Critical Path Computing

关键路径预测是一种处理器架构技术，它使用指令流中指令的过去行为来预测获取的指令将在关键路径上；也就是说，哪些指令会对处理器性能产生重大影响，哪些不会。这些信息可以用来指导各种处理器优化的选择性应用。现代处理器消除了对执行吞吐量的大多数人为限制。因此，当前处理器上的许多工作负载的瓶颈是代码中的真正依赖项。相互依赖的指令链限制了机器的总体吞吐量，常常使先进的处理器技术得不到充分利用。通过代码，这些相互依赖的指令链构成了关键性能路径或关键路径（CP）。因此，处理器的性能是由它在这条关键路径上执行指令的速度决定的。在我们努力从处理器获得最大性能的过程中，不再合理地对待所有指令都是相同的。如果我们知道哪些指令对性能至关重要，我们就可以加速它们的执行，可能会以不在关键路径上的指令为代价。本研究将尝试在硬件中动态地识别这些关键指令。我们称之为关键路径预测。这种预测是基于管道中先前调用指令的行为。这种预测将使处理器能够更好地决定在何处应用某些策略和优化。各种关键路径预测将被检查。在许多情况下，关键路径预测将能够更有效地应用其他资源或进行优化。关键路径预测的可能应用包括指导性值预测、指令重用、指令发布优先级、集群架构上的指令调度、功率受限处理器上的推测控制、多线程架构上指令或线程之间的仲裁，或者在推测性多线程处理器中指导推测性线程的生成。

Critical path prediction is a processor architecture technique that uses the past behavior of instructions in the instruction stream to predict whichfetched instructions will be on the critical path; that is, which instructions will have a significant impact on processor performance, and which will not. This information can then be used to guide the selective application of a variety of processor optimizations.Modern processors remove most artificial constraints on executionthroughput. Therefore, the bottleneck for many workloads on current processors is the true dependences in the code. Chains of dependent instructions constrain the overall throughput of the machine, often leaving aggressive processor technology highly underutilized. These chains of dependent instructions constitute the critical performance path, or critical path (CP), though the code.The performance of the processor is thus determined by the speed atwhich it executes the instructions along this critical path. In ourefforts to get the maximum performance from the processor, it is nolonger reasonable to treat all instructions the same. If we can knowwhich instructions are critical to performance, we can acceleratetheir execution, possibly at the expense of instructions not on thecritical path.This research will attempt to identify these critical instructionsdynamically in hardware. We call this critical path prediction. This prediction is based on the behavior of previous invocations of the instruction in the pipeline. This prediction will enable the processor to make better decisions about where to apply certain policies and optimizations. A variety of critical path predictors will be examined.In many cases, critical path prediction will enable more effective application of other resources or optimizations. Possible applications of critical path prediction include guiding value prediction, instruction reuse, instruction issue priority, instruction scheduling on a clustered architecture, speculation control on a power-constrained processor, arbitration between instructions or threads on a multithreaded architecture, or to guide the spawning of speculative threads in a speculative multithreaded processor.