Rethinking DRAM Power Modes for Energy Proportionality

We re-think DRAM power modes by modeling and characterizing inter-arrival times for memory requests to determine the properties an ideal power mode should have. This analysis indicates that even the most responsive of today's power modes are rarely used. Up to 88% of memory is spent idling in an active mode. This analysis indicates that power modes must have much shorter exit latencies than they have today. Wake-up latencies less than 100ns are ideal. To address these challenges, we present MemBlaze, an architecture with DRAMs and links that are capable of fast power up, which provides more opportunities to power down memories. By eliminating DRAM chip timing circuitry, a key contributor to power up latency, and by shifting timing responsibility to the controller, MemBlaze permits data transfers immediately after wake-up and reduces energy per transfer by 50% with no performance impact. Alternatively, in scenarios where DRAM timing circuitry must remain, we explore mechanisms to accommodate DRAMs that power up with less than perfect interface timing. We present MemCorrect which detects timing errors while MemDrowsy lowers transfer rates and widens sampling margins to accommodate timing uncertainty in situations where the interface circuitry must recalibrate after exit from power down state. Combined, MemCorrect and MemDrowsy still reduce energy per transfer by 50% but incur modest (e.g., 10%) performance penalties.