The UVM virtual memory system

We introduce UVM, a new virtual memory system for the BSD kernel that has an improved design that increases system performance over the old Mach-based 4.4BSD VM system. In this paper we present an overview of both UVM and the BSD VM system. We focus our discussion on the design decisions made when creating UVM and contrast the UVM design with the less efficient BSD VM design. Topics covered include mapping, memory object management, anonymous memory and copy-on-write mechanisms, and pager design. We also present an overview of virtual memory based data movement mechanisms that have been introduced in BSD by UVM. We believe that the lessons we learned from designing and implementing UVM can be applied to other kernels and large software systems. Implemented in the NetBSD operating system, UVM will completely replace BSD VM in NetBSD 1.4.