[CMake] VS2012 builds using v3.1.0 are a lot slower than v2.8.12

[Yves Frederix]

Hi,

I also tested with the build of an 'open' project (the crypto example
was part of a work-related project). The results below are for the
compilation of OpenCV (latest git from
https://github.com/itseez/opencv), again with VS2012 Win64, Release
only and no tweaking of the CMake settings.

3.1.0:    10m50s
2.8.12:   4m02s
3.1.0 (/MP): 6m29s
2.8.12 (/MP): 3m48s

This does not look entirely the same as for the Crypto example, but
the performance decrease is at least very much visible.

Best regards,
Yves



On Fri, Jan 16, 2015 at 5:24 PM, Yves Frederix
<yves.frederix+cmake at gmail.com> wrote:
> Hi,
>
>>> 3.1.0:
>>>         ALL                 Crypto          Crypto_s        Crypto_test
>>> real    0m38.211s           0m14.023s       0m7.003s        0m14.691s
>>>
>>> 2.8.12:
>>>         ALL                 Crypto          Crypto_s        Crypto_test
>>> real    0m27.230s           0m17.110s       0m10.231s       0m18.618s
>>
>> Interesting, thanks for the timings.
>>
>>> This made me wonder if it maybe had to do with default settings for
>>> parallelization in the builds. So I tried, and if I explicitly "set
>>> CL=/MP" for the 3.1.0 build, build time reduces from 38 seconds to 24
>>> seconds which is what I would expect. Could this be the cause and was
>>> there a change in default settings?
>>
>> I don't think such a change was made intentionally.  To help narrow
>> it down, could you please do the timings for CMake 3.0.2 as well?
>
> The behavior seems to be the same for 3.0.2:
>
> 3.0.2:
>           ALL                 Crypto          Crypto_s        Crypto_test
> real    0m38.756s           0m14.113s       0m7.150s        0m15.106s
>
>
> Regards,
> Yves