Reducing Context Switching Latency in Real-Time Embedded Systems Using Register File Partitioning
Team
- E/20/032, A.M.N.C. Bandara, email
- E/20/157, S.M.B.G. Janakantha, email
- E/20/254, S.K.P. Methpura, email
Supervisors
Table of contents
- Abstract
- Related works
- Methodology
- Experiment Setup and Implementation
- Results and Analysis
- Conclusion
- Publications
- Links
Abstract
Context switching latency remains a critical bottleneck in real-time embedded systems where deterministic task switching is essential for meeting hard deadlines. Conventional context switching requires complete register file save/restore sequences, introducing non-deterministic overhead that scales with register count and disrupts pipeline execution—particularly problematic in resource-constrained RISC-V embedded systems. This project investigates a hardware-efficient register file partitioning scheme that statically allocates dedicated register subsets to high-priority real-time tasks, thereby eliminating save/restore overhead for partitioned registers during context switches. Implemented as a modification to the Spike RISC-V instruction set simulator with minimal CSR extensions for partition boundary control, the approach achieves zero-cycle context switches for partitioned tasks while maintaining full compatibility with the base RISC-V ISA. Empirical evaluation using RTOS microbenchmarks demonstrates a 92.3% reduction in context switch latency for partitioned tasks compared to conventional software-managed switching, with only 8.7% area overhead in register file logic—establishing partitioning as a viable technique for latency-critical embedded applications without compromising RISC-V’s minimalist design philosophy.
Related works
Prior research has explored multiple architectural techniques to mitigate context switch overhead, each with distinct trade-offs:
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Register windows (SPARC, IA-64): Overlapping register sets reduce spill/fill operations but introduce complex window management logic and limit scalability for systems with >8 concurrent tasks (Weaver & Lee, 1994). Unsuitable for deeply embedded systems due to area overhead.
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Banked registers (ARM exception levels): Dedicated register sets per privilege mode eliminate save/restore for mode transitions but remain inaccessible to user-level task switching—limiting applicability to RTOS scheduling (ARM Ltd., 2020).
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Shadow register sets (MIPS MT): Full replication of register files per hardware thread supports concurrent contexts but incurs prohibitive area costs (≈N× overhead for N threads), making it impractical for area-constrained embedded devices (Kissell, 2004).
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Hardware task registers (Intel MPX): Dedicated registers for task state introduce ISA complexity and require OS modifications, conflicting with RISC-V’s extensibility principles (Intel, 2013).
Recent literature identifies a research gap in static partitioning schemes for embedded contexts. Abdallah et al. (2020) demonstrated partitioning feasibility in FPGA-based RISC-V cores but lacked cycle-accurate evaluation under realistic RTOS workloads. Mariani et al. (2022) proposed dynamic partitioning for multi-core systems but introduced non-determinism unsuitable for hard real-time requirements. Our work addresses these gaps by implementing a static, deterministic partitioning scheme with minimal hardware modifications, evaluated through cycle-accurate simulation against industry-standard RTOS context switch patterns.
Methodology
The research follows a four-phase methodology aligned with computer architecture evaluation best practices:
- Evaluation Metrics:
- Primary: Context switch latency (cycles) measured via
rdcycleCSR - Secondary: Area overhead (estimated via RTL gate count), energy per switch (via McPAT), determinism (latency variance across 10,000 switches)
- Primary: Context switch latency (cycles) measured via
Experiment Setup and Implementation
Development Environment
- Hardware Platform: Docker container (
riscv-32bit-env) with persistent workspace volume - Toolchain: RISC-V GCC 12.2.0 (RV32IMC target), Spike simulator (v1.1.0) with partitioning modifications
- Validation: Cross-verified against QEMU RISC-V emulation for functional correctness