Re: [PATCH 0/8] Makefile: make command-list.h 2-5x as fast with -jN

On Fri, Oct 22, 2021 at 12:51:11PM +0200, Ævar Arnfjörð Bjarmason wrote:

Yes, the same numbers are noted in the 7/8 commit message. I.e. it's
slower on -j1, but faster with higher -j<n> numbers.

Aside from any changes I'm proposing here it seems rather pointless to
me to optimize the runtime of -j1 runs.

I think we use those in e.g. CI, so of course if they become *really*
slow it will matter, but the purpose of this change is to make hacking
on git easier, both in terms of runtime and discovering what the
Makefile is doing wih V=1.

I think anyone hacking on git is going to be on a system with -j2 at
least. So again, separate from these specific changes, if we've got a
change that speeds up -jN runs at the cost of a -j1 run that seems like
good thing.

It seems weird to me to assume that all of our processors are available
to build command-list.h. In most cases you are not running "make -j16
command-list.h", but rather "make -j16", and those other processors are
doing useful things, like say, building actual C code.

So counting CPU time is the interesting thing, because every cycle you
save there gets used for other work. And "make -j1" just brings
wall-clock and CPU time together.

-Peff