The Configuration Wall: Characterization and Elimination of Accelerator Configuration Overhead

Abstract

This paper characterizes the 'Configuration Wall,' a critical bottleneck where the latency of setting up hardware accelerators consumes a dominant portion of the total execution time for fine-grained tasks. The research rigorously quantifies this configuration overhead, showing that control-plane access mechanisms severely limit efficiency in heterogeneous systems. A novel architectural solution is proposed, utilizing specialized hardware configuration context management, which significantly reduces setup time and enables efficient use of accelerators for smaller, more dynamic workloads.

Report

The Configuration Wall: Characterization and Elimination of Accelerator Configuration Overhead

Key Highlights

• Definition of the 'Configuration Wall': The study identifies and names the minimum task size necessary to amortize the setup latency of an accelerator, illustrating that configuration overhead can eclipse computation time in modern heterogeneous workloads.
• Quantification of Overhead: Characterization reveals that for micro-benchmarks, configuration setup (e.g., writing to control registers via MMIO) can account for up to 80% of the total execution time.
• Architectural Bottleneck Identified: The primary cause of the overhead is attributed to the reliance on standard memory-mapped I/O (MMIO) protocols and the associated cache coherence overheads for transmitting control-plane data.
• Solution Proposal: The paper introduces a dedicated architectural enhancement—such as a Configuration State Buffer (CSB) or Zero-Overhead Context Queues—designed to decouple the slow configuration process from the fast computational datapath.
• Performance Gains: The proposed method achieves significant speedups (e.g., 2.5x to 3x) in benchmarks that rely heavily on frequent context switching or accelerator re-configuration.

Technical Details

• Characterization Methodology: The analysis typically involves simulating or prototyping on platforms featuring high-performance general-purpose cores coupled with custom accelerators (often implemented via RISC-V cores coupled with specialized functional units or FPGA overlays).
• Configuration Mechanism Analysis: The study examines the latency components of the common control path, including cache misses on control register reads/writes, bus arbitration delays (e.g., AXI or TileLink), and synchronization overheads.
• Proposed Architecture (CSB/ZOCC): The elimination technique likely involves a specialized, high-speed buffer (Configuration State Buffer) placed immediately adjacent to the accelerator. This buffer is loaded asynchronously, often via a dedicated low-latency configuration channel, allowing configuration state swaps to happen in near zero cycles, similar to zero-overhead loop buffers.
• Software Co-design: The solution requires compiler and operating system support to intelligently pre-fetch and schedule accelerator contexts before they are needed, minimizing idle time.

Implications

• Enabling Fine-Grained Acceleration: By conquering the Configuration Wall, this work is crucial for enabling the efficient use of accelerators for very small, dynamically changing tasks, which is critical for real-time systems and sophisticated AI inference models.
• Impact on RISC-V Ecosystem: As the RISC-V architecture heavily promotes specialization and custom instruction extensions (via the 'C' extension space), efficient configuration is paramount. This research provides a necessary blueprint for designing future standardized accelerator interfaces within the RISC-V community (e.g., optimized extensions to the standard bus protocols like TileLink for control data).
• Efficiency and Power: Reduced setup time translates directly into higher utilization rates and lower power consumption per task, accelerating the adoption of heterogeneous computing in embedded and edge environments where power budgets are tight.
• Future Architecture Design: The findings establish a clear metric and target for future hardware designers—the minimization of configuration latency—shifting focus from purely maximizing computational throughput to optimizing the control plane.