show-branch: use prio_queue

Building a list using commit_list_insert_by_date() has quadratic worst
case complexity.  Avoid it by using prio_queue.

Use prio_queue_peek()+prio_queue_replace() instead of prio_queue_get()+
prio_queue_put() if possible, as the former only rebalance the
prio_queue heap once instead of twice.

In sane repositories this won't make much of a difference because the
number of items in the list or queue won't be very high:

Benchmark 1: ./git_v2.52.0 show-branch origin/main origin/next origin/seen origin/todo
  Time (mean ± σ):     538.2 ms ±   0.8 ms    [User: 527.6 ms, System: 9.6 ms]
  Range (min … max):   537.0 ms … 539.2 ms    10 runs

Benchmark 2: ./git show-branch origin/main origin/next origin/seen origin/todo
  Time (mean ± σ):     530.6 ms ±   0.4 ms    [User: 519.8 ms, System: 9.8 ms]
  Range (min … max):   530.1 ms … 531.3 ms    10 runs

Summary
  ./git show-branch origin/main origin/next origin/seen origin/todo ran
    1.01 ± 0.00 times faster than ./git_v2.52.0 show-branch origin/main origin/next origin/seen origin/todo

That number is not limited, though, and in pathological cases like the
one in p6010 we see a sizable improvement:

Test                      v2.52.0           HEAD
------------------------------------------------------------------
6010.4: git show-branch   2.19(2.19+0.00)   0.03(0.02+0.00) -98.6%

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index 441babf2e350f94477422e9d1381c7e2b21dd939..9e4efbaaed7f4dd7f77fa007883286156f093339 100644 (file)
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -18,6 +18,7 @@
 #include "commit-slab.h"
 #include "date.h"
 #include "wildmatch.h"
+#include "prio-queue.h"
 
 static const char*const show_branch_usage[] = {
     N_("git show-branch [-a | --all] [-r | --remotes] [--topo-order | --date-order]\n"
@@ -59,11 +60,10 @@ static const char *get_color_reset_code(void)
        return "";
 }
 
-static struct commit *interesting(struct commit_list *list)
+static struct commit *interesting(struct prio_queue *queue)
 {
-       while (list) {
-               struct commit *commit = list->item;
-               list = list->next;
+       for (size_t i = 0; i < queue->nr; i++) {
+               struct commit *commit = queue->array[i].data;
                if (commit->object.flags & UNINTERESTING)
                        continue;
                return commit;
@@ -222,17 +222,18 @@ static int mark_seen(struct commit *commit, struct commit_list **seen_p)
        return 0;
 }
 
-static void join_revs(struct commit_list **list_p,
+static void join_revs(struct prio_queue *queue,
                      struct commit_list **seen_p,
                      int num_rev, int extra)
 {
        int all_mask = ((1u << (REV_SHIFT + num_rev)) - 1);
        int all_revs = all_mask & ~((1u << REV_SHIFT) - 1);
 
-       while (*list_p) {
+       while (queue->nr) {
                struct commit_list *parents;
-               int still_interesting = !!interesting(*list_p);
-               struct commit *commit = pop_commit(list_p);
+               int still_interesting = !!interesting(queue);
+               struct commit *commit = prio_queue_peek(queue);
+               bool get_pending = true;
                int flags = commit->object.flags & all_mask;
 
                if (!still_interesting && extra <= 0)
@@ -253,8 +254,14 @@ static void join_revs(struct commit_list **list_p,
                        if (mark_seen(p, seen_p) && !still_interesting)
                                extra--;
                        p->object.flags |= flags;
-                       commit_list_insert_by_date(p, list_p);
+                       if (get_pending)
+                               prio_queue_replace(queue, p);
+                       else
+                               prio_queue_put(queue, p);
+                       get_pending = false;
                }
+               if (get_pending)
+                       prio_queue_get(queue);
        }
 
        /*
@@ -642,7 +649,8 @@ int cmd_show_branch(int ac,
 {
        struct commit *rev[MAX_REVS], *commit;
        char *reflog_msg[MAX_REVS] = {0};
-       struct commit_list *list = NULL, *seen = NULL;
+       struct commit_list *seen = NULL;
+       struct prio_queue queue = { compare_commits_by_commit_date };
        unsigned int rev_mask[MAX_REVS];
        int num_rev, i, extra = 0;
        int all_heads = 0, all_remotes = 0;
@@ -886,14 +894,14 @@ int cmd_show_branch(int ac,
                 */
                commit->object.flags |= flag;
                if (commit->object.flags == flag)
-                       commit_list_insert_by_date(commit, &list);
+                       prio_queue_put(&queue, commit);
                rev[num_rev] = commit;
        }
        for (i = 0; i < num_rev; i++)
                rev_mask[i] = rev[i]->object.flags;
 
        if (0 <= extra)
-               join_revs(&list, &seen, num_rev, extra);
+               join_revs(&queue, &seen, num_rev, extra);
 
        commit_list_sort_by_date(&seen);
 
@@ -1004,7 +1012,7 @@ out:
        for (size_t i = 0; i < ARRAY_SIZE(reflog_msg); i++)
                free(reflog_msg[i]);
        free_commit_list(seen);
-       free_commit_list(list);
+       clear_prio_queue(&queue);
        free(args_copy);
        free(head);
        return ret;

diff --git a/t/perf/p6010-merge-base.sh b/t/perf/p6010-merge-base.sh
index 54f52fa23ee1e7a637f10a8531d22448eb5e6d0c..08212dd0377db01e61d2d8425833b4d95a81b88f 100755 (executable)
--- a/t/perf/p6010-merge-base.sh
+++ b/t/perf/p6010-merge-base.sh
@@ -83,9 +83,9 @@ build_history2 () {
 test_expect_success 'setup' '
        max_level=15 &&
        build_history $max_level | git fast-import --export-marks=marks &&
-       git tag one &&
+       git branch one &&
        build_history2 $max_level | git fast-import --import-marks=marks --force &&
-       git tag two &&
+       git branch two &&
        git gc &&
        git log --format=%H --no-merges >expect
 '
@@ -98,4 +98,8 @@ test_expect_success 'verify result' '
        test_cmp expect actual
 '
 
+test_perf 'git show-branch' '
+       git show-branch one two
+'
+
 test_done