t/perf/p3400: speed up setup using fast-import

The setup phase in 't/perf/p3400-rebase.sh' generates 100 commits to
simulate a noisy history. It currently uses a shell loop that invokes
'git add', 'git commit', 'test_seq', and 'sort' in each iteration.
This incurs significant overhead due to repeated process spawning.

Optimize the setup by using 'git fast-import' to generate the commit
history. Additionally, pre-compute the forward and reversed file contents
to avoid repetitive execution of 'seq' and 'sort'.

To ensure the test measures rebase performance against a consistent
object layout (rather than the suboptimal pack/loose objects created
by the raw import), perform a full repack (`git repack -a -d`) at the
end of the setup.

This reduces the setup time significantly while maintaining the validity
of the subsequent performance tests.

Performance enhancement (Average value of 5 tests):
            Real        Rebase
  Before:  29.045s      13.34s
   After:  21.989s      12.84s

Measured on Lenovo Yoga 2020, Ubuntu 24.04.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/perf/p3400-rebase.sh b/t/perf/p3400-rebase.sh
index e6b0277729b19631ad49740070b092ff08121972..56dc3e1d319c7d990ae1a9b52f1a1671105e5aa0 100755 (executable)
--- a/t/perf/p3400-rebase.sh
+++ b/t/perf/p3400-rebase.sh
@@ -9,21 +9,44 @@ test_expect_success 'setup rebasing on top of a lot of changes' '
        git checkout -f -B base &&
        git checkout -B to-rebase &&
        git checkout -B upstream &&
-       for i in $(test_seq 100)
-       do
-               # simulate huge diffs
-               echo change$i >unrelated-file$i &&
-               test_seq 1000 >>unrelated-file$i &&
-               git add unrelated-file$i &&
-               test_tick &&
-               git commit -m commit$i unrelated-file$i &&
-               echo change$i >unrelated-file$i &&
-               test_seq 1000 | sort -nr >>unrelated-file$i &&
-               git add unrelated-file$i &&
-               test_tick &&
-               git commit -m commit$i-reverse unrelated-file$i ||
-               return 1
-       done &&
+       test_seq 1000 >content_fwd &&
+       sort -nr content_fwd >content_rev &&
+       (
+               for i in $(test_seq 100)
+               do
+                       test_tick &&
+                       echo "commit refs/heads/upstream" &&
+                       echo "committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE" &&
+                       echo "data <<EOF" &&
+                       echo "commit$i" &&
+                       echo "EOF" &&
+
+                       if test "$i" = 1; then
+                               echo "from refs/heads/upstream^0"
+                       fi &&
+
+                       echo "M 100644 inline unrelated-file$i" &&
+                       echo "data <<EOF" &&
+                       echo "change$i" &&
+                       cat content_fwd &&
+                       echo "EOF" &&
+
+                       echo "commit refs/heads/upstream" &&
+                       echo "committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE" &&
+                       echo "data <<EOF" &&
+                       echo "commit$i-reverse" &&
+                       echo "EOF" &&
+                       echo "M 100644 inline unrelated-file$i" &&
+                       echo "data <<EOF" &&
+                       echo "change$i" &&
+                       cat content_rev &&
+                       echo "EOF" || exit 1
+               done
+       ) >fast_import_stream &&
+
+       git fast-import <fast_import_stream &&
+       git repack -a -d &&
+       git checkout -f upstream &&
        git checkout to-rebase &&
        test_commit our-patch interesting-file
 '