Utopia: Fast and Efficient Address Translation via Hybrid Restrictive & Flexible Virtual-to-Physical Address Mappings

Conventional virtual memory (VM) frameworks enable a virtual address to flexibly map to any physical address. This flexibility necessitates large data structures to store virtual-to-physical mappings, which leads to high address translation latency and large translation-induced interference in the memory hierarchy, especially in data-intensive workloads. On the other hand, restricting the address mapping so that a virtual address can only map to a specific set of physical addresses can significantly reduce address translation overheads by making use of compact and efficient translation structures. However, restricting the address mapping flexibility across the entire main memory severely limits data sharing across different processes and increases data accesses to the swap space of the storage device even in the presence of free memory. We propose Utopia, a new hybrid virtual-to-physical address mapping scheme that allows both flexible and restrictive hash-based address mapping schemes to harmoniously co-exist in the system. The key idea of Utopia is to manage physical memory using two types of physical memory segments: restrictive segments and flexible segments. A restrictive segment uses a restrictive, hash-based address mapping scheme that maps virtual addresses to only a specific set of physical addresses and enables faster address translation using compact translation structures. A flexible segment employs the conventional fully-flexible address mapping scheme. By mapping data to a restrictive segment, Utopia enables faster address translation with lower translation-induced interference. At the same time, Utopia retains the ability to use the flexible address mapping to (i) support conventional VM features such as data sharing and (ii) avoid storing data in the swap space of the storage device when program data does not fit inside a restrictive segment. Our evaluation using 11 diverse data-intensive workloads shows that Utopia improves performance by 24% in a single-core system over the baseline conventional four-level radix-tree page table design, whereas the best prior state-of-the-art contiguity-aware translation scheme improves performance by 13%. Utopia provides 95% of the performance benefits of an ideal address translation scheme where every translation request hits in the firstlevel TLB. All of Utopia's benefits come at a modest cost of 0.64% area overhead and 0.72% power overhead compared to a modern high-end CPU. The source code of Utopia is freely available at https://github.com/CMU-SAFARI/Utopia.