[CMake] Tips on reducing configure time

[Eric Noulard]

Le mer. 14 nov. 2018 à 13:25, Poughon Victor <Victor.Poughon at cnes.fr> a
écrit :

> Yes you are correct on all your observations. We already use ninja and
> ccache wherever we can. In fact we have an issue about the whole end-to-end
> build performance where we track all effort on this throughout the project
> (if you're interested:
> https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb/issues/1649)
>

End-to-end build performance is a subject of interest to me, I'll have a
look.


> There are indeed quick and easy wins to be had with those tools (and we
> are working on it). But my original question is not about that. It's about
> speeding up the configure step with makefile generator. I still don't think
> it's normal that it does hundreds of thousands of I/O on files that are a
> few bytes or even empty. However it's possible that it's because we do
> something incorrect in our CMakeLists.txt and not CMake's fault.
>

I don't really know makefile-generator internals so I cannot tell.
For sur the current OTB build dir constructed with Makefile generator is
spitting out around 180 Makefile.
May be you can profile the cmake execution by building a debug version of
CMake and collect more precise insight on where the bottlenecks may reside.

You may track try_compile which could obviously be slow and may be
configure_file as well.
Otherwise I don't know, I guess you'll have to profile the cmake run.

It's true that a few minutes of configure is not much when doing a full
> build, but consider incremental builds where all you do is change one cxx
> file (and cmake is triggered because you changed git branch or something).
> Then the generate step is a significant part of the time you wait.
>

Yes that right.
Note however that changing git branch should nor per-se trigger a cmake run.
Only changing a CMakeLists.txt or configured file or explicitely specified
by
CMAKE_CONFIGURE_DEPENDS.

Otherwise changing branch with git that only modifies source shouldn't
trigger cmake rebuild.
I do that all the time in a classical git branch dev model.



-- 
Eric
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