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macOS: Better approximation for AvailablePhysicalMemory

nvartolomei requested to merge nvartolomei/cmake:nv/macos-available_memory into master

On macOS, free pages does not seem to be a good indicatur for "available physical memory". Neither inactive ones, but it answers better questions like: "How much memory could this process use in best case scenario".

Free + Inactive aproximately matches Activity Monitor.app output.

https://developer.apple.com/library/archive/documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html#//apple_ref/doc/uid/20001880-99406-TPXREF103

I can't find better documentation for this values.

Motivation behind this Merge Request is caused by CMakeFiles that limit number of compiler/linker jobs based on available memory (as reported by CMake). On my machine with 16GB RAM and ~10GB in use (as reported by Activity Monitor), CMake reports 100MB free which is correct for some definition of free, but if requested OS can give way more memory.

Topic-rename: macos-available_memory

Edited by Brad King

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