Why people recommend the -j3 option for make when having a dual-core CPU?

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http://unix.stackexchange.com – In Gentoo Linux it is possible to set the MAKEOPTS variable in /etc/portage/make.conf to tell make how many jobs it should run in parallel when building packages. Since I have a dual-core CPU, I naively chose to use the -j2 option: one job per core, so both have something to do. The "problem" is there are a lot of references that tell users having a dual-core CPU to set the -j3 option instead. Some of them are: Gentoo handbook Gentoo wiki make.conf(5) man page For example, the Gentoo handbook says: A good choice is the number of CPUs (or CPU cores) in your system plus one, but this gui (HowTos)