[WIP] spdlog for multi-level multi-sink logging

[C0nsultant]

TBD

Close #461.

To Do

• push implementation
• use spdlog 1.4+ and include link compiled .cpp file instead of increasing compile-time via header-only lib?
• Docs:
  • README.md
  • docs/source/dev/buildoptions.rst
  • docs/source/dev/dependencies.rst

[ax3l]

Python 3 is only needed if the python bindings are build

[C0nsultant]

Well in this case, we require Python due to run-clang-format.py. It's 2019 already and I'd say it's time to leave 2.7 behind altogether.

[ax3l]

Ok, then you only need to change the entries in the matrix that use run-clang-format.py if it is not backwards compatible :)
But I think all of this can be done in another PR and is independent of this change, no?
update: just saw the comment above, yep.

[ax3l]

We can do the control this way, but it's slightly different as the other dependencies then.

For example, if the openPMD_USE_XY is AUTO and set to EXTERNAL and not found, this will not fail in our current logic. Here it would.

[C0nsultant]

Shamelessly copied from Pybind11 as that probably is the most consistent with the other CMake _USE_ logic.

[ax3l]

Yes, I just wonder if we should couple it to the Debug build logic since we will use log levels on various intensity also in Release builds and vice versa, might want to disable it for some Debug runs.

So maybe just handle it's default activation just like any other functional extension? Just a thought.

[ax3l]

little typo:

Suggested change

	In order to enable this feature unconditionally, pass ``-DopenPMD_USE_LOGGING=ON`` or ``-DopenPMD_USE_LOGGING=OFF`` to disable it unconditinoally.
	In order to enable this feature unconditionally, pass ``-DopenPMD_USE_LOGGING=ON`` or ``-DopenPMD_USE_LOGGING=OFF`` to disable it unconditionally.

[ax3l]

can we also add a "timing" mode in this or another level? "throughput" (bytes/s) would also be awesome.
cc @franzpoeschel

[C0nsultant]

Interesting idea. That would mean we count bytes during all backend IO and take timestamps in the fronend triggering calls.

Intuitively, this would result in a different code structure to the other log facilities.

auto series = openPMD::Series("data%T.h5", openPMD::AccessType::CREATE);
RecordComponent ideas = ...

std::shared_prt< double > patch = ...

// assume DEFER
ideas.store_chunk(patch, {0}, {patch_size});
LOG_TIME(series.flush());     /* example.cpp:8 series.flush() took 0.314s */

std::array< std::shared_prt< double >, 42 > lots_of_patches = ...
for( auto patch : lots_of_patches )
    ideas.store_chunk(patch, ...);
LOG_THROUGHPUT(series.flush());  /* example.cpp:13 series.flush() averaged 314MB/s */

I'll see what I can do. This will be pretty invasive, though.

[ax3l]

Doesn't need to go into the first log version, we can add those in a follow-up if you like.