Re: [PATCH v2] t/perf/p3400: speed up setup using fast-import

[Phillip Wood <phillip.wood123@gmail.com>]

On 28/01/2026 16:07, Tian Yuchen wrote:
> 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 in a single stream. 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:
>            Real         Rebase
>    Before: 29.045s      13.34s
>    After:  22.231s      12.78s

That's a nice speedup in the test setup

> diff --git a/t/perf/p3400-rebase.sh b/t/perf/p3400-rebase.sh
> index e6b0277729..9f4251aed6 100755
> --- a/t/perf/p3400-rebase.sh
> +++ b/t/perf/p3400-rebase.sh
> @@ -9,25 +9,47 @@ 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 &&
> +	test_seq 1000 | sort -nr >content_rev &&
> +
> +	(
> +		for i in $(test_seq 100)
> +		do
> +			echo "commit refs/heads/upstream" &&
> +			echo "committer WGYDY <author@mock.com> $i +0000" &&

You can keep the same author and committer as the original with

	test_tick &&
	echo "author $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> $GIT_AUTHOR_DATE" &&
	echo "committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 
$GIT_COMMITTER_DATE" &&

here and below

> +			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 WGYDY <author@mock.com> $i +0000" &&
> +			echo "data <<EOF" &&
> +			echo "commit$i-reversed" &&

The commit message in the original is "commit$i-reverse", not "reversed"

> +			echo "EOF" &&
> +			echo "M 100644 inline unrelated-file$i" &&
> +			echo "data <<EOF" &&
> +			echo "change$i" &&
> +			cat content_rev &&
> +			echo "EOF" || return 1
> +		done

As Johannes pointed out we'll ignore the any failure above. We can 
address that by adding "echo done" here and adding "--done" to "git 
fast-import" below. That will cause "git fast-import" to fail because if 
there is an error in the loop as the last line of input to fast-import 
will not be "done"

Thanks

Phillip

> +	) | git fast-import &&
> +
> +	git repack -a -d &&
> +	git checkout -f upstream &&
>   	git checkout to-rebase &&
>   	test_commit our-patch interesting-file
>   '
> -
>   test_perf 'rebase on top of a lot of unrelated changes' '
>   	git rebase --onto upstream HEAD^ &&
>   	git rebase --onto base HEAD^