Partial-SET: Write speedup of PCM main memory

Phase change memory (PCM) is a promising nonvolatile memory technology developed as a possible DRAM replacement. Although it offers the read latency close to that of DRAM, PCM generally suffers from the long write latency. Long write request may block the read requests on the critical path of cache/memory access, incurring adverse impact on the system performance. Besides, the write performance of PCM is very asymmetric, i.e, the SET operation (writing `1') is much slower than that of the RESET operation (writing `0'). In this work, we re-examine the resistance transform process during the SET operation of PCM and propose a novel Partial-SET scheme to alleviate the long write latency issue of PCM. During a write access to a memory line, a short Partial-SET pulse is applied first to program the PCM cells to a pre-stable state, achieving the same write latency as RESET. The partially-SET cells are then fully programmed within the retention window to preserve the data integrity. Experimental results show that our Partial-SET scheme can improve the memory access performance of PCM by more than 45% averagely with very marginal storage overhead.