Data tiering in heterogeneous memory systems.

Memory-based data center applications require increasingly large memory capacities, but face the challenges posed by the inherent difficulties in scaling DRAM and also the cost of DRAM. Future systems are attempting to address these demands with heterogeneous memory architectures coupling DRAM with high capacity, low cost, but also lower performance, non-volatile memories (NVM) such as PCM and RRAM. A key usage model intended for NVM is as cheaper high capacity volatile memory. Data center operators are bound to ask whether this model for the usage of NVM to replace the majority of DRAM memory leads to a large slowdown in their applications? It is crucial to answer this question because a large performance impact will be an impediment to the adoption of such systems. This paper presents a thorough study of representative applications -- including a key-value store (MemC3), an in-memory database (VoltDB), and a graph analytics framework (GraphMat) -- on a platform that is capable of emulating a mix of memory technologies. Our conclusions are that it is indeed possible to use a mix of a small amount of fast DRAM and large amounts of slower NVM without a proportional impact to an application's performance. The caveat is that this result can only be achieved through careful placement of data structures. The contribution of this paper is the design and implementation of a set of libraries and automatic tools that enables programmers to achieve optimal data placement with minimal effort on their part. With such guided placement and with DRAM constituting only 6% of the total memory footprint for GraphMat and 25% for VoltDB and MemC3 (remaining memory is NVM with 4x higher latency and 8x lower bandwidth than DRAM), we show that our target applications demonstrate only a 13% to 40% slowdown. Without guided placement, these applications see, in the worst case, 1.5x to 5.9x slowdown on the same configuration. Based on a realistic assumption that NVM will be 5x cheaper (per bit) than DRAM, this hybrid solution also results in 2x to 2.8x better performance/$ than a DRAM-only system.