Software-level Analysis and Optimization to Mitigate the Cost of Write Operations on Non-Volatile Memories

Rabab Bouziane 1
1 PACAP - Pushing Architecture and Compilation for Application Performance
Inria Rennes – Bretagne Atlantique , IRISA_D3 - ARCHITECTURE
Abstract : Traditional memories such as SRAM, DRAM and Flash have faced during the last years, critical challenges related to what modern computing systems required: high performance, high storage density and low power. As the number of CMOS transistors is increasing, the leakage power consumption becomes a critical issue for energy-efficient systems. SRAM and DRAM consume too much energy and have low density and Flash memories have a limited write endurance. Therefore, these technologies can no longer ensure the needs in both embedded and high-performance computing domains. The future memory systems must respect the energy and performance requirements. Since Non Volatile Memories (NVMs) appeared, many studies have shown prominent features where such technologies can be a potential replacement of the conventional memories used on-chip and off-chip. NVMs have important qualities in storage density, scalability, leakage power, access performance and write endurance. Many research works have proposed designs based on NVMs, whether on main memory or on cache memories. Nevertheless, there are still some critical drawbacks of these new technologies. The main drawback is the cost of write operations in terms of latency and energy consumption. Ideally, we want to replace traditional technologies with NVMs to benefit from storage density and very low leakage but eventually without the write operations overhead. The scope of this work is to exploit the advantages of NVMs employed mainly on cache memories by mitigating the cost of write operations. Obviously, reducing the number of write operations in a program will help in reducing the energy consumption of that program. Many approaches about reducing writes operations exist at circuit level, architectural level and software level. We propose a compiler-level optimization that reduces the number of write operations by eliminating the execution of redundant stores, called silent stores. A store is silent if it’s writing in a memory address the same value that is already stored at this address. The LLVM-based optimization eliminates the identified silent stores in a program by not executing them. Furthermore, the cost of a write operation is highly dependent on the used NVM and its non-volatility called retention time; when the retention time is high then the latency and the energetic cost of a write operation are considerably high and vice versa. Based on this characteristic, we propose an approach applicable in a multi-bank NVM where each bank is designed with a specific retention time. We analyze a program and we compute the worst-case lifetime of a store instruction. The worst-case lifetime will help to allocate data to the most appropriate NVM bank.
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Rabab Bouziane. Software-level Analysis and Optimization to Mitigate the Cost of Write Operations on Non-Volatile Memories. Computer Science [cs]. Université de Rennes 1 [UR1], 2018. English. ⟨tel-01954076⟩

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