parallel builds - From CMake setup 'make' to use '-j' option by default

Question

I want my CMake project to be built by make -j N, whenever I call make from the terminal. I don't want to set -j option manually every time.

For that, I set CMAKE_MAKE_PROGRAM variable to the specific command line. I use the ProcessorCount() function, which gives the number of procesors to perform build in parallel.

When I do make, I do not see any speed up. However if I do make -j N, then it is built definitely faster.

Would you please help me on this issue? (I am developing this on Linux.)

Here is the snippet of the code that I use in CMakeList.txt:

include(ProcessorCount)
ProcessorCount(N)
message("number of processors: "  ${N})
if(NOT N EQUAL 0)
  set(CTEST_BUILD_FLAGS -j${N})
  set(ctest_test_args ${ctest_test_args} PARALLEL_LEVEL ${N})
  set(CMAKE_MAKE_PROGRAM "${CMAKE_MAKE_PROGRAM} -j ${N}")      
endif()
message("cmake make program" ${CMAKE_MAKE_PROGRAM})

Thank you very much.

Answer 1

In case you want to speed up the build you can run multiple make processes in parallel but not cmake. To perform every build with predefined number of parallel processes you can define this in MAKEFLAGS.

Set MAKEFLAGS in your environment script, e.g. ~/.bashrc as you want:

export MAKEFLAGS=-j8

On Linux the following sets MAKEFLAGS to the number of CPU - 1: (Keep one CPU free for other tasks while build) and is useful in environments with dynamic ressources, e.g. VMware:

export MAKEFLAGS=-j$(($(grep -c ^processor /proc/cpuinfo) - 1))