic to work in this sparse way is to start caring about which paths introduce new trees. While it is not possible to generate a diff between the frontier boundary and all of the interesting commits, we can simulate that behavior by inspecting all of the root trees as a whole, then recursing down to the set of trees at each path.

We already had taken the first step by passing an oidset to mark_trees_uninteresting_sparse(). We now create a dictionary whose keys are paths and values are oidsets. We consider the set of trees that appear at each path. While we inspect a tree, we add its subtrees to the oidsets corresponding to the tree entry's path. We also mark trees as UNINTERESTING if the tree we are parsing is UNINTERESTING.

To actually improve the peformance, we need to terminate our recursion unless the oidset contains some intersting trees and some uninteresting trees. Technically, we only need one interesting tree for this to speed up in most cases, but we also will not mark anything UNINTERESTING if there are no uninteresting trees, so that would be wasted effort.

There are a few ways that this is not a universally better option.

First, we can pack extra objects. If someone copies a subtree from one tree to another, the first tree will appear UNINTERESTING and we will not recurse to see that the subtree should also be UNINTERESTING. We will walk the new tree and see the subtree as a "new" object and add it to the pack. We add a test case that demonstrates this as a way to prove that the --sparse option is actually working.

Second, we can have extra memory pressure. If instead of being a single user pushing a small topic we are a server sending new objects from across the entire working directory, then we will gain very little (the recursion will rarely terminate early) but will spend extra time maintaining the path-oidset dictionaries.

Despite these potential drawbacks, the benefits of the algorithm are clear. By adding a counter to 'add_children_by_path' and 'mark_tree_contents_uninteresting', I measured the number of parsed trees for the two algorithms in a variety of repos.

For git.git, I used the following input:

	v2.19.0
	^v2.19.0~10

 Objects to pack: 550
Walked (old alg): 282
Walked (new alg): 130

For the Linux repo, I used the following input:

	v4.18
	^v4.18~10

 Objects to pack:   518
Walked (old alg): 4,836
Walked (new alg):   188

The two repos above are rather "wide and flat" compared to other repos that I have used in the past. As a comparison, I tested an old topic branch in the Azure DevOps repo, which has a much deeper folder structure than the Linux repo.

 Objects to pack:    220
Walked (old alg): 22,804
Walked (new alg):    129

I used the number of walked trees the main metric above because it is consistent across multiple runs. When I ran my tests, the performance of the pack-objects command with the same options could change the end-to-end time by 10x depending on the file system being warm. However, by repeating the same test on repeat I could get more consistent timing results. The git.git and Linux tests were too fast overall (less than 0.5s) to measure an end-to-end difference. The Azure DevOps case was slow enough to see the time improve from 15s to 1s in the warm case. The cold case was 90s to 9s in my testing.

These improvements will have even larger benefits in the super- large Windows repository. In our experiments, we see the "Enumerate objects" phase of pack-objects taking 60-80% of the end-to-end time of non-trivial pushes, taking longer than the network time to send the pack and the server time to verify the pack.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 revision.c                     | 111 ++++++++++++++++++++++++++++++---
 t/t5322-pack-objects-sparse.sh |  21 +++++--
 2 files changed, 116 insertions(+), 16 deletions(-)

diff --git a/revision.c b/revision.c
index 3a62c7c187..7e4bfe621a 100644
--- a/revision.c
+++ b/revision.c
@@ -99,26 +99,117 @@ void mark_tree_uninteresting(struct repository *r, struct tree *tree)
 	mark_tree_contents_uninteresting(r, tree);
 }
 
+struct paths_and_oids {
+	struct string_list list;
+};
+
+static void paths_and_oids_init(struct paths_and_oids *po)
+{
+	string_list_init(&po->list, 1);
+}
+
+static void paths_and_oids_clear(struct paths_and_oids *po)
+{
+	int i;
+	for (i = 0; i < po->list.nr; i++) {
+		oidset_clear(po->list.items[i].util);
+		free(po->list.items[i].util);
+	}
+
+	string_list_clear(&po->list, 0);
+}
+
+static void paths_and_oids_insert(struct paths_and_oids *po,
+				  const char *path,
+				  const struct object_id *oid)
+{
+	struct string_list_item *item = string_list_insert(&po->list, path);
+	struct oidset *set;
+
+	if (!item->util) {
+		set = xcalloc(1, sizeof(struct oidset));
+		oidset_init(set, 16);
+		item->util = set;
+	} else {
+		set = item->util;
+	}
+
+	oidset_insert(set, oid);
+}
+
+static void add_children_by_path(struct repository *r,
+				 struct tree *tree,
+				 struct paths_and_oids *po)
+{
+	struct tree_desc desc;
+	struct name_entry entry;
+
+	if (parse_tree_gently(tree, 1) < 0)
+		return;
+
+	init_tree_desc(&desc, tree->buffer, tree->size);
+	while (tree_entry(&desc, &entry)) {
+		switch (object_type(entry.mode)) {
+		case OBJ_TREE:
+			paths_and_oids_insert(po, entry.path, entry.oid);
+
+			if (tree->object.flags & UNINTERESTING) {
+				struct tree *child = lookup_tree(r, entry.oid);
+				child->object.flags |= UNINTERESTING;
+			}
+			break;
+		case OBJ_BLOB:
+			if (tree->object.flags & UNINTERESTING) {
+				struct blob *child = lookup_blob(r, entry.oid);
+				child->object.flags |= UNINTERESTING;
+			}
+			break;
+		default:
+			/* Subproject commit - not in this repository */
+			break;
+		}
+	}
+
+	free_tree_buffer(tree);
+}
+
 void mark_trees_uninteresting_sparse(struct repository *r,
 				     struct oidset *set)
 {
+	int i;
+	unsigned has_interesting = 0, has_uninteresting = 0;
+	struct paths_and_oids po;
 	struct object_id *oid;
 	struct oidset_iter iter;
 
 	oidset_iter_init(set, &iter);
-	while ((oid = oidset_iter_next(&iter))) {
+	while ((!has_interesting || !has_uninteresting) &&
+	       (oid = oidset_iter_next(&iter))) {
 		struct tree *tree = lookup_tree(r, oid);
 
-		if (tree->object.flags & UNINTERESTING) {
-			/*
-			 * Remove the flag so the next call
-			 * is not a no-op. The flag is added
-			 * in mark_tree_unintersting().
-			 */
-			tree->object.flags ^= UNINTERESTING;
-			mark_tree_uninteresting(r, tree);
-		}
+		if (tree->object.flags & UNINTERESTING)
+			has_uninteresting = 1;
+		else
+			has_interesting = 1;
 	}
+
+	/* Do not walk unless we have both types of trees. */
+	if (!has_uninteresting || !has_interesting)
+		return;
+
+	paths_and_oids_init(&po);
+
+	oidset_iter_init(set, &iter);
+	while ((oid = oidset_iter_next(&iter))) {
+		struct tree *tree = lookup_tree(r, oid);
+		add_children_by_path(r, tree, &po);
+	}
+
+	for (i = 0; i < po.list.nr; i++)
+		mark_trees_uninteresting_sparse(
+			r, (struct oidset *)po.list.items[i].util);
+
+	paths_and_oids_clear(&po);
 }
 
 struct commit_stack {
diff --git a/t/t5322-pack-objects-sparse.sh b/t/t5322-pack-objects-sparse.sh
index 81f6805bc3..45dba6e014 100755
--- a/t/t5322-pack-objects-sparse.sh
+++ b/t/t5322-pack-objects-sparse.sh
@@ -83,22 +83,25 @@ test_expect_success 'sparse pack-objects' '
 	test_cmp expect_objects.txt sparse_objects.txt
 '
 
+# Demonstrate that the algorithms differ when we copy a tree wholesale
+# from one folder to another.
+
 test_expect_success 'duplicate a folder from f1 into f3' '
 	mkdir f3/f4 &&
 	cp -r f1/f1/* f3/f4 &&
 	git add f3/f4 &&
 	git commit -m "Copied f1/f1 to f3/f4" &&
-	cat >packinput.txt <<-EOF &&
+	cat >packinput.txt <<-EOF
 	topic1
 	^topic1~1
 	EOF
-	git rev-parse		\
-		topic1		\
-		topic1^{tree}	\
-		topic1:f3 | sort >expect_objects.txt
 '
 
 test_expect_success 'non-sparse pack-objects' '
+	git rev-parse			\
+		topic1			\
+		topic1^{tree}		\
+		topic1:f3 | sort >expect_objects.txt &&
 	git pack-objects --stdout --revs <packinput.txt >nonsparse.pack &&
 	git index-pack -o nonsparse.idx nonsparse.pack &&
 	git show-index <nonsparse.idx | awk "{print \$2}" >nonsparse_objects.txt &&
@@ -106,10 +109,16 @@ test_expect_success 'non-sparse pack-objects' '
 '
 
 test_expect_success 'sparse pack-objects' '
+	git rev-parse			\
+		topic1			\
+		topic1^{tree}		\
+		topic1:f3		\
+		topic1:f3/f4		\
+		topic1:f3/f4/data.txt | sort >expect_sparse_objects.txt &&
 	git pack-objects --stdout --revs --sparse <packinput.txt >sparse.pack &&
 	git index-pack -o sparse.idx sparse.pack &&
 	git show-index <sparse.idx | awk "{print \$2}" >sparse_objects.txt &&
-	test_cmp expect_objects.txt sparse_objects.txt
+	test_cmp expect_sparse_objects.txt sparse_objects.txt
 '
 
 test_done

-- 
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