A caching model of operating system kernel functionality
Authors: David R. Cheriton, Kenneth J. Duda
Venue: ACM SIGOPS Operating Systems Review (1995)

Operating system design has had limited success in providing adequate application functionality and a poor record in avoiding excessive growth in size and complexity, especially with protected operating systems. Applications require far greater control over memory, I/O and processing resources to meet their requirements. For example, database transaction processing systems include their own "kernel" which can much better manage resources for the application than can the application-ignorant general-purpose conventional operating system mechanisms. Large-scale parallel applications have similar requirements. The same requirements arise with servers implemented outside the operating system kernel.In our research, we have been exploring the approach of making the operating system kernel a cache for active operating systems objects such as processes, address spaces and communication channels, rather than a complete manager of these objects. The resulting system is smaller than recent so-called micro-kernels, and also provides greater flexibility for applications, including real-time applications, database management systems and large-scale simulations. As part of this research, we have developed what we call a cache kernel, a new type of micro-kernel that supports operating system configurations across these dimensions.The cache kernel can also be regarded as providing a hardware adaptation layer (HAL) to operating system services rather than trying to just provide a key subset of OS services, as has been the common approach in previous micro-kernel work. However, in contrast to conventional HALs, the cache kernel is fault-tolerant because it is protected from the rest of the operating system (and applications), it is replicated in large-scale configurations and it includes audit and recovery mechanisms. A cache kernel has been implemented on a scalable shared-memory and networked multi-computer [2] hardware which provides architectural support for the cache kernel approach.Figure 1 illustrates a typical target configuration. There is an instance of the cache kernel per multi-processor module (MPM), each managing the processors, second-level cache and network interface of that MPM. The cache kernel executes out of PROM and local memory of the MPM, making it hardware-independent of the rest of the system except for power. That is, the separate cache kernels and MPMs fail independently. Operating system services are provided by application kernels, server kernels and conventional operating system emulation kernels in conjunction with privileged MPM resource managers (MRM) that execute on top of the cache kernel. These kernels may be in separate protected address spaces or a shared library within a sophisticated application address space. A system bus connects the MPMs to each other and the memory modules. A high-speed network interface per MPM connects this node to file servers and other similarly configured processing nodes. This overall design can be simplified for real-time applications and similar restricted scenarios. For example, with relatively static partitioning of resources, an embedded real-time application could be structured as one or more application spaces incorporating application kernels as shared libraries executing directly on top of the cache kernel.