Overview

The virtual memory (VM) subsystem was completely rewritten by Digital in V2.0 in order to improve upon the Mach design adopted by the OSF. Specifically, Digital added the following functionality to improve performance and maintainability (V4.0 added several enhancements to these areas):

• Improved upon the lazy allocation policy

• Improved upon the eager reservation policy

• Unified Buffer Cache

• Round-robin swapping algorithm

• Page in and page out clustering

• Memory-mapped device interface

• Mach mmap MAP_PRIVATE semantics to make them compatible with Sun Microsystems and System V Release 4.0

• Ensured that processes cannot read or write other processes' shared memory segments

• Support for shared text segments

• Support for page coloring

• New kernel memory allocator

• Improved memory reclamation policy

• Restructured swap allocation mechanism