  Traditional DRAM-based main memory systems face several challenges with
memory refresh overhead, high latency, and low throughput as the industry moves
towards smaller DRAM cells. These issues have been exacerbated by the emergence
of data-intensive applications in recent years. Memories based on phase change
materials (PCMs) offer promising solutions to these challenges. PCMs store data
in the material's phase, which can shift between amorphous and crystalline
states when external thermal energy is supplied. This is often achieved using
electrical pulses. Alternatively, using laser pulses and integration with
silicon photonics offers a unique opportunity to realize high-bandwidth and
low-latency photonic memories. Such a memory system may in turn open the
possibility of realizing fully photonic computing systems. But to realize
photonic memories, several challenges that are unique to the photonic domain
such as crosstalk, optical loss management, and laser power overhead have to be
addressed. In this work, we present COMET, the first cross-layer optimized
optical main memory architecture that uses PCMs. In architecting COMET, we
explore how to use silicon photonics and PCMs together to design a large-scale
main memory system while addressing associated challenges. We explore
challenges and propose solutions at the PCM cell, photonic memory circuit, and
memory architecture levels. Based on our evaluations, COMET offers 7.1x better
bandwidth, 15.1x lower EPB, and 3x lower latencies than the best-known prior
work on photonic main memory architecture design.
