Publications by Year: 2009

Supply voltage fluctuations that result from inductive noise are increasingly troublesome in modern microprocessors. A voltage “emergency”, i.e., a swing beyond tolerable operating margins, jeopardizes the safe and correct operation of the processor. Techniques aimed at reducing power consumption, e.g., by clock gating or by reducing nominal supply voltage, exacerbate this noise problem, requiring ever-wider operating margins. We propose an event-guided, adaptive method for avoiding voltage emergencies, which exploits the fact that most emergencies are correlated with unique microarchitectural events, such as cache misses or the pipeline flushes that follow branch mispredictions. Using checkpoint and rollback to handle unavoidable emergencies, our method adapts dynamically by learning to trigger avoidance mechanisms when emergency-prone events recur. After tightening supply voltage margins to increase clock frequency and accounting for all costs, the net result is a performance improvement of 8% across a suite of fifteen SPEC CPU2000 benchmarks.

Transient faults are emerging as a critical concern in the reliability of general-purpose microprocessors. As architectural trends point towards multi-core designs, there is substantial interest in adapting such parallel hardware resources for transient fault tolerance. This paper presents process-level redundancy (PLR), a software technique for transient fault tolerance which leverages multiple cores for low overhead. PLR creates a set of redundant processes per application process, and systematically compares the processes to guarantee correct execution. Redundancy at the process level allows the operating system to freely schedule the processes across all available hardware resources. PLR uses a software-centric approach to transient fault tolerance which shifts the focus from ensuring correct hardware execution to ensuring correct software execution. As a result, many benign faults that do not propagate to affect program correctness can be safely ignored. A real prototype is presented that is designed to be transparent to the application and can run on general-purpose single-threaded programs without modifications to the program, operating system, or underlying hardware. The system is evaluated for fault coverage and performance on 4-way SMP machine, and provides improved performance over existing software transient fault tolerance techniques with an 16.9% overhead for fault detection on a set of optimized SPEC2000 binaries.

Index Terms—fault tolerance, reliability, transient faults, soft errors, process-level redundancy

Power constrained designs are becoming increasingly sensitive to supply voltage noise. We propose a hardware-software collaborative approach to enable aggressive operating margins: a checkpoint-recovery mechanism corrects margin violations, while a run-time software layer reschedules the program’s instruction stream to prevent recurring margin crossings at the same program location. The run-time layer removes 60% of these events with minimal overhead, thereby significantly improving overall performance.

Categories and Subject Descriptors

C.0 [Computer Systems Organization]: General— Hardware/Software interfaces and System architectures.

General Terms

Performance, Reliability.

Keywords

Runtime Optimization, Hardware Software Co-Design.

Inductive noise forces microprocessor designers to sacrifice performance in order to ensure correct and reliable operation of their designs. The possibility of wide fluctuations in supply voltage means that timing margins throughout the processor must be set pessimistically to protect against worst-case droops and surges. While sensor-based reactive schemes have been proposed to deal with voltage noise, inherent sensor delays limit their effectiveness. Instead, this paper describes a voltage emergency predictor that learns the signatures of voltage emergencies (the combinations of control flow and microarchitectural events leading up to them) and uses these signatures to prevent recurrence of the corresponding emergencies. In simulations of a representative superscalar microprocessor in which fluctuations beyond 4% of nominal voltage are treated as emergencies (an aggressive configuration), these signatures can pinpoint the likelihood of an emergency some 16 cycles ahead of time with 90% accuracy. This lead time allows machines to operate with much tighter voltage margins (4% instead of 13%) and up to 13.5% higher performance, which closely approaches the 14.2% performance improvement possible with an ideal oracle-based predictor.

Power constrained designs are becoming increasingly sensitive to supply voltage noise. We propose hardware-software collaboration to enable aggressive voltage margins: a fail-safe hardware mechanism tolerates margin violations in order to train a run-time software layer that reschedules instructions to avoid recurring violations. Additionally, the software controls an emergency signature-based predictor that throttles to suppress emergencies that code rescheduling cannot eliminate.