> On May 23, 2023, at 5:13 AM, Zoltán Turányi <zoltan.tura...@ericsson.com>
> wrote:
>
> Hello,
>
> I have looked through the mail archives and the manual (to some degree of
> thoroughness) and have not found this idea yet. If, it exists, I apologize
> for the spam in advance.
>
> I use make with autotools in multiple directories and have observed that
> parallel builds are limited to each directory, as autotools invoked make
> separately for each directory.
This is not a requirement of autotools. Instead, it's a common mistake made by
users of automake.
Re-running "make" in each subdirectory is called "recursive make". Recursive
make *seems* reasonable,
but it's terrible. What it *means* is that each make has incorrect (incomplete)
information.
In *practice* this produces makefiles that are fragile, do the wrong thing, and
take a long time to run. See:
"Recursive Make Considered Harmful" by Peter Miller, e.g.:
https://accu.org/journals/overload/14/71/miller_2004/
I discuss how to avoid recursive make (and what to do instead) in my autotools
tutorial:
https://www.youtube.com/watch?v=4q_inV9M_us
The solution is to *NOT* use recursive make. Have *ONE* process run the
makefile, with the correct data.
Now you can enable parallel jobs, and have it run really quickly, because the
make process has the
correct information. Using this approach you can routinely run large make jobs
in a fraction of a second.
You don't need to rig make to work with a bad design; change the design.
--- David A. Wheeler
