diff: halt tree-diff early after max_changes

When computing the changed-paths bloom filters for the commit-graph,
we limit the size of the filter by restricting the number of paths
in the diff. Instead of computing a large diff and then ignoring the
result, it is better to halt the diff computation early.

Create a new "max_changes" option in struct diff_options. If non-zero,
then halt the diff computation after discovering strictly more changed
paths. This includes paths corresponding to trees that change.

Use this max_changes option in the bloom filter calculations. This
reduces the time taken to compute the filters for the Linux kernel
repo from 2m50s to 2m35s. On a large internal repository with ~500
commits that perform tree-wide changes, the time reduced from
6m15s to 3m48s.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/bloom.c b/bloom.c
index 881a984..a16eee9 100644
--- a/bloom.c
+++ b/bloom.c
@@ -133,6 +133,7 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 	struct bloom_filter_settings settings = DEFAULT_BLOOM_FILTER_SETTINGS;
 	int i;
 	struct diff_options diffopt;
+	int max_changes = 512;
 
 	if (bloom_filters.slab_size == 0)
 		return NULL;
@@ -141,6 +142,7 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 
 	repo_diff_setup(r, &diffopt);
 	diffopt.flags.recursive = 1;
+	diffopt.max_changes = max_changes;
 	diff_setup_done(&diffopt);
 
 	if (c->parents)
@@ -149,7 +151,7 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
 		diff_tree_oid(NULL, &c->object.oid, "", &diffopt);
 	diffcore_std(&diffopt);
 
-	if (diff_queued_diff.nr <= 512) {
+	if (diff_queued_diff.nr <= max_changes) {
 		struct hashmap pathmap;
 		struct pathmap_hash_entry *e;
 		struct hashmap_iter iter;