Translation Ranger: Operating System Support for Contiguity-Aware TLBs

Virtual memory (VM) eases programming effort but can suffer from high address translation overheads. Architects have traditionally coped by increasing Translation Lookaside Buffer (TLB) capacity; this approach, however, requires considerable hardware resources. One promising alternative is to rely on software-generated translation contiguity to compress page translation encodings within the TLB. To enable this, operating systems (OSes) have to assign spatially-adjacent groups of physical frames to contiguous groups of virtual pages, as doing so allows compression or coalescing of these contiguous translations in hardware. Unfortunately, modern OSes do not currently guarantee translation contiguity in many real-world scenarios; as systems remain online for long periods of time, their memory can and does become fragmented. We propose Translation Ranger, an OS service that recovers lost translation contiguity even where previous contiguity- generation proposals struggle with memory fragmentation. Translation Ranger increases contiguity by actively coalescing scattered physical frames into contiguous regions and can be leveraged by any contiguity-aware TLB without requiring changes to applications. We implement and evaluate Translation Ranger in Linux on real hardware and find that it generates contiguous memory regions 40x larger than the Linux default configuration, permitting TLB coverage of 120GB memory with typically no more than 128 contiguous translation regions. This is achieved with less than 2% run time overhead, a number that is outweighed by the TLB coverage improvements that Translation Ranger provides.