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Editorial Team
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Asked: 2026-05-24T20:21:55+00:00

I’m hoping someone points out something wrong or a workaround for what we are

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I’m hoping someone points out something wrong or a workaround for what we are experiencing.

When compiling a project with /MP it appears that only files in the same folder are compiled concurrently. I used process explorer to swipe the command line and confirm the behavior.

Project filters seem to have all no impact on the what gets compiled concurrently.

Project structure on disk: 

Folder\
  project.vcxproj  
  source\  
    foo.cpp  
    foo1.cpp  
  other_folder\  
    bar.cpp
    bar1.cpp
    bar3.cpp

Initial process tree:

MSBuild.exe
  cl.exe  ( passed: source\foo.cpp source\foo1.cpp )
    cl.exe  ( passed: source\foo.cpp )
    cl.exe  ( passed: source\foo1.cpp )

After the 2 child instances of cl.exe complete the parent closes and the following process tree appears:

MSBuild.exe
  cl.exe  ( passed: other_folder\bar.cpp other_folder\bar1.cpp other_folder\bar2.cpp )
    cl.exe  ( passed: other_folder\bar.cpp )
    cl.exe  ( passed: other_folder\bar1.cpp )
    cl.exe  ( passed: other_folder\bar2.cpp )

Our source is nicely organized in many levels of nested folders that match the layout of the headers on disk – I would hate to have to give that up to take advantage of /MP.

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1 Answer

  1. Editorial Team

    The use of %(RelativeDir) in “Object File Name” ( in the vcxproj XML, /Fo on the CL.exe command line ) project causes msbuild to batch the cpp files to cl.exe on a per directory basis. This can have a significant impact on the benefits gained from using /MP.

    Note that if your project uses %(RelativeDir) for object files its likely that the configuration is trying to avoid colliding .obj files that from cpp files with the same name in different folders.

    The /Fo command line parameter is typically a folder that the compiler dumps the obj files into – there is only ONE passed, so all of the cpp files for a given directory can only be passed to CL.exe at a time.

    That was a pain – but I’m glad there is a reason and a solution. Hope it helps.

    Update

    A team mate found that anytime an MSBuild parameter is sent to CL.exe it seems to break or severely limit /MP. This is most likely because for /MP to work well the top level CL.exe needs to have a bundle of cpp files.

    Our solution was to not use any msbuild params ( I think its the %params% ) for ‘Object File Name’. This required that we rename some cpp files so they did not collide.

    Hope this has changed in VS2012 or VS2013.

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