CAST: Content-Aware STT-MRAM Cache Write Management for Different Levels of Approximation

Spin transfer torque magnetic RAM (STT-MRAM) technology is one of the most promising alternative for static RAM (SRAM) for implementing on-chip memories. Compared with SRAMs, STT-MRAMs benefit from higher density and near-zero leakage power, nonetheless they impose high energy consumption for reliable write operations. However, in many applications, absolute data integrity is not required; thus, acting on the current applied in the write operations may represent a novel knob for disciplined approximate computing to obtain energy saving with a minimal quality loss in applications' outputs. This article proposes CAST, a hardware/software approach to adjust the energy/quality of write operations in STT-MRAM caches in multicore systems based on the content of requested write operations. CAST utilizes fine-grained cache-line-level actuation knobs with different levels of quality for individual write operations. This unique feature of STT-MRAMs allows to avoid interapplication actuation interference suffered by SRAMs, and makes the approach particularly suitable for systems running multiple applications with mixed accuracy sensitivity. Moreover, CAST exploits another peculiarity of STT-MRAMs represented by the asymmetry and transition-dependency of the write error rate, to further tune in a fine-grained manner the write current to achieve an additional energy saving, even in full-accurate applications. Our evaluations on workloads of full-approximate, mixed-criticality, and full-accurate applications demonstrate up to 57%, 34%, and 21% energy savings over a baseline STT-MRAM cache, respectively, with an acceptable quality of the generated outputs.