Implementing caches in a 3D technology for high performance processors

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Abstract
3D integration is an emergent technology that has the potential to greatly increase device density while simultaneously providing faster on-chip communication. 3D fabrication involves stacking two or more die connected with a very high-density and low-latency interface. The die-to-die vias that comprise this interface can be treated like regular on-chip metal due to their small size (on the order of 1µm) and high speed (sub-FO4 die-to-die communication delay). The increased device density and the ability to place and route in the third dimension provide new opportunities for microarchitecture design. In this paper, we first present a brief overview of 3D integration technology. We then focus on the design of onchip caches using 3D integration. In particular, we show that the dense die-to-die vias enable caches that are 3Dpartitioned at the level of individual wordlines or bitlines. This results in a wire length reduction within SRAM arrays, and a reduction in the footprint of individual SRAM banks, which reduces the global routing from the edge of the cache to the banks and back. The wire length reduction provides both power and performance benefits, e.g., 21.5% latency reduction and 30.9% energy reduction for a 512KB cache. We also report that implementing only the caches in 3D, without accounting for possible benefits from implementing other components of the processor in 3D, results in a 12% IPC gain. These results demonstrate some of the potential of this new technology, and motivate further research in 3D microarchitectures.

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