Group contexts: an architectural approach to virtual sharing

Software managed translation look-aside buffers (TLBs) provide fine grain address translations at a virtual page. The paper presents a novel scheme that exploits the aforementioned property to allow a group of processes to share system resources of write-protected memory segments. These resources consist of segment and address mapping data structures, and software translation cache and translation look-aside buffer entries. While this feature reduces allocation of kernel memory, it also better utilizes address translation caches by coalescing multiple entries into a single entry. The idea of virtual sharing is complicated by two issues. First, the virtual memory sub-system may not map shareable segments at the same virtual addresses and access permissions for all participants. The authors present a simple and flexible VM policy which enforces this requirement in the presence of dynamic linking and permission changes. Second, the underlying hardware address translation architecture may not explicitly support group sharing. They propose a group context TLB architecture which allows the system to share TLB entries of virtually shared pages. They also present a judicious software multiplexing mechanism that enables the operating system to share software address translation mappings independent of the underlying hardware characteristics