Exploiting eager register release in a redundantly multi-threaded processor

Journal ArticleDue to shrinking transistor sizes and lower supply voltages, transient faults (soft errors) in computer systems are projected to increase by orders of magnitude. Fault detection and recovery can be achieved through redundancy. Redundant multithreading (RMT) is one attractive approach to detect and recover from these errors. However, redundant threads can impose significant performance overheads by competing with the main program for resources such as the register file. In this paper, we propose using eager register release in the main program thread by exploiting the availability of register values in the trailing thread's register space. This performance optimization can help support a smaller register file and potentially reduce register file access time, power consumption, and increase its immunity towards soft errors

Reducing soft errors through operand width aware policies

Soft errors are an important challenge in contemporary microprocessors. Particle hits on the components of a processor are expected to create an increasing number of transient errors with each new microprocessor generation. In this paper, we propose simple mechanisms that effectively reduce the vulnerability to soft errors in a processor. Our designs are generally motivated by the fact that many of the produced and consumed values in the processors are narrow and their upper order bits are meaningless. Soft errors caused by any particle strike to these higher order bits can be avoided by simply identifying these narrow values. Alternatively, soft errors can be detected or corrected on the narrow values by replicating the vulnerable portion of the value inside the storage space provided for the upper order bits of these operands. As a faster but less fault tolerant alternative to ECC and parity, we offer a variety of schemes that make use of narrow values and analyze their efficiency in reducing soft error vulnerability of different data-holding components of a processor. On average, techniques that make use of the narrowness of the values can provide 49 percent error detection, 45 percent error correction, or 27 percent error avoidance coverage for single bit upsets in the first level data cache across all Spec2K. In other structures such as the immediate field of the issue queue, an average error detection rate of 64 percent is achieved.Peer ReviewedPostprint (published version

Soft-error resilient on-chip memory structures

Soft errors induced by energetic particle strikes in on-chip memory structures, such as L1 data/instruction caches and register files, have become an increasing challenge in designing new generation reliable microprocessors. Due to their transient/random nature, soft errors cannot be captured by traditional verification and testing process due to the irrelevancy to the correctness of the logic. This dissertation is thus focusing on the reliability characterization and cost-effective reliable design of on-chip memories against soft errors. Due to various performance, area/size, and energy constraints in various target systems, many existing unoptimized protection schemes on cache memories may eventually prove significantly inadequate and ineffective. This work develops new lifetime models for data and tag arrays residing in both the data and instruction caches. These models facilitate the characterization of cache vulnerability of the stored items at various lifetime phases. The design methodology is further exemplified by the proposed reliability schemes targeting at specific vulnerable phases. Benchmarking is carried out to showcase the effectiveness of these approaches. The tag array demands high reliability against soft errors while the data array is fully protected in on-chip caches, because of its crucial importance to the correctness of cache accesses. Exploiting the address locality of memory accesses, this work proposes a Tag Replication Buffer (TRB) to protect information integrity of the tag array in the data cache with low performance, energy and area overheads. To provide a comprehensive evaluation of the tag array reliability, this work also proposes a refined evaluation metric, detected-without-replica-TVF (DOR-TVF), which combines the TVF and access-with-replica (AWR) analysis. Based on the DOR-TVF analysis, a TRB scheme with early write-back (TRB-EWB) is proposed, which achieves a zero DOR-TVF at a negligible performance overhead. Recent research, as well as the proposed optimization schemes in this cache vulnerability study, have focused on the design of cost-effective reliable data caches in terms of performance, energy, and area overheads based on the assumption of fixed error rates. However, for systems in operating environments that vary with time or location, those schemes will be either insufficient or over-designed for the changing error rates. This work explores the design of a self-adaptive reliable data cache that dynamically adapts its employed reliability schemes to the changing operating environments in order to maintain a target reliability. The experimental evaluation shows that the self-adaptive data cache achieves similar reliability to a cache protected by the most reliable scheme, while simultaneously minimizing the performance and power overheads. Besides the data/instruction caches, protecting the register file and its data buses is crucial to reliable computing in high-performance microprocessors. Since the register file is in the critical path of the processor pipeline, any reliable design that increases either the pressure on the register file or the register file access latency is not desirable. This work proposes to exploit narrow-width register values, which represent the majority of generated values, for making the duplicates within the same register data item. A detailed architectural vulnerability factor (AVF) analysis shows that this in-register duplication (IRD) scheme significantly reduces the AVF in the register file compared to the conventional design. The experimental evaluation also shows that IRD provides superior read-with-duplicate (RWD) and error detection/recovery rates under heavy error injection as compared to previous reliability schemes, while only incurring a small power overhead. By integrating the proposed reliable designs in data/instruction caches and register files, the vulnerability of the entire microprocessor is dramatically reduced. The new lifetime model, the self-adaptive design and the narrow-width value duplication scheme proposed in this work can also provide guidance to architects toward highly efficient reliable system design

Exploiting narrow values for soft error tolerance

Soft errors are an important challenge in contemporary microprocessors. Particle hits on the components of a processor are expected to create an increasing number of transient errors with each new microprocessor generation. In this paper we propose simple mechanisms that effectively reduce the vulnerability to soft errors in a processor. Our designs are generally motivated by the fact that many of the produced and consumed values in the processors are narrow and their upper order bits are meaningless. Soft errors canted by any particle strike to these higher order bits can be avoided by simply identifying these narrow values. Alternatively soft errors can be detected or corrected on the narrow values by replicating the vulnerable portion of the value inside the storage space provided for the upper order bits of these operands. We offer a variety of schemes that make use of narrow values and analyze their efficiency in reducing soft error vulnerability of level-1 data cache of the processor.Peer Reviewe