Minimal Multi-threading: Finding and Removing Redundant Instructions in Multi-threaded Processors

• Authors: Guoping Long;Diana Franklin;Susmit Biswas;Pablo Ortiz;Jason Oberg;Dongrui Fan;Frederic T. Chong
• Venue: MICRO '43 Proceedings of the 2010 43rd Annual IEEE/ACM International Symposium on Microarchitecture
• Year: 2010

Parallelism is the key to continued performance scaling in modern microprocessors. Yet we observe that this parallelism can often contain a surprising amount of instruction redundancy. We propose to exploit this redundancy to improve performance and decrease energy consumption. We propose a multi-threading micro-architecture, Minimal Multi-Threading (MMT), that leverages register renaming and the instruction window to combine the fetch and execution of identical instructions between threads in SPMD applications. While many techniques exploit intra-thread similarities by detecting when a later instruction may use an earlier result, MMT exploits inter-thread similarities by, whenever possible, fetching instructions from different threads together and only splitting them if the computation is unique. With two threads, our design achieves a speedup of 1.15(geometric mean) over a two-thread traditional SMT with a trace cache. With four threads, our design achieves a speedup of 1.25 (geometric mean) over a traditional SMT processor with four-threads and a trace cache. These correspond to speedups of 1.5 and 1.84 over a traditional out-of-order processor. Moreover, our performance increases inmost applications with no power increase because the increase in overhead is countered with a decrease in cache accesses, leading to a decrease in energy consumption for all applications.