A Caching Model of Operating System Kernel Functionality

One-line summary: Scheduler activations part II, the micro-kernel.

Overview/Main Points

• Concepts:
  • Micro-kernel OS's should provide modularity, reliability, and security over monolithic kernels. Some have turned out to be slower, larger, and more error-prone, and do not support domain-specific resource allocation policies any better. (due to VM system, exception handling, hardware, etc)

  • Cache Kernel caches OS system objects such as threads, address spaces, and application kernels. The higher level application kernels provide the management functions for the application, managing thread priority and address spaces and reducing supervisor-level complexity. Libraries are available for resource management (memory, processing, and communication), so that the application writer can customize selectively.

  • When a thread faults, it s loaded into the cache kernel address space, from which it jumps to the application-kernel address space, where the fault is handled.

  • Memory-based messaging used to communicate between address spaces, as well as to access devices controlled by the cache kernel (ie ethernet). Applications interested in receiving messages spawn a "signal thread" and register it with the service. Minimal copying and protection boundary crossing. An example is given for accessing devices directly from the application kernel, but I am not convinced that this is good/safe.

  • Cache Kernel handles resource allocation, application kernel mappings, and monitoring through the system resource manager, which is the first application kernel (loaded on boot-up). SRM serves as the owner for the other app kernel object, handling their write-back and granting requests for processing capacity, memory pages, and network capacity. Each application kernel handles its own resource mappings.

  • Complex relationships can occur between thread, address space, and message objects, requiring complex replacement mechanisms.

  • Code size is small in comparison to a monolithic kernel. Trap handling, signal delivery, and page fault handling compare favorably to Mach 2.5 and monolithic kernels.

Conclusions

The authors criticize micro-kernel OSs and point out the language dependence of SPIN and Aegis, but leave out performance comparisons to justify their statements. It would seem that they would suffer from the same problems with hardware support (They criticize microkernels for their hardware support, but then state that they only I/O that they support is high-speed networking!?). I also do not buy that their support for different policies is much different from features such as micro-kernel user-level virtual memory support.