On Windows, MB_OF_MEMORY is calculated in jdk/make/common/shared/Platform.gmk with explicit reference to C:/cygwin/bin/{free,bash}.exe. AFAIK, it is not required anywhere else in the build that cygwin be installed at that location, and I can find no copy of free.exe available in any of my cygwin installations, nor do I see how to get it from cygwin itself. (Perhaps it is part of some bundle, but I don't know which).

The default MB_OF_MEMORY, 512, causes smaller amounts of VM to be used. My Windows box has 2048 mb, and IIRC the JPRT boxes are pretty beefy, but unfortunately the current MB_OF_MEMORY limit causes them to be throttled back, and underutilized.