The need for customizable operating systems

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. We contend that there is a very general issue here, which operating systems have been among the first kinds of software to have to face head-on: some implementation decisions are crucial strategy decisions whose resolution will invariably bias the performance of the resulting implementation. 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