[CMake] cmake + ninja how to use several CPU cores?

[Alan W. Irwin]

On 2019-07-28 23:39-0700 Alan W. Irwin wrote:

> @Both: I also plan to look at whether this issue exists for
> the internal library case so more later when I get that
> corresponding simple test project implemented.

@Brad and David:

I have now implemented a simple test project for the internal library
case.  See the attached tarball (and please let me know if your input
mail software chain allows you to receive that tarball).  In general I
think this test case improves somewhat on Dave's test project by
making the sleep more explicit at the CMake level, generating the
headers explicitly rather than including them in the tarball,
generating all header and source code files and the library (internal
in this case) within the build tree rather than source tree (which
considerably simplifies making a clean start), and by allowing the
user to specify the -DBUILD_SHARED_LIBS=ON (or OFF) cmake option to
choose whether the library is built shared on static.  (Of course,
none of these improvements should affect the delayed compilation
issue.)

For this new simple test project, here are the results for the -G"Ninja" case (when -DBUILD_SHARED_LIBS=ON is specified):

irwin at merlin> ninja -j16 -v app
[1/10] cd /home/irwin/David.Milter/20190728/test_ninja/add_internal_library/build_dir/lib && sh /home/irwin/David.Milter/20190728/test_ninja/add_internal_library/lib/include_generator.sh
[2/10] cd /home/irwin/David.Milter/20190728/test_ninja/add_internal_library/build_dir/lib && sh /home/irwin/David.Milter/20190728/test_ninja/add_internal_library/lib/lib_generator.sh
[3/10] cd /home/irwin/David.Milter/20190728/test_ninja/add_internal_library/build_dir && sleep 5
[4/10] /usr/bin/c++  -Dinternal_lib_EXPORTS -Ilib -fPIC -MD -MT CMakeFiles/internal_lib.dir/lib/lib1.cpp.o -MF CMakeFiles/internal_lib.dir/lib/lib1.cpp.o.d -o CMakeFiles/internal_lib.dir/lib/lib1.cpp.o -c lib/lib1.cpp
[5/10] /usr/bin/c++  -Dinternal_lib_EXPORTS -Ilib -fPIC -MD -MT CMakeFiles/internal_lib.dir/lib/lib3.cpp.o -MF CMakeFiles/internal_lib.dir/lib/lib3.cpp.o.d -o CMakeFiles/internal_lib.dir/lib/lib3.cpp.o -c lib/lib3.cpp
[6/10] /usr/bin/c++  -Dinternal_lib_EXPORTS -Ilib -fPIC -MD -MT CMakeFiles/internal_lib.dir/lib/lib4.cpp.o -MF CMakeFiles/internal_lib.dir/lib/lib4.cpp.o.d -o CMakeFiles/internal_lib.dir/lib/lib4.cpp.o -c lib/lib4.cpp
[7/10] /usr/bin/c++  -Dinternal_lib_EXPORTS -Ilib -fPIC -MD -MT CMakeFiles/internal_lib.dir/lib/lib2.cpp.o -MF CMakeFiles/internal_lib.dir/lib/lib2.cpp.o.d -o CMakeFiles/internal_lib.dir/lib/lib2.cpp.o -c lib/lib2.cpp
[8/10] : && /usr/bin/c++ -fPIC    -shared -Wl,-soname,libinternal_lib.so -o libinternal_lib.so CMakeFiles/internal_lib.dir/lib/lib1.cpp.o CMakeFiles/internal_lib.dir/lib/lib2.cpp.o CMakeFiles/internal_lib.dir/lib/lib3.cpp.o CMakeFiles/internal_lib.dir/lib/lib4.cpp.o   && :
[9/10] /usr/bin/c++   -Ilib  -MD -MT CMakeFiles/app.dir/main.cpp.o -MF CMakeFiles/app.dir/main.cpp.o.d -o CMakeFiles/app.dir/main.cpp.o -c ../main.cpp
[10/10] : && /usr/bin/c++     CMakeFiles/app.dir/main.cpp.o  -o app  -Wl,-rpath,/home/irwin/David.Milter/20190728/test_ninja/add_internal_library/build_dir libinternal_lib.so && :

Note that the build of main.cpp.o is (unreasonably in my opinion) delayed to step 9.

And that build is similarly delayed in the -G"Unix Makefiles" case.
That is this newly implemented test case for the internal library
build shows exactly the same unexplained compilation delays for
main.cpp for both -G"Ninja" and -G"Unix Makefiles" as David's test
case for the external library build.

@Brad and other CMake developers who might be lurking here:

Can anybody explain why CMake has this delayed compilation "feature"
for apps that depend on internal or external libraries, and if there
is no compelling reason for it would it be straightforward to remove
it?

Alan
__________________________
Alan W. Irwin

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.org); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
__________________________

Linux-powered Science
__________________________
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