Re: Are prerequisites made in deterministic order when parallelism is disabled?

[Henrik Carlqvist]

On Wed, 14 Jun 2017 11:25:35 -0400
Kyle Rose <address@hidden> wrote:

> The right answer is always to write your makefiles so the rules execute
> in the required order whether run in parallel or not. Relying on
> whichever arbitrary ordering GNU make employs, even if that behavior is
> stable(either historically or by design), is likely to result in sadness
> at some point, such as when someone calls into your makefile recursively
> from their own -j build.

Sometimes I write Makefiles considering the performance at parallel builds
and those Makefiles get their prerequisites ordered by approximately how
much time each prerequisite needs to be built. Let me give you an example
of such a rule:

final_target: 3_hour_prerequisite 2_hour_prerequisite 1_hour_prerequisite
        do_something_quickly

With the above example calling make without -j will take about 6 hours for
all prerequisites to build. On a machine with 3 or more cores calling make
with "-j 3" all prerequisites will be finished in 3 hours when also the
most time consuming one is done.

But what if we have a build machine with only 2 CPU cores? If so, calling
make with "-j 2" will be the fastest way to compile, it will still be done
in 3 hours. First the 3 hour job and the 2 hour job will be started in
parallell, then after 2 hours one job will be finished and the remaining 1
hour job will be started. After a total of 3 hours the 3 hour job and 1
hour job will finish about at the same time.

What would happen with "-j 2" if GNU Make changed the order of how
prerequisites where compiled? Then it might start with the 1 hour job and
the 2 hour job. Not until the 1 hour job where finished the 3 hour job
would start and the total build time would be 4 hours instead.

Even though the compile would complete successfully I think that not being
able to depend on job start order would give a significant lack of
performance.

I can also come to think of examples when non parallel builds would
benefit from a deterministic build order of prerequisites even though none
of the prerequisites depend on each other.

regards Henrik