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

[Patrick Steinhardt <ps@pks.im>]

On Thu, Jul 02, 2026 at 05:16:14PM -0400, Jeff King wrote:
> On Thu, Jul 02, 2026 at 02:00:56PM +0200, Patrick Steinhardt wrote:
> 
> > 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.
> 
> Seems like a good fix for this case, where we can skip the extra process
> entirely.
> 
> It feels like test_copy_bytes should be able to do much better in
> general. The obvious thing to reach for is "head -c", but the function
> was originally added because that wasn't portable. The "-c" option is
> not in POSIX, though the original comment claims IRIX was the problem,
> so I wonder if "head -c" is de facto portable these days.

An alternative could be to implement a simple helper as part of our
test-tool. But I doubt it's really worth it: almost all callers only
want to copy a small number of bytes. The only exception seems to be
t0021, where we copy up to 65kB. But that whole test suite still only
takes ~3 seconds, so optimizing that feels like wasted time to me.

Patrick