How many jobs should i start when using make?

I wonder what number of jobs would be the most effective one when i run 开发者_如何学编程'make -j x'. Is it the number of cores or would it be better to use even more jobs?

A good value is number of CPUs + 1.

So if you have a recent quadcore CPU which has hyperthreading, use 9. If you have a dualcore without hyperthreading, use 3.

I've seen recommendations for -j(2N+1) where N is the number of cores. I have no idea where the number came from, sorry.

I have a 3rd Generation Intel Core i7 (quad-core) on my laptop. I realized that make -j5 does not take advantage of all the parallelism the i7 can offer.

make -j9, however, does not go 100% cpu usage either while compiling most packages. Compiling the GNU kernel is way faster here (and uses 100% cpu) if I do something bigger than 10, like make -j18 or something.

I guess it's trial and error.