Automatically setting jobs (-j) flag for a multico

Question

I have a Makefile on a machine that has a ton of cores in it, but I always seem to forget to write -jX when compiling my project and it takes way longer than it should.

Is there some way I can set the -j flag through an environment variable or some other persistent config file so that make will automatically execute multiple jobs in parallel on this machine?

Answer 1

I would just put

alias make='make -j'

in ~/.profile or ~/.bashrc.

According to the manual:

If there is nothing looking like an integer after the ‘-j’ option, there is no limit on the number of job slots.

Answer 2

As Jeremiah Willcock said, use MAKEFLAGS, but here is how to do it:

export MAKEFLAGS="-j $(grep -c ^processor /proc/cpuinfo)"

or you could just set a fixed value like this:

export MAKEFLAGS="-j 8"

If you really want to boost performance you should use ccache by adding something like the following to your Makefile:

CCACHE_EXISTS := $(shell ccache -V)
ifdef CCACHE_EXISTS
    CC := ccache $(CC)
    CXX := ccache $(CXX)
endif

Answer 3

You can add a line to your Makefile similar to the following:

NUMJOBS=${NUMJOBS:-" -j4 "}

Then add a ${NUMJOBS} line in your rules, or add it into another Makefile var (like MAKEFLAGS). This will use the NUMJOBS envvar, if it exists; if it doesn't, automatically use -j4. You can tune or rename it to your taste.

(N.B.: Personally, I'd prefer the default to be -j1 or "", especially if it's going to be distributed to others, because although I have multiple cores also, I compile on many different platforms, and often forget to dis-able the -jX setting.)

Answer 4

I usually do this as follows in my bash scripts:

make -j$(nproc)

Answer 5

Aliases are not expanded inside scripts. It's better to create a separate make script and place it into one of the $PATH directories:

#!/bin/sh

if [ -f /proc/cpuinfo ]; then
    CPUS=`grep processor /proc/cpuinfo | wc -l`
else
    CPUS=1
fi
/usr/bin/make -j`expr $CPUS + 1` "$@"

Answer 6

On Ubuntu 16.4 using all CPU cores:

export MAKEFLAGS='-j$(nproc)'

or

export MAKEFLAGS='-j 2'