  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. 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 using 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 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. Utopia improves performance by 24% in a
single-core system over the baseline system, whereas the best prior
state-of-the-art contiguity-aware translation scheme improves performance by
13%.
