On 07/07/2016 04:59 PM, Robert Dailey wrote: > I notice on ninja builds generated via CMake 3.6, the link step blocks > compilation threads. What I expect is that if I have 32 threads > available for compiling, those are also shared via the link step. In > other words, if I reach a link step, I want it to consume 1 thread and > the other 31 threads start compiling translation units for the next > library. > > Is there a reason it isn't behaving as I describe? Is there a setting > or CMake option I can set to enable this?
This happens because CMake generates order-only dependencies on the compilation rules of each exe/lib's objects to make sure the do not compile until linking of dependencies is done. This makes the build semantics match that of other generators where each logical target is treated as its own isolated build that is evaluated only when its logical target dependencies have been finished. This is unfortunately necessary to get correct builds for CMake projects in general because we support cases where add_custom_command is used in library "foo" to generate a header file that is included during compilation in library "bar" that links to "foo", but we have no good way to express this dependency besides the ordering dependency of bar on foo. It is likely possible to detect causes automatically when this is not necessary, such as when the transitive closure of a target's dependencies contains no custom commands. A dedicated effort will be needed to investigate this further and implement such optimizations on the build graph generation. -Brad -- Powered by www.kitware.com Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ Kitware offers various services to support the CMake community. For more information on each offering, please visit: CMake Support: http://cmake.org/cmake/help/support.html CMake Consulting: http://cmake.org/cmake/help/consulting.html CMake Training Courses: http://cmake.org/cmake/help/training.html Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html Follow this link to subscribe/unsubscribe: http://public.kitware.com/mailman/listinfo/cmake
