[CMake] ExternalProject and redundant update/configure/compile actions.

[Jean-Christophe Fillion-Robin]

Hi David,

I should have specify the context  ...

..for example, in the case you change the SHA1 associated with your
external project. If update is disabled, doing a full rebuild will take
longer than simply doing an update + incremental build.  Especially if
other projects depend on the fully rebuild project.

With CMake 2.8.8 and lower, wihtin Slicer we had to disable the UPDATE
command since it wasn't working reliably. By requiring CMake 2.8.9, things
seems to work better on all platform. See [1], [2] and [3]

Thanks
Jc


[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=20615

[2]
http://cmake.org/gitweb?p=cmake.git;a=blobdiff;f=Modules/ExternalProject.cmake;h=1997572196997614f152bc7da886beffb85f01ab;hp=301d1fc4d50d850d5abe219eab3a230d55769910;hb=987c017f6b1b49ab832c244166e30e07260432da;hpb=8bb46ee787828c75b825d72b7121296d91bef7d6

[3] http://cmake.org/gitweb?p=cmake.git;a=commit;h=99f6055fe4f047


On Tue, Sep 17, 2013 at 6:06 PM, David Cole <dlrdave at aol.com> wrote:

>  Some background about the Slicer build system that inspired
>> the NamicExternalProjects system discussed by Kent  ...
>>
>> Before CMake 2.8.8, we disabled the UPDATE of external project
>> because it was not working well with Visual Studio. While this was
>> working as expected, the overall build time was also increased
>> because it wasn't doing an incremental rebuild.
>>
>
> The build time increased compared to what?
>
> A second build, in a no update situation, should yield "no work to do"
> because nothing got updated.
>
>
>
>  As of today, we re-enabled the UPDATE part of external project
>> and also require CMake >= 2.8.9 and it works quite well. The
>> overall build time is largely decreased.
>>
>
> Again, I don't understand how updating as part of the superbuild could
> ever result in a decreased build time...
>
>
> What am I missing here?
> D
>
>


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