Enable CMake parallel build

[mvhulten]

For components using the CMake build tool; tested with ParFlow.

Four threads appears to be the sweet spot. Walltimes (ParFlow only):

-j 1 -> 164 s
-j 2 -> 100 s
-j 4 -> 64 s
-j 8 -> 50 s

[mvhulten]

I don't know how to do this for ParFlow alone, but it should not break building of any other component as it is CMake native.

Maybe CI is also faster now. Can this be checked?

[kvrigor]

Not a good idea to hardcode --parallel 4 as it would pose a problem for coupled builds (see #62). Instead what I'd suggest is to parameterize the # of build threads, e.g.:

# default.env should provide a default setting for TSMP2_PARALLEL_JOBS.
# Either default=1 or a machine-specific default value.
export TSMP2_PARALLEL_JOBS=4

# Should be aware of TSMP2_PARALLEL_JOBS setting
./build_tsmp2.sh ...

This way you can still enable parallel builds while keeping the default build behavior to a known working state (i.e. -j1).

[mvhulten]

Firstly, I don't see what TSMP2_PARALLEL_JOBS does (assuming we're not going to write somewhere later --parallel $TSMP2_PARALLEL_JOBS, because then I'm missing the point).

Then, I suppose it is still not good enough, because your amended solution would mean that we have to test anyway if "-j 4" works on JSC machines. This is where I want to use it, at least sometimes.

I suppose, by default it must everywhere be "-j 1", and we could introduce build_tsmp2.sh --jobs and then add to the build command --parallel $jobs.

[mvhulten]

• It's important here to squash all commits
• There is the -j 8 of ICON to consider, but I don't think it matters (just in that the --help is lying)

[kvrigor]

assuming we're not going to write somewhere later --parallel $TSMP2_PARALLEL_JOBS

I was thinking to use TSMP2_PARALLEL_JOBS to parallelize builds within Build*.cmake scripts. Currently each component model uses a hardcoded -j setting. These hardcoded settings can be replaced by reading from the TSMP2_PARALLEL_JOBS env var.

Then, I suppose it is still not good enough, because your amended solution would mean that we have to test anyway if "-j 4" works on JSC machines. This is where I want to use it, at least sometimes.

It makes more sense to set the optimal TSMP2_PARALLEL_JOBS per machine thru environment files. If we do it this way, the default env file should set a conservative setting of TSMP2_PARALLEL_JOBS=1.

[kvrigor]

CMAKE_BUILD_PARALLEL_LEVEL can trigger intercomponent parallel builds which we don't want (see #62)