Major Description
When a library (in cmake's sense) contains several sources with different paths but the same filename (for example, slave/validation.cpp and resource_provider/validation.cpp), the build generated by CMake for MSVC does not allow for building those files in parallel (presumably, because the .obj files will be located in the same directory).
This has been tested observed with both cmake 3.9 and 3.17, with "Visual Studio 15 2017 Win64" generator.
It seems to be a known behaviour - see https://stackoverflow.com/questions/7033855/msvc10-mp-builds-not-multicore-across-folders-in-a-project.
Two options for fixing this in a way that will work with these cmake/MSVC configurations are:
- splitting the build into small static libraries (a library per directory)
- introducing an intermediate code-generation-like step optionally flattening the directory structure (slave/validation.cpp -> slave_validation.cpp)
Both options have their drawbacks:
- The first will result in changing the layout the static build artifacts (mesos.lib will be replaced with a ton of smaller libraries), that will pose integration cahllenges, and potentially will result in worse parallelism.
- The second will result in being unable to use #include without a path (right now there are three or four such #include's in the whole Mesos/libprocess code buildable on Windows) in changed value of _FILE_ macro (as a consequence, in the example above, `validation.cpp` in logs will be replaced either with `slave_validation.cpp` or with `resource_provider_validation.cpp`)
Note that the second approach will need to deal with potential collisions when the source tree has filenames with underscores. If, for example, we had both slave/validation.cpp and slave_validation.cpp, then either some additional escaping will be needed or, alternatively, such layout could be just forbidden (and made to fail the build).
Preliminary testing shows that on a 8-core AWS instance flattening source trees of libprocess, mesos-protobufs and libmesos results in clean build speedup from ~1 hour to ~30 minutes.