This is the user space slab memory allocator, umem, first available in Solaris 9 (SunOS 5.4) now the default allocator on Solaris and Illumos. This implementation has been ported to other popular operating systems, such as Linux, Windows and BSDish systems (including Darwin/OSX) by OmniTI (portableumem) and includes changes made by Joyent as part of their ongoing work to improve SmartOS.
