DRAM Translation Layer: Software-Transparent DRAM Power Savings for Disaggregated Memory

Memory disaggregation is a promising solution to scale memory capacity and bandwidth shared by multiple server nodes in a flexible and cost-effective manner. DRAM power consumption, which is reported to be around 40% of the total system power in the datacenter server, will become an even more serious concern in this high-capacity environment. Exploiting the low average utilization of DRAM capacity in today's datacenters, it is appealing to put unallocated/cold DRAM ranks into a power-saving mode. However, the conventional DRAM address mapping with fine-grained interleaving to maximize rank-level parallelism is incompatible with such rank-level DRAM power management techniques. Furthermore, existing DRAM power-saving techniques often require intrusive changes to the system stack, including OS, memory controller (MC), or even DRAM devices, to pose additional challenges for deployment. Thus, we propose DRAM Translation Layer (DTL) for host software/MC-transparent DRAM power management with commodity DRAM devices. Inspired by Flash Translation Layer (FTL) in modern SSDs, DTL is placed in the CXL memory controller to provide (i) flexible address mappings between host physical address and DRAM device physical address and (ii) host-transparent memory page migration. Leveraging DTL, we propose two DRAM power-saving techniques with different temporal granularities to maximize the number of DRAM ranks that can enter low-power states while provisioning sufficient DRAM bandwidth: rank-level power-down and hotness-aware self-refresh. The first technique consolidates unallocated memory pages into a subset of ranks at deallocation of a virtual machine (VM) and turns them off transparently to both OS and host MC. Our evaluation with CloudSuite benchmarks demonstrates that this technique saves DRAM power by 31.6% on average at a 1.6% performance cost. The hotness-aware self-refresh scheme further reduces DRAM energy consumption by up to 14.9% with negligible performance loss via opportunistically migrating cold pages into a rank and making it enter self-refresh mode.