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

> On Thu, Sep 24, 2026 at 2:13 AM Patrick Steinhardt <ps@pks.im> wrote:
> >
> > On Wed, Sep 23, 2026 at 01:13:28PM -0400, Tamir Duberstein wrote:
> > > 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].

> > > 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

> > > Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> > > ---
> > >  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?