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Using Multiple Processors to Build Projects

Visual Studio 2008 can take advantage of systems that have multiple processors, or multiple-core processors. A separate build process is created for each available processor. For example, if the system has four processors, then four build processes are created. MSBuild can process these builds simultaneously, and therefore overall build time is reduced. However, parallel building introduces some changes in how build processes occur. This topic discusses those changes.

When the Microsoft Build Engine encounters a project-to-project (P2P) reference while it is using parallel builds to build a project, it builds the reference only one time. If two projects have the same P2P reference, the reference is not rebuilt for each project. Instead, the build engine returns the same P2P reference to both projects that depend on it. Future requests in the session for the same target are provided the same P2P reference.

Cycle detection functions the same as it did in MSBuild 2.0, except that now MSBuild can report the detection of the cycle at a different time or in the build.

In parallel builds, errors and exceptions can occur at different times than they do in a non-parallel build, and when one project does not build, the other project builds continue. MSBuild will not stop any project build that is building in parallel with the one that failed. Other projects continue to build until they either succeed or fail. However, if ContinueOnError has been enabled, then no builds will stop even if an error occurs.

Both Visual C++ projects (.vcproj) and solution (.sln) files can be passed to the MSBuild Task. For Visual C++ projects, VCWrapperProject is called, and then the internal MSBuild project is created. For Visual C++ solutions, a SolutionWrapperProject is created, and then the internal MSBuild project is created. In both cases, the resulting project is treated the same as any other MSBuild project.

Almost all build-related activities require the current directory to remain constant throughout the build process to prevent path-related errors. Therefore, projects cannot run on different threads in MSBuild because they would cause multiple directories to be created.

To avoid this problem but still enable multi-processor builds, MSBuild uses "process isolation." By using process isolation, MSBuild can create a maximum of n processes, where n equals the number of processors available on the system. For example, if MSBuild builds a solution on a system that has two processors, then only two build processes are created. These processes are re-used to build all the projects in the solution.

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