c - Using make with -j4 or -j8

Question

I have 4 processors and am compiling a processor-hungry application. I read that using make with the -j4 switch was recommended for OpenCV; should I instead use -j8? What is the advantage of making for multiple processors?

Answer 1

The answers above are all mostly correct. However, the details are a bit misleading. For example, there's no need to add an extra job for a "managing thread" (note: make is not actually multithreaded). make never counts itself as a job for the purposes of -j, so, as Huygens says above, if you say -j5 you'll get 5 compile jobs running, not 4 plus make.

The reason most people use [number of cores] + [some padding] has nothing to do with make or what it needs, but rather with the nature of the compiler. A compiler is really just a very complicated text translation tool: it reads in text in one form and converts it to "text" (binary) in another form. A lot of this (especially as your language gets more complex, like C++), requires a lot of CPU. But it also requires a lot of disk I/O. Disk I/O is slow, so while one compiler is waiting for some data from the disk, the kernel schedules other jobs to run. That is why you can usefully have more than the number of cores compiles running at the same time.

Exactly how large you can get -j before you start seeing diminishing returns (your builds actually start going slower, at some point, with more -j) depends completely on your hardware, the kinds of builds you're doing, etc. The only way to know for sure is experimentation.

However, [number of cores]+[a few] is typically a good approximation.