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Re: -j/-l : minimum of jobs running under max-load with auto = cpu+1 def
Re: -j/-l : minimum of jobs running under max-load with auto = cpu+1 default
Mon, 21 May 2018 08:23:16 -0400
On Mon, 2018-05-21 at 08:36 +0200, Garreau, Alexandre wrote:
> Then I discovered --load-average, and I’m asking myself wether the
> optimum is -j n+1, -l 1.0, or -l n or n+1?
IMO, there are too many problems with choosing any value as the default
* It's not so simple to determine the number of CPUs, portably.
* Many makefiles invoke commands that themselves are multi-threaded
and for these commands you definitely don't want to choose a -j
value with "# of CPUs" parallelism.
* Many users of makefiles want to do other things with their systems
in addition to builds, and don't want to choose a -j or -l value
that makes their system slow or unusable. They'd rather their
builds take longer.
* There are other metrics besides # of CPUs that need to be taken into
consideration; for example memory. I have a build environment which
builds 200 different C++ programs, each of which ends up to be 200M
or so, and the linker takes huge amounts of memory. If I use -j8 on
my 8-core system and the rebuild only needs to re-link, my entire
system will hang for many minutes swapping RAM (I can't even move
the mouse), even though I have enough CPUs. If I choose -j5 or -j6,
it works much better.