Although modern operating systems provide powerful abstractions to application programs, they often fail to implement those abstractions in a way that provides applications programs, especially specialized application programs, with the best utilization of the physical resources of the computer system. The operating system community has implicitly recognized this problem by providing mechanisms that give client programmers more access to the physical substrate. The Mach External Pager allows clients to replace the paging mechanism. More recent work allows client replacement of the paging policy as well.
Scheduler activations share the job of thread management between clients and the system.
Apertos allows these and other aspects of operating system implementation to be client-controlled. Object-oriented operating systems under development also provide these kinds of control. Explicitly recognizing this issue helps to make sense of current trends and suggests new directions to explore. We consider the implications of this issue for operating systems, providing a framework with which to analyze systems such as those mentioned above, and suggesting connections with similar problems in other domains. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Xerox's products or services.