DynaMOS: Dynamic Schedule Migration for Heterogeneous Cores

InOrder (InO) cores achieve limited performance be-cause their inability to dynamically reorder instructions prevents them from exploiting Instruction-Level-Parallelism. Conversely, Out-of-Order (OoO) cores achieve high per-formance by aggressively speculating past stalled in-structions and creating highly optimized issue sched-ules. It has been observed that these issue schedules tend to repeat for sequences of instructions with pre-dictable control and data-flow. An equally provisioned InO core can potentially achieve OoO’s performance at a fraction of the energy cost if provided with an OoO schedule. In the context of a fine-grained heterogeneous multicore system composed of a big (OoO) core and a little (InO) core, we could offload recurring issue sched-ules from the big to the little core, to achieve energy-efficiency while maintaining performance. To this end, we introduce the DynaMOS architecture. Recurring issue schedules may contain instructions that speculate across branches, utilize renamed registers to eliminate false dependencies, and reorder memory oper-ations. DynaMOS provisions little with an OinO mode to replay a speculative schedule while ensuring program correctness. Any divergence from the recorded instruc-tion sequence causes execution to restart in program order from a previously checkpointed state. On a sys-tem capable of switching between big and little cores rapidly with low overheads, DynaMOS schedules 38% of execution on the little on average, increasing utiliza-tion of the energy-efficient core by 2.9X over prior work. This amounts to energy savings of 32 % over execution on only big core, with an allowable 5 % performance loss