* Application-controlled physical memory using external page-cache management * Authors: Kieran Harty, David R. Cheriton * Field: OS/Virtual Memory * Reference: Fifth ASPLOS, October 1992 * Theme: Kernel extensions for application-specific resource management, in this case paging and vm * Implementation: Segment manager for each segment, several segments make up an app's VA space. Syscalls for page migration between segments & information about pages. A system segment manager manages globally. * Examples: DB systems, scientific applications, systems that use more memory than available in core. * Problems: Complex, many trips in/out of kernel for certain types of faults * Quote: Conventional virtual memory makes main memory vs. secondary storage division transparent except for performance. * Take: Applications which warrant the design effort needed to provide custom kernel extensions may need to bypass/customize more aspects of the system than can be securely allowed by a general-purpose OS. Example is many modern DB systems, which basically run as miniature OSs in themselves, bypassing almost all OS services entirely, preferring to manage resources by themselves. Kernel extensions are complex because of the tension between safety, speed, flexibility, the need to maintain critical and complex system state. Maintaining internal system state is probably the most vexing and troublesome in implementation.