Notes on thread models in Mach 3.0

Journal ArticleDuring the Mach In-Kernel Servers work, we explored two alternate thread models that could be used to support traps to in-kernel servers. In the "migrating threads" model we used, the client's thread temporarily moves into the server's task for the duration of the call. In t h e "thread switching" model, an actual server thread is dispatched to handle client traps. Based on our experience, we find that the migrating threads model is quite complex and difficult to implement in t h e context of the current design of Mach and the Unix single server. The thread switching model would fit more naturally and would probably be much simpler and more robust than migrating threads, making it a valuable approach to explore in the near future. However, we believe migrating threads inherently to be faster than thread switching, and ultimately to be the best long term direction

In-Kernel Servers on Mach 3.0: Implementation and Performance

The advantages in modularity and power of microkernel-based operating systems such as Mach 3.0 are well known. The existing performance problems of these systems, however, are significant. Much of the performance degradation is due to the cost of maintaining separate protection domains, traversing software layers, and using a semantically rich inter-process communication mechanism. An approach that optimizes the common case is to permit merging of protection domains in performance critical applications, while maintaining protection boundaries for debugging or in situations that demand robustness. In our system, client calls to the server are effectively bound either to a simple system call interface, or to a full RPC mechanism, depending on the server's location. The optimization reduces argument copies, as well as work done in the control path to handle complex and infrequently encountered message types. In this paper we present a general method of doing this for Mach 3.0 and the resu..