Re: [PATCH 1/2] t4205: compare huge output without diff

On Thu, Sep 24, 2026 at 2:13 AM Patrick Steinhardt [off-list ref] wrote:

On Wed, Sep 23, 2026 at 01:13:28PM -0400, Tamir Duberstein wrote:

quoted

The huge-commit test compares two files with a line larger than 2 GiB.
In Linux GitHub Actions jobs, git log produces its huge output but
its subsequent diff process is killed with SIGKILL.

I've never seen that failure before. Do you maybe have a link to it?

The failures happened on a private repo that I've since lost access to
- but I believe it was precipitated by GitHub runners having half the
memory in private repos as in public ones [1].

quoted

Use test_cmp_bin to compare the output byte for byte without constructing
a line-oriented diff. Remove the two large files after a successful
comparison, releasing more than 4 GiB before subsequent tests.

It would be great to back up the claim that test_cmp_bin is better than
test_cmp, e.g. by comparing peak RSS and its runtime.

As for the comparison: on Linux arm64 with GNU
diffutils 3.8 using two identical files containing 2,147,483,649 "1" bytes
followed by "0\n" (matching this test's expected output) gave:

Command              Mean +/- stddev       Maximum RSS (KiB)
diff -u expect actual  5.276 +/- 0.572 s              4199924
cmp expect actual      0.506 +/- 0.099 s                 1264

quoted

Signed-off-by: Tamir Duberstein <redacted>
---
 t/t4205-log-pretty-formats.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 4be5c51489..6279a7e9bc 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh

@@ -1189,7 +1189,8 @@ test_expect_success EXPENSIVE,SIZE_T_IS_64BIT 'set up huge commit' '
 test_expect_success EXPENSIVE,SIZE_T_IS_64BIT 'log --pretty with huge commit message' '
      git log -1 --format="%B%<(1)%x30" $huge_commit >actual &&
      echo 0 >>expect &&
-     test_cmp expect actual
+     test_cmp_bin expect actual &&
+     rm expect actual
 '

Hm. Sure, releasing these files isn't a bad idea by itself. But we
rewrite "expect" in the next test anyway, and "actual" will be rewritten
two tests further down. So does it really buy us that much...?

You're right, this probably does not buy much.

Would you like me to include the performance comparison in v2? As for
the deletion: would you prefer I drop it?

Link: https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for--private-repositories
[1]