From: Jeff King <peff@peff.net>
Date: Fri, 25 Sep 2026 00:13:32 GMT
Subject: Re: [PATCH 1/2] t4205: compare huge output without diff
Message-ID: <20260925001332.GA1408107@coredump.intra.peff.net>
In-Reply-To: <CAJ-ks9kJWc0e7aEX4vAL-RoJ5kVvfjDABqV86_hZf2Fn-085GA@mail.gmail.com>

On Thu, Sep 24, 2026 at 04:41:05PM -0400, Tamir Duberstein wrote:

> > 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

Yeah, that's a big difference. This is probably an outlier because of
the giant files, but I've wondered if test_cmp() ought to be doing
something like:

  # first check quickly if there is any difference
  cmp "$@" && return 0
  # if not, then we can spend time to produce a useful output
  diff "$@"

But I never pursued it because I figured most of the things we compare
are not big enough to matter (and really the comparison itself is
dwarfed by the process startup time).

So I am happy just marking this particular case as cmp_bin, too.

-Peff