How do you design and implement an efficient out-of-order execution scheduler?

3 Priority Selection

The scheduler needs to select the best instruction to execute at each cycle, based on the availability of operands, functional units, and other factors. The priority selection algorithm determines the order in which the instructions are issued to the functional units, and can have a significant impact on the performance and power consumption of the processor. A simple priority selection algorithm is to use a first-in first-out (FIFO) policy, where the oldest instruction in the instruction window is selected first. However, this may not be optimal for some scenarios, such as when younger instructions have lower latency or higher priority. A more sophisticated priority selection algorithm is to use a critical path analysis, where the scheduler estimates the impact of each instruction on the overall execution time, and selects the most critical instruction first.

4 Wakeup and Select

The wakeup and select logic is the part of the scheduler that implements the dependency tracking and priority selection functions. The wakeup logic monitors the completion of instructions and operands, and updates the readiness status of the dependent instructions in the instruction window. The select logic chooses the best instruction to execute from the ready instructions, based on the priority selection algorithm. The wakeup and select logic is often implemented as a large combinational circuit that operates in a single cycle, which can be a bottleneck for the performance and scalability of the scheduler. Some techniques to reduce the complexity and power consumption of the wakeup and select logic are to use clustering, partitioning, or pipelining.

5 Reorder Buffer

The reorder buffer is the part of the processor that ensures that the out-of-order execution does not affect the architectural state of the processor, such as the register file and the program counter. The reorder buffer stores the instructions in program order, along with their results and status. When an instruction completes its execution, it writes its result to the reorder buffer, but not to the register file. When an instruction reaches the head of the reorder buffer, it commits its result to the register file, and updates the program counter if it is a branch instruction. The reorder buffer also handles exceptions and mispredictions, by discarding or flushing the instructions that are affected, and restoring the correct state of the processor.

6 Performance Evaluation

The performance of an out-of-order execution scheduler depends on many factors, such as the instruction mix, the memory hierarchy, the branch predictor, and the functional unit configuration. To evaluate the performance of a scheduler, one can use analytical models, simulation tools, or hardware prototypes. Analytical models can provide insights into the trade-offs and limitations of different design choices, but they may not capture all the details and interactions of a real system. Simulation tools can provide more accurate and detailed results, but they may be slow and complex to use. Hardware prototypes can provide the most realistic and reliable results, but they may be costly and time-consuming to build and test.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?