Towards Accurate Performance Modeling of RISC-V Designs

Abstract

This paper investigates the critical challenge of achieving high performance modeling accuracy using microarchitecture-level simulators, which traditionally prioritize speed over fidelity compared to RTL simulation. The authors conduct a detailed study using the widely adopted gem5 simulator, diligently adjusting parameters to accurately model a RISC-V out-of-order superscalar microprocessor core. The research successfully demonstrates the main sources of errors that prevent high accuracy levels in microarchitecture modeling, providing crucial insights for future RISC-V design validation.

Report

Key Highlights

• The study focuses on closing the accuracy gap between high-throughput microarchitecture-level simulators and highly accurate, but slow, lower-level (RTL) simulation.
• The primary research subject is achieving accurate performance modeling for a modern RISC-V out-of-order superscalar microprocessor core.
• The methodology involves a detailed microarchitecture-level simulation study where key parameters of the simulator are meticulously adjusted to match the target design.
• The paper successfully identifies and demonstrates the principal sources of errors that limit the accuracy of microarchitecture modeling, particularly when compared against RTL results.

Technical Details

• Target Architecture: A RISC-V out-of-order superscalar microprocessor core.
• Simulation Tool: The widely used microarchitectural simulator, gem5, is the basis for the modeling study.
• Goal of Modeling: To accurately express the design standards and capture how various aspects of the microarchitecture change during design refinement.
• Validation Method: Accuracy is benchmarked by comparing the microarchitecture-level simulation results against the true accuracy provided by RTL simulation of the target design.
• Technical Focus: Analyzing the trade-off between simulation speed and hardware accuracy, specifically identifying factors that introduce uncertainty in performance measurements.

Implications

• Accelerated Design Space Exploration (DSE): By improving the accuracy of high-speed simulators like gem5, designers can perform DSE far faster than relying on time-consuming RTL simulation, drastically shortening the RISC-V development cycle.
• Improved Validation Reliability: Pinpointing and correcting the main sources of modeling errors enhances the reliability of performance measurements taken from high-level models, ensuring that RISC-V architects make design decisions based on more trustworthy data.
• Advancing Simulation Tooling: The findings guide the necessary refinements for tools like gem5, pushing the state-of-the-art in microarchitectural simulation to better handle the complexities of modern, superscalar, and out-of-order RISC-V implementations.
• Wider Ecosystem Benefit: More robust and accurate modeling tools lower the barrier for entry and accelerate innovation for the entire RISC-V hardware ecosystem.