Kernel Support for the Wisconsin Wind Tunnel

This paper describes a kernel interface that provides an untrusted user-level process (an executive) with protected access to memory management functions, including the ability to create, manipulate, and execute within subservient contexts (address spaces). Page motion callbacks not only give the executive limited control over physical memory management, but also shift certain responsibilities out of the kernel, greatly reducing kernel state and complexity. The executive interface was motivated by the requirements of the Wisconsin Wind Tunnel (WWT), a system for evaluating cache-coherent shared-memory parallel architectures. WWT uses the executive interface to implement a ne-grain user-level extension of Li's shared virtual memory on a Thinking Machines CM-5, a message-passing multicomputer. However, the interface is suuciently general that an executive could act as a multipro-grammed operating system, exporting an alternative interface to the threads running in its subservient contexts. The executive interface is currently implemented as an extension to CMOST, the standard operating system for the CM-5. In CMOST, policy decisions are made on a central, distinct control processor (CP) and broadcast to the processing nodes (PNs). The PNs execute a minimal kernel suucient only to implement the CP's policy. While this structure eeciently supports some parallel application models, the lack of autonomy on the PNs restricts its generality. Adding the executive interface provides limited autonomy to the PNs, creating a structure that supports multiple models of application parallelism. This structure, with autonomy on top of centralization, is in stark contrast to most microkernel-based parallel operating systems in which the nodes are fundamentally autonomous. the copies are not made or distributed for commercial advantage, the USENIX Association copyright notice and the title and date of publication appear, and that notice is given that copying is by permission of the USENIX Association. To copy or republish otherwise requires speciic permission from the USENIX Association.