Protected Shared Libraries

Protected shared libraries are a new mechanism for building fast user-level operating system services. They retain the isolation properties of other userlevel implementation approaches with improved performance and flexibility. The approach can be used with either microkernel-based or monolithic operating systems. Operating system services are built as shared libraries that can be linked into client tasks. However, the classic notion of dynamically linkable shared libraries is extended to isolate the services from the clients, from other services and from the kernel. Thus, protected shared library services avoid the interdependencies that inhibit changes and complicate maintenance in monolithic systems. However, they also eliminate the overhead of full context switches that limit the performance of classic microkernels. Since they retain the key protection features of microkernels, they represent a much more attractive performance improvement approach than co-location. Unlike migratable threads or hand-off scheduling, they provide an ideal basis for flexible system services through the use of common memory. A prototype implementation of protected shared libraries is being built for IBM RS/6000 workstations running AIX 3.2.5. Early results indicates that the performance of protected shared libraries is better than for other service invocation mechanisms, including thread migration. Finally, system services that are implemented as protected shared libraries are well-matched to the structure of classic objects.