Prolonging Lifetime of Non-volatile Last Level Caches with Cluster Mapping

Recently, work has been done on using nonvolatile cells, such as Spin Transfer Torque RAM (STT-RAM) or Magnetic RAM (M-RAM), to construct last level caches (LLC). These structures mitigate the leakage power and density problem found in traditional SRAM cells. However, the low endurance of nonvolatile caches decreases the lifetime of the LLC. Therefore, an effective wear-leveling technique is required to tackle this issue. In this paper, we propose the inter-set algorithm that distributes the write traffic to all portions of the cache. Our method is based on cluster mapping that dynamically replaces two clusters during the operation of system. Since the inter-set algorithm is based on data movement, a large amount of data must transfer in each replacement. For an efficient data movement with a minimum effect on performance, we develop the novel scheduling technique that utilizes the idle time of the LLC in the computation phase of the processors. Our approach effectively improves the lifetime of LLC with negligible performance and area overhead. Using these methods in a quad core system with 2MB LLC, we can improve the lifetime of non-volatile LLC by 30% on average.