| Description |
Like most operating systems, Mach 3.0 views threads as statically associated with a single task. An alternative model is that of migrating threads, in which a single thread abstraction moves between tasks with the logical flow of control, and "server" code is passively executed. We have compatibly replaced Mach's static threads with migrating threads, isolating that aspect of operating system design and implementation. The key element of our design is a decoupling of the thread abstraction into the controllable execution context and the schedulable thread of control, consisting of a chain of contexts. A key element of our implementation is that threads are now "based" in the kernel, and temporarily make excursions into tasks via upcalls. The new system provides cleaner and more powerful semantics for thread manipulation, allows scheduling and accounting attributes to follow threads, simplifies both kernel and server code, and improves RPC performance. We have retained the old thread and IPC interfaces for backwards compatibility, with no changes required to existing client programs and only a minimal change to servers, as demonstrated by a functional Unix single server and clients. Code size along the critical RPC path has been reduced by a factor of three, while its logical complexity has been reduced by an order of magnitude. Initial timings show that the performance of local RPC, doing normal marshaling, has also improved by a factor of three. We conclude that a migrating thread model is superior to a static model, and that it is feasible to improve existing operating systems in this manner. |
