Question

I am using GCC 4.7 and compiling my C++ code-project. The code consists of files distributed in directories.

On RHEL server I use, there are 16 cores, still the compilation speed is quite slow. Can you suggest possible alternatives or options of makefile, which may help in compiling fast. I have tried -j but it only compiles some folders and stops; does not compile the main binary.

I will be grateful for any help.

Answer 1

if your makefile fails when you compile with -j but works fine without, then you may need to fix your makefile to work properly with parallel compilation. Otherwise, those other 15 cores are of no use to you.

It’s not uncommon for less experienced makefile writers to write something like:

final: step1 step2 step3

to mean “to build final, first build step1, then step2, then step3”. This works fine when you are running with the default setup of -j 1 because make happens to build each of the dependencies in left-to-right order. But if you use -j 20 (say) then it will attempt to build them in parallel. It will attempt to start building all 3 steps at once, without first waiting until each successive step is complete.

The correct way to write this is:

final: step3
step3: step2
step2: step1

This tells make exactly what’s happening: to build final you first need to build step3, for which you need step2, for which you need step1.