Parallel commands in cmake

Question

I have a (simplified) CMAKE file like so:

cmake_minimum_required( VERSION 3.6 ) function(BUILD_SIMULINK model) set(OUTPUT_FILE ${model}.dll) set(SOURCE_FILE ${model}.slx) set(EXECUTE_COMMAND matlab -nojvm -nodisplay -nodesktop -nosplash -wait -r "rtwbuild(${model})" ) add_custom_target( ${model} ALL DEPENDS ${OUTPUT_FILE} ) add_custom_command( COMMAND ${EXECUTE_COMMAND} DEPENDS ${SOURCE_FILE} OUTPUT ${OUTPUT_FILE} ) install( TARGETS ${OUTPUT_FILE} DESTINATION lib ) endfunction(BUILD_SIMULINK) project(SimulinkBuild VERSION 1.0.0) build_simulink( model1 ) build_simulink( model2 ) build_simulink( model3 ) ... build_simulink( modeln ) # Arbitrarily large number of models

My problem is that it takes several hours to do a clean build because I am calling one command at a time, using only one core at a time. I'd like to build my files in parallel, taking advantage of the multiple cores on my machine. How can we achieve executing parallel commands from cmake?

When I build I do this:

cmake .. cmake --build . --target install

Build is fully controlled by the build tool, against which you configure the CMake project. So it depends whether your build tool supports parallel builds. E.g., make has -j option for that case. — Tsyvarev
– Tsyvarev, Commented Jun 8, 2017 at 11:56
I use cmake --build to invoke the build too. I think I just pass cmake --build . --target install --j4 when using make. However, I'm using visual studio to test this right now which requires a /MP directive in the command-line options. I'm hoping that there is a tool-independent way to do both. — Stewart
– Stewart, Commented Jun 8, 2017 at 12:14
Visual Studio (the generator) doesn't appear to support one of these options. The closest thing is MSVC supports a /MP switch, but I'm not using MSVC, I'm executing my own commands so this doesn't really work. — Stewart
– Stewart, Commented Jun 8, 2017 at 12:25
Nevermind, I'm wrong. You're a genius! The solution is to run: cmake --build . --target install -- /m for visual studio and cmake --build . --target install -- j for make/ninja. This solves my issue. Though it would still be nice to have a generator-independant solution. — Stewart
– Stewart, Commented Jun 8, 2017 at 12:37

Stewart · Accepted Answer · 2017-06-08 15:21:16Z

1

The solution is how to invoke cmake, and isn't part of the cmake file itself.

cmake .. cmake --build . -- /m # Visual Studio cmake --build . -- -j # Gnu make

answered Jun 8, 2017 at 15:21

Stewart

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2 Comments

Florian Over a year ago

Just wanted to hint to blog.kitware.com/cmake-building-with-all-your-cores

KRoy Over a year ago

Be aware of race conditions when doing targets in parallel that depend on same library.

johnb003 · Accepted Answer · 2018-07-20 08:21:44Z

In CMake 3.12 support was added so that regardless of what generator you used, you can still kick off parallel builds (if supported) with:

cmake --build . --target install -j <number of threads>

But for earlier versions of cmake, any arguments after -- are passed to the build system being used. So for Makefiles:

cmake --build . --target install -- -j <number of threads>

Or for msvc builds:

cmake --build . --target install -- /m <number of threads>

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