# \[MSVC\] CMAKE\_CXX\_MP\_FLAG default value

**URL:** <https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786>\
**Category:** Development\
**Created:** [January 8, 2018, 7:50pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786 "2018-01-08T19:50:05Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![adamrankin](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/adamrankin/32/155_2.png) [@adamrankin](https://discourse.slicer.org/u/adamrankin)\
**Post date:** [January 8, 2018, 7:50pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/1 "2018-01-08T19:50:05Z")

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Hello,

Just wanted to start a conversation around the default value of CMAKE\_CXX\_MP\_FLAG on MSVC builds. Would there be any objection to setting it to ON?

Adam

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [January 8, 2018, 8:02pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/2 "2018-01-08T20:02:08Z")

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This has been discussed in the past, see [http://slicer-devel-archive.65872.n3.nabble.com/Reducing-Slicer-build-time-on-Windows-using-CMAKE-CXX-MP-FLAG-td4035926.html](http://slicer-devel-archive.65872.n3.nabble.com/Reducing-Slicer-build-time-on-Windows-using-CMAKE-CXX-MP-FLAG-td4035926.html)

@adamrankin How much does it reduce the build time for you?

As I remember, there were some potential issues in parallel builds - but maybe it was about parallel execution of test. @jcfr is there any risk or disadvantage of enabling parallel builds?

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**Author:** ![adamrankin](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/adamrankin/32/155_2.png) [@adamrankin](https://discourse.slicer.org/u/adamrankin)\
**Post date:** [January 8, 2018, 8:04pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/3 "2018-01-08T20:04:21Z")

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I have not quantified it, but it is noticeable. Is it worth me quantifying it in addition to your results?

Edit: it will be more and more noticeable with the introduction of higher and higher multicore count (see Ryzen Threadripper 16 core).

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**Author:** ![adamrankin](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/adamrankin/32/155_2.png) [@adamrankin](https://discourse.slicer.org/u/adamrankin)\
**Post date:** [January 8, 2018, 8:18pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/4 "2018-01-08T20:18:32Z")

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From [here](https://blogs.msdn.microsoft.com/vcblog/2016/10/26/recommendations-to-speed-c-builds-in-visual-studio/)

> /MP – Parallelize compilation of source files  
> Invokes multiple instances of cl.exe to compile project source files in parallel. See documentation for /MP for a detailed discussion of the switch including conflicts with other compiler features. In addition to documentation this blog post has good information about the switch.
> 
> Resolving conflicts with other compiler features  
> /Gm (enable minimal rebuild): I recommend using /MP over /Gm to reduce build time.  
> #import: Documentation for /MP discusses one option to resolve this conflict. Another option is to move all import directives to precompiled header.  
> /Yc (create precompiled header): /MP does not help with creating precompiled header so not an issue.  
> /EP, /E, /showIncludes: These switches are typically used to diagnose issues hence should not be an issue.

Depending on the [incompatibility](https://docs.microsoft.com/en-ca/cpp/build/reference/mp-build-with-multiple-processes):

> If you use an incompatible compiler option with the /MP option, the compiler issues warning D9030 and ignores the /MP option. If you use an incompatible language feature, the compiler issues error C2813 then ends or continues depending on the current compiler warning level option.

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**Author:** ![jcfr](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/jcfr/32/17825_2.png) [@jcfr](https://discourse.slicer.org/u/jcfr)\
**Post date:** [January 8, 2018, 8:18pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/5 "2018-01-08T20:18:47Z")

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The flag is set to do the nightly build. See [https://github.com/Slicer/DashboardScripts/blob/4efec4e330f4e79da9d9eef3ac445a350bde4618/overload-vs2013-slicer\_release\_nightly.cmake#L57](https://github.com/Slicer/DashboardScripts/blob/4efec4e330f4e79da9d9eef3ac445a350bde4618/overload-vs2013-slicer_release_nightly.cmake#L57)

Since having four cores (or more) is fairly common, may be we could make this the default ?

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**Author:** ![ihnorton](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/ihnorton/32/9_2.png) [@ihnorton](https://discourse.slicer.org/u/ihnorton)\
**Post date:** [January 8, 2018, 9:09pm UTC](https://discourse.slicer.org/t/msvc-cmake-cxx-mp-flag-default-value/1786/6 "2018-01-08T21:09:14Z")

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> [@lassoan](#):
>
> As I remember, there were some potential issues in parallel builds

The problem I’ve seen with ninja (mac) is nested external project builds. For example, building with `-j4` at the superbuild level means potentially four top-level projects can build in parallel. But when the flag is passed to the child process, in ninja _each_ one of those tries to use four cores so the computer ends up swapping/thrashing. I don’t know if `msbuild` is susceptible to such an issue.

(there is a “job pool” concept in GNU make, and now available in ninja, to avoid this problem)