Skip It: Take Control of Your Cache!

Mechanisms to explicitly manage the presence of data in caches are fundamental for the correctness and performance of modern systems. These operations, while critical, often incur significant performance penalties even when carefully used. Moreover, these mechanisms are implemented in proprietary and often undocumented hardware, so research into optimizations and novel designs is mostly limited to slow, simplified software simulations. In this paper, we design microarchitectural extensions to support two types of user-controlled cache writebacks to main memory. Furthermore, we propose Skip It, a mechanism built on top of our extensions that substantially reduces redundant writebacks. We implemented these designs on the open-source BOOM out-of-order RISC-V CPU. The performance in hardware is ≈ 100 cycles which favorably compares to similar operations in commercially available server-class platforms. In addition, Skip It performs as well as or better than state-of-the-art software techniques for avoiding unnecessary writebacks.