Re: [PATCH 3/9] t4141: fix inefficient use of dd(1)

[Patrick Steinhardt <ps@pks.im>]

On Thu, Jul 02, 2026 at 07:49:50PM +0200, SZEDER Gábor wrote:
> On Thu, Jul 02, 2026 at 02:00:56PM +0200, Patrick Steinhardt wrote:
> > In t4141 we generate a patch that is roughly 1GB in size to verify that
> > git-apply(1) indeed rejects that patch. We generate that patch by
> > prepending a patch header and then executing `test-tool genzeros`
> > without a limit. This causes us to print infinitely many zeros, and we
> > limit the overall amount of generated bytes via `test_copy_bytes`.
> > 
> > This test setup is extremely expensive, as `test_copy_bytes` is
> > implemented via `dd ibs=1 count="$1"`, which copies data one byte at a
> > time. So as we write 1GB of data, we end up doing 1 billion reads and
> > writes. This naturally takes a while: it takes 6 minutes on my system,
> > and around 40 minutes in some CI jobs!
> > 
> > We can do much better though, as genzeros already knows to handle an
> > optional limit of how much data it is supposed to write, which allows us
> > to remove the call to `test_copy_bytes`. Furthermore, it has already
> > been optimized to generate the data fast.
> > 
> > And indeed, doing this conversion drops the test execution to less than
> > a second on my machine, so that we can drop the EXPENSIVE prerequisite.
> 
> EXPENSIVE is not only about execution time, but about resources in
> general.  While the modified test finishes quite fast indeed, 'git
> apply' uses over 1GB of RSS.  Therefore, the EXPENSIVE prerequisite
> should be kept.

That's fair. It's questionable whether 1GB of RSS really is expensive
nowadays anymore, but I don't mind reinstating the prerequisite.

Patrick