[PATCH 7/7] merge-ort: restart merge with cached renames to reduce process entry cost

From: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>
Date: Jul 1, 2021, 03:46 UTC
Message-ID: <76bc73262c4d01db12b2c0fa401b7ccad2988e1c.1625111177.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.988.git.1625111177.gitgitgadget@gmail.com>

From: Elijah Newren <newren@gmail.com>

The merge algorithm mostly consists of the following three functions:
   collect_merge_info()
   detect_and_process_renames()
   process_entries()
Prior to the trivial directory resolution optimization of the last half
dozen commits, process_entries() was consistently the slowest, followed
by collect_merge_info(), then detect_and_process_renames().  When the
trivial directory resolution applies, it often dramatically decreases
the amount of time spent in the two slower functions.

Looking at the performance results in the previous commit, the trivial directory resolution optimization helps amazingly well when there are no relevant renames. It also helps really well when reapplying a long series of linear commits (such as in a rebase or cherry-pick), since the relevant renames may well be cached from the first reapplied commit. But when there are any relevant renames that are not cached (represented by the just-one-mega testcase), then the optimization does not help at all.

Often, I noticed that when the optimization does not apply, it is because there are a handful of relevant sources -- maybe even only one. It felt frustrating to need to recurse into potentially hundreds or even thousands of directories just for a single rename, but it was needed for correctness.

However, staring at this list of functions and noticing that
process_entries() is the most expensive and knowing I could avoid it if
I had cached renames suggested a simple idea: change
   collect_merge_info()
   detect_and_process_renames()
   process_entries()
into
   collect_merge_info()
   detect_and_process_renames()
   <cache all the renames, and restart>
   collect_merge_info()
   detect_and_process_renames()
   process_entries()

This may seem odd and look like more work. However, note that although we run collect_merge_info() twice, the second time we get to employ trivial directory resolves, which makes it much faster, so the increased time in collect_merge_info() is small. While we run detect_and_process_renames() again, all renames are cached so it's nearly a no-op (we don't call into diffcore_rename_extended() but we do have a little bit of data structure checking and fixing up). And the big payoff comes from the fact that process_entries(), will be much faster due to having far fewer entries to process.

This restarting only makes sense if we can save recursing into enough directories to make it worth our while. Introduce a simple heuristic to guide this. Note that this heuristic uses a "wanted_factor" that I have virtually no actual real world data for, just some back-of-the-envelope quasi-scientific calculations that I included in some comments and then plucked a simple round number out of thin air. It could be that tweaking this number to make it either higher or lower improves the optimization. (There's slightly more here; when I first introduced this optimization, I used a factor of 10, because I was completely confident it was big enough to not cause slowdowns in special cases. I was certain it was higher than needed. Several months later, I added the rough calculations which make me think the optimal number is close to 2; but instead of pushing to the limit, I just bumped it to 3 to reduce the risk that there are special cases where this optimization can result in slowing down the code a little. If the ratio of path counts is below 3, we probably will only see minor performance improvements at best anyway.)

Also, note that while the diffstat looks kind of long (nearly 100 lines), more than half of it is in two comments explaining how things work.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin performance work; instrument with trace2_region_* calls", 2020-10-28), this change improves the performance as follows:

                            Before                  After
    no-renames:      205.1  ms ±  3.8  ms   204.2  ms ±  3.0  ms
    mega-renames:      1.564 s ±  0.010 s     1.076 s ±  0.015 s
    just-one-mega:   479.5  ms ±  3.9  ms   364.1  ms ±  7.0  ms

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c                         | 106 ++++++++++++++++++++++++++--
 t/t6423-merge-rename-directories.sh |   2 +-
 2 files changed, 101 insertions(+), 7 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index c9cf7a158c8..51274f40059 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -209,6 +209,7 @@ struct rename_info {
 	 *   MERGE_SIDE2: cached data from side2 can be reused
 	 *   MERGE_SIDE1: cached data from side1 can be reused
 	 *   0:           no cached data can be reused
+	 *   -1:          See redo_after_renames; both sides can be reused.
 	 */
 	int cached_pairs_valid_side;
 
@@ -254,6 +255,28 @@ struct rename_info {
 	 */
 	struct strset cached_irrelevant[3];
 
+	/*
+	 * redo_after_renames: optimization flag for "restarting" the merge
+	 *
+	 * Sometimes it pays to detect renames, cache them, and then
+	 * restart the merge operation from the beginning.  The reason for
+	 * this is that when we know where all the renames are, we know
+	 * whether a certain directory has any paths under it affected --
+	 * and if a directory is not affected then it permits us to do
+	 * trivial tree merging in more cases.  Doing trivial tree merging
+	 * prevents the need to run process_entry() on every path
+	 * underneath trees that can be trivially merged, and
+	 * process_entry() is more expensive than collect_merge_info() --
+	 * plus, the second collect_merge_info() will be much faster since
+	 * it doesn't have to recurse into the relevant trees.
+	 *
+	 * Values for this flag:
+	 *   0 = don't bother, not worth it (or conditions not yet checked)
+	 *   1 = conditions for optimization met, optimization worthwhile
+	 *   2 = we already did it (don't restart merge yet again)
+	 */
+	unsigned redo_after_renames;
+
 	/*
 	 * needed_limit: value needed for inexact rename detection to run
 	 *
@@ -540,7 +563,8 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
 		renames->trivial_merges_okay[i] = 1; /* 1 == maybe */
 		if (!reinitialize)
 			assert(renames->cached_pairs_valid_side == 0);
-		if (i != renames->cached_pairs_valid_side) {
+		if (i != renames->cached_pairs_valid_side &&
+		    -1 != renames->cached_pairs_valid_side) {
 			strset_func(&renames->cached_target_names[i]);
 			strmap_func(&renames->cached_pairs[i], 1);
 			strset_func(&renames->cached_irrelevant[i]);
@@ -1242,7 +1266,9 @@ static int handle_deferred_entries(struct merge_options *opt,
 	struct hashmap_iter iter;
 	struct strmap_entry *entry;
 	int side, ret = 0;
+	int path_count_before, path_count_after = 0;
 
+	path_count_before = strmap_get_size(&opt->priv->paths);
 	for (side = MERGE_SIDE1; side <= MERGE_SIDE2; side++) {
 		unsigned optimization_okay = 1;
 		struct strintmap copy;
@@ -1377,7 +1403,51 @@ static int handle_deferred_entries(struct merge_options *opt,
 						path));
 			resolve_trivial_directory_merge(ci, side);
 		}
+		if (!optimization_okay || path_count_after)
+			path_count_after = strmap_get_size(&opt->priv->paths);
 	}
+	if (path_count_after) {
+		/*
+		 * Not sure were the right cut-off is for the optimization
+		 * to redo collect_merge_info after we've cached the
+		 * regular renames is.  Basically, collect_merge_info(),
+		 * detect_regular_renames(), and process_entries() are
+		 * similar costs and all big tent poles.  Caching the
+		 * result of detect_regular_renames() means that redoing
+		 * that one function will cost us virtually 0 extra, so it
+		 * depends on the other two functions, which are both O(N)
+		 * cost in the number of paths.  Thus, it makes sense that
+		 * if we can cut the number of paths in half, then redoing
+		 * collect_merge_info() at half cost in order to get
+		 * process_entries() at half cost should be about equal
+		 * cost.  If we can cut by more than half, then we would
+		 * win.  The fact that process_entries() is about 10%-20%
+		 * more expensive than collect_merge_info() suggests we
+		 * could make the factor be less than two.  The fact that
+		 * even when we have renames cached, we still have to
+		 * traverse down to the individual (relevant) renames,
+		 * which suggests we should perhaps use a bigger factor.
+		 *
+		 * The exact number isn't critical, since the code will
+		 * work even if we get the factor wrong -- it just might be
+		 * slightly slower if we're a bit off.  For now, just error
+		 * on the side of a bigger fudge.  For the linux kernel
+		 * testcases I was looking at with massive renames, the
+		 * ratio came in around 50 to 250, which clearly would
+		 * trigger this optimization and provided some *very* nice
+		 * speedups.
+		 */
+		int wanted_factor = 3;
+
+		/* We should only redo collect_merge_info one time */
+		assert(renames->redo_after_renames == 0);
+
+		if (path_count_after / path_count_before > wanted_factor) {
+			renames->redo_after_renames = 1;
+			renames->cached_pairs_valid_side = -1;
+		}
+	} else if (renames->redo_after_renames == 2)
+		renames->redo_after_renames = 0;
 	return ret;
 }
 
@@ -2820,8 +2890,8 @@ static int compare_pairs(const void *a_, const void *b_)
 }
 
 /* Call diffcore_rename() to update deleted/added pairs into rename pairs */
-static void detect_regular_renames(struct merge_options *opt,
-				   unsigned side_index)
+static int detect_regular_renames(struct merge_options *opt,
+				  unsigned side_index)
 {
 	struct diff_options diff_opts;
 	struct rename_info *renames = &opt->priv->renames;
@@ -2834,7 +2904,7 @@ static void detect_regular_renames(struct merge_options *opt,
 		 * side had directory renames.
 		 */
 		resolve_diffpair_statuses(&renames->pairs[side_index]);
-		return;
+		return 0;
 	}
 
 	partial_clear_dir_rename_count(&renames->dir_rename_count[side_index]);
@@ -2869,6 +2939,18 @@ static void detect_regular_renames(struct merge_options *opt,
 	diff_queued_diff.nr = 0;
 	diff_queued_diff.queue = NULL;
 	diff_flush(&diff_opts);
+
+	if (renames->redo_after_renames) {
+		int i;
+		struct diff_filepair *p;
+
+		renames->redo_after_renames = 2;
+		for (i = 0; i < renames->pairs[side_index].nr; ++i) {
+			p = renames->pairs[side_index].queue[i];
+			possibly_cache_new_pair(renames, p, side_index, NULL);
+		}
+	}
+	return 1;
 }
 
 /*
@@ -2958,14 +3040,19 @@ static int detect_and_process_renames(struct merge_options *opt,
 	struct diff_queue_struct combined;
 	struct rename_info *renames = &opt->priv->renames;
 	int need_dir_renames, s, clean = 1;
+	unsigned detection_run = 0;
 
 	memset(&combined, 0, sizeof(combined));
 	if (!possible_renames(renames))
 		goto cleanup;
 
 	trace2_region_enter("merge", "regular renames", opt->repo);
-	detect_regular_renames(opt, MERGE_SIDE1);
-	detect_regular_renames(opt, MERGE_SIDE2);
+	detection_run |= detect_regular_renames(opt, MERGE_SIDE1);
+	detection_run |= detect_regular_renames(opt, MERGE_SIDE2);
+	if (renames->redo_after_renames && detection_run) {
+		trace2_region_leave("merge", "regular renames", opt->repo);
+		goto cleanup;
+	}
 	use_cached_pairs(opt, &renames->cached_pairs[1], &renames->pairs[1]);
 	use_cached_pairs(opt, &renames->cached_pairs[2], &renames->pairs[2]);
 	trace2_region_leave("merge", "regular renames", opt->repo);
@@ -4380,6 +4467,7 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 					       opt->subtree_shift);
 	}
 
+redo:
 	trace2_region_enter("merge", "collect_merge_info", opt->repo);
 	if (collect_merge_info(opt, merge_base, side1, side2) != 0) {
 		/*
@@ -4399,6 +4487,12 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 	result->clean = detect_and_process_renames(opt, merge_base,
 						   side1, side2);
 	trace2_region_leave("merge", "renames", opt->repo);
+	if (opt->priv->renames.redo_after_renames == 2) {
+		trace2_region_enter("merge", "reset_maps", opt->repo);
+		clear_or_reinit_internal_opts(opt->priv, 1);
+		trace2_region_leave("merge", "reset_maps", opt->repo);
+		goto redo;
+	}
 
 	trace2_region_enter("merge", "process_entries", opt->repo);
 	process_entries(opt, &working_tree_oid);
diff --git a/t/t6423-merge-rename-directories.sh b/t/t6423-merge-rename-directories.sh
index e834b7e6efe..d8919d276a1 100755
--- a/t/t6423-merge-rename-directories.sh
+++ b/t/t6423-merge-rename-directories.sh
@@ -4797,7 +4797,7 @@ test_setup_12f () {
 	)
 }
 
-test_expect_merge_algorithm failure failure '12f: Trivial directory resolve, caching, all kinds of fun' '
+test_expect_merge_algorithm failure success '12f: Trivial directory resolve, caching, all kinds of fun' '
 	test_setup_12f &&
 	(
 		cd 12f &&

-- 
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