Rethinking Dynamic Instruction Scheduling and Retirement for Efficient Microarchitectures
Abstract: Out-of-order execution is one of the main micro-architectural techniques used to improve the performance of both single- and multi-threaded processors. The application of such a processor varies from mobile devices to server computers. This technique achieves higher performance by finding independent instructions and hiding execution latency and uses the cycles which otherwise would be wasted or caused a CPU stall. To accomplish this, it uses scheduling resources including the ROB, IQ, LSQ and physical registers, to store and prioritize instructions.The pipeline of an out-of-order processor has three macro-stages: the front-end, the scheduler, and the back-end. The front-end fetches instructions, places them in the out-of-order resources, and analyzes them to prepare for their execution. The scheduler identifies which instructions are ready for execution and prioritizes them for scheduling. The back-end updates the processor state with the results of the oldest completed instructions, deallocates the resources and commits the instructions in the program order to maintain correct execution.Since out-of-order execution needs to be able to choose any available instructions for execution, its scheduling resources must have complex circuits for identifying and prioritizing instructions, which makes them very expansive, therefore, limited. Due to their cost, the scheduling resources are constrained in size. This limited size leads to two stall points respectively at the front-end and the back-end of the pipeline. The front-end can stall due to fully allocated resources and therefore no more new instructions can be placed in the scheduler. The back-end can stall due to the unfinished execution of an instruction at the head of the ROB which prevents other resources from being deallocated, preventing new instructions from being inserted into the pipeline.To address these two stalls, this thesis focuses on reducing the time instructions occupy the scheduling resources. Our front-end technique tackles IQ pressure while our back-end approach considers the rest of the resources. To reduce front-end stalls we reduce the pressure on the IQ for both storing (depth) and issuing (width) instructions by bypassing them to cheaper storage structures. To reduce back-end stalls, we explore how we can retire instructions earlier, and out-of-order, to reduce the pressure on the out-of-order resource.
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