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authorLibravatar Elijah Newren <newren@gmail.com>2021-07-16 05:22:37 +0000
committerLibravatar Junio C Hamano <gitster@pobox.com>2021-07-20 14:47:40 -0700
commit8b09a900a1f1f00d4deb04f567994ae8f1804b5e (patch)
tree88bd8c59e19afc938611353341dd31c764d5ee35
parentmerge-ort: avoid recursing into directories when we don't need to (diff)
downloadtgif-8b09a900a1f1f00d4deb04f567994ae8f1804b5e.tar.xz
merge-ort: restart merge with cached renames to reduce process entry cost
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> Signed-off-by: Junio C Hamano <gitster@pobox.com>
-rw-r--r--merge-ort.c92
-rwxr-xr-xt/t6423-merge-rename-directories.sh2
2 files changed, 87 insertions, 7 deletions
diff --git a/merge-ort.c b/merge-ort.c
index ad94b30a76..e75b524153 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -213,6 +213,7 @@ 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;
@@ -259,6 +260,28 @@ 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
*
* If the current rename limit wasn't high enough for inexact
@@ -541,7 +564,8 @@ static void clear_or_reinit_internal_opts(struct merge_options_internal *opti,
strintmap_func(&renames->relevant_sources[i]);
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]);
@@ -1249,7 +1273,9 @@ 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;
@@ -1385,7 +1411,32 @@ 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) {
+ /*
+ * The choice of wanted_factor here does not affect
+ * correctness, only performance. When the
+ * path_count_after / path_count_before
+ * ratio is high, redoing after renames is a big
+ * performance boost. I suspect that redoing is a wash
+ * somewhere near a value of 2, and below that redoing will
+ * slow things down. I applied a fudge factor and picked
+ * 3; see the commit message when this was introduced for
+ * back of the envelope calculations for this ratio.
+ */
+ const 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;
}
@@ -2828,8 +2879,8 @@ 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;
@@ -2842,7 +2893,7 @@ 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]);
@@ -2868,6 +2919,8 @@ static void detect_regular_renames(struct merge_options *opt,
trace2_region_leave("diff", "diffcore_rename", opt->repo);
resolve_diffpair_statuses(&diff_queued_diff);
+ if (diff_opts.needed_rename_limit > 0)
+ renames->redo_after_renames = 0;
if (diff_opts.needed_rename_limit > renames->needed_limit)
renames->needed_limit = diff_opts.needed_rename_limit;
@@ -2877,6 +2930,8 @@ static void detect_regular_renames(struct merge_options *opt,
diff_queued_diff.nr = 0;
diff_queued_diff.queue = NULL;
diff_flush(&diff_opts);
+
+ return 1;
}
/*
@@ -2966,14 +3021,32 @@ 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) {
+ int i, side;
+ struct diff_filepair *p;
+
+ /* Cache the renames, we found */
+ for (side = MERGE_SIDE1; side <= MERGE_SIDE2; side++) {
+ for (i = 0; i < renames->pairs[side].nr; ++i) {
+ p = renames->pairs[side].queue[i];
+ possibly_cache_new_pair(renames, p, side, NULL);
+ }
+ }
+
+ /* Restart the merge with the cached renames */
+ renames->redo_after_renames = 2;
+ 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);
@@ -4403,6 +4476,7 @@ 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) {
/*
@@ -4422,6 +4496,12 @@ 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 4af4fb0038..5b81a130e9 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 &&