Optimizing MLC-based STT-RAM caches by dynamic block size reconfiguration

The use of STT-RAM as on-chip caches has been widely studied. However, existing works focused mainly on single-level cell (SLC) design while the potential of multi-level cell (MLC) STT-RAM has not yet been fully explored. It is expected that MLC STT-RAM can achieve 2× the storage density of SLC and thus improves system performance. Unfortunately, at the device level, the two-step read/write scheme introduces performance and energy overhead. In this paper, we propose an architectural design to dynamically reconfigure the cache block size for a MLC STT-RAM last-level cache. Our approach place certain hot data chunks in smaller blocks so as to benefit from the lower latency and energy, while keeping the rest in larger blocks to maintain an overall hit rate. Experiment shows that our strategy reduces the performance and energy penalty of MLC STT-RAM caches with a slightly higher miss rate. On average, IPC is increased by 4.6% while energy consumption is reduced by 23.5% compared to the conventional MLC STT-RAM cache.