Approximate Memory Compression

Memory subsystems are a major energy bottleneck in computing platforms due to frequent transfers between processors and off-chip memory. We propose approximate memory compression, a technique that leverages the intrinsic resilience of emerging workloads such as machine learning and data analytics to reduce off-chip memory traffic, thereby improving energy and performance. We realize approximate memory compression by enhancing the memory controller to be aware of approximate memory regions—regions in memory that contain approximation-resilient data—and to transparently compress (decompress) the data written to (read from) these regions. To provide control over approximations, each approximate memory region is associated with an error constraint such as the maximum error that may be introduced in each data element. The quality-aware memory controller subjects memory transactions to a compression scheme that introduces approximations, thereby reducing memory traffic, while adhering to the specified error constraint for each approximate memory region. A software interface is provided to allow programmers to identify data structures (DSs) that are resilient to approximations. A runtime quality control framework automatically determines the error constraints for the identified DSs such that a given target application-level quality is maintained. We evaluate our proposal by applying it to three different main memory technologies in the context of a general-purpose computing system—DDR3 DRAM, LPDDR3 DRAM, and spin-transfer torque magnetic RAM (STT-MRAM). To demonstrate the feasibility of the proposed concepts, we also implement a hardware prototype using the Intel UniPHY-DDR3 memory controller and Nios-II processor, a Hynix DDR3 DRAM module, and a Stratix-IV field-programmable gate array (FPGA) development board. Across a wide range of machine learning benchmarks, approximate memory compression obtains significant benefits in main memory energy ( $1.18\times $ for DDR3 DRAM, $1.52\times $ for LPDDR3 DRAM, and $2.0\times $ for STT-MRAM) and a simultaneous improvement in execution time (5.2% for DDR3 DRAM, 5.4% for LPDDR3 DRAM, and 9.3% for STT-MRAM) with nearly identical application output quality.