EnVM: virtual memory design for new memory architectures

Virtual memory is optimized for SRAM-based memory devices in which memory accesses are symmetric, i.e., the latency of read and write accesses are similar. Unfortunately, with the emergence of newer non-volatile memory (NVM) technologies that are denser and more energy efficient, this assumption is no longer valid. For example, STT-RAMs are known to have high write latencies and limited write endurance which the virtual memory is unaware of. A popular architecture is a hybrid cache that uses both SRAM and NVM. There are a number of proposals for such architectures at nearly all the levels of the cache. However, these proposals are often self-contained with monitoring and management schemes implemented with special hardware at the level where the cache is deployed. With moves to use NVM at several levels of the memory hierarchy, such solutions may lead to duplication and higher overheads. Worse, because the management algorithms implemented can be different at different levels of memory, it may lead to negative interference between them resulting in impaired efficiency.

In this paper, we propose a virtual memory design, EnVM, that takes into consideration the idiosyncrasies of NVM-based hybrid caches. The new virtual memory layout is implicitly used to allocate data to NVM and SRAM at any level of the memory hierarchy and is not dependant on the particular arrangements of the two partitions. The proposed design successfully filters out write operations and allocates them to SRAM. Moreover, it can be applied to any existing fine-grained data allocation technique to enhance the efficiency of these memories.