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Subject: Re: Hint to Ada newbies
From: Hadrien Grasland <hadrien.grasland@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:33322
Date: 2017-02-11T01:25:42-08:00
List-Id: <comp.lang.ada>

Le vendredi 10 f=C3=A9vrier 2017 10:56:53 UTC+1, Simon Wright a =C3=A9crit=
=C2=A0:
> Robert A Duff writes:
>=20
> > Simon Wright writes:
> >
> >> I have had to use the power switch to stop a build using -j0! '-j<1 le=
ss
> >> than the number of cores>' does pretty well.
> >
> > Which operating system?
>=20
> macOS (actually a few releases ago, Mavericks maybe). And it's just
> possible it was the equivalent switch in Make.

There's actually a major difference between gnatmake's -j0 and make's -j sw=
itches.

gnatmake -j0 means "Try to keep all my CPU cores busy", i.e. use 4 processe=
s on a 4-core machine. This is reasonable, as long as you have enough RAM t=
o handle it.

make -j means "Spawn new processes as soon as you can", i.e. use an unbound=
ed amount of processes. Not only is this strategy stupid (your build will a=
ctually run slower if too many processes compete for system resources), it =
is also the easiest way to get your machine to run out of memory.

make -l ("Do not spawn new processes if the CPU load is higher than x") was=
 an attempt to address this broken design, which fails because it cannot ha=
ndle the system load inertia: stating a new process may only increase the C=
PU load a couple of seconds later, and this is enough for make to spawn ton=
s of build processes in meantime.