This article presents a chip multiprocessor (CMP) design that mixes coarse- and fine-grained reconfigurability to increase core availability of safety-critical embedded systems in the presence of hard errors. The authors conducted a comprehensive design-space exploration to identify the granularity mixes that maximize CMP fault tolerance and minimize performance and energy overheads. The authors added fine-grained reconfigurable logic to a coarse-grained sparing approach. Their resulting design can tolerate 3 times more hard errors than core redundancy and 1.5 times more than any other purely coarse-grained solution.

Permanent faults on a chip are often tolerated using spare resources. In the past, sparing has been applied to Chip Multiprocessors (CMPs) at various granularities of substitutable units (SUs). Entire processors, pipeline stages or even individual functional units are isolated when faulty and replaced by spare ones using flexible, reconfigurable interconnects. Although spare resources increase systems fault tolerance, the extra delay imposed by the reconfigurable interconnects limits performance. In this paper, we study two options for dealing with this delay: (i) pipelining the reconfigurable interconnects and (ii) scaling down operating frequency. The former keeps a frequency close to the one of the baseline processor, but increases the number of cycles required for executing a program. The latter maintains the number of execution cycles constant, but requires a slower clock. We investigate the above performance tradeoff using an adaptive 4-core CMP design with substitutable pipeline stages. We retrieve post place and route results of different designs running two sets of benchmarks and evaluate their performance. Our experiments indicate that adding reconfigurable interconnects for wiring the SUs of a 4-core CMP pose significant delay increasing the critical path of the design almost by 3.5 times. On the other hand, pipelining the reconfigurable interconnects increases cycle time by 41% and - depending on the processor configuration - reduces performance overhead to 1.4-2.9× the execution time of the baseline.

Recent trends in semiconductor technology have dictated the constant reduction of device size. One negative effect stemming from the reduction in size and increased complexity is the reduced device reliability. This paper is centered around the matter of permanent fault tolerance and graceful system degradation in the presence of permanent faults. We take advantage of the natural redundancy of homogeneous multicores following a sparing strategy to reuse functional pipeline stages of faulty cores. This is done by incorporating reconfigurable interconnects next to which the cores of the system are placed, providing the flexibility to redirect the data-flow from the faulty pipeline stages of damaged cores to spare (still) functional ones. Several micro-architectural changes are introduced to decouple the processor stages and allow them to be interchangeable. The proposed approach is a clear departure from previous ones by offering full flexibility as well as highly graceful performance degradation at reasonable costs. More specifically, our coarsegrain faulttolerant multicore array provides up to ×4 better availability compared to a conventional multicore and up to ×2 higher probability to deliver at least one functioning core in high fault densities. For our benchmarks, our design (synthesized for STM 65nm SP technology) incurs a total execution-time overhead for the complete system ranging from ×1.37 to ×3.3 compared to a (baseline) non-fault-tolerant system, depending on the permanent-fault density. The area overhead is 19.5% and the energy consumption, without incorporating any power/energy- saving technique, is estimated on average to be 20.9% higher compared to the baseline, unprotected design.

Reconfigurable hardware can be employed to tolerate permanent faults. Hardware components comprising a System-on-Chip can be partitioned into a handful of substitutable units interconnected with reconfigurable wires to allow isolation and replacement of faulty parts. This paper offers a probabilistic analysis of reconfigurable designs estimating for different fault densities the average number of fault-free components that can be constructed as well as the probability to guarantee a particular availability of components. Considering the area overheads of reconfigurability, we evaluate the resilience of various reconfigurable designs with different granularities. Based on this analysis, we conduct a comprehensive design-space exploration to identify the granularity mixes that maximize the fault-tolerance of a system. Our findings reveal that mixing fine-grain logic with a coarse-grain sparing approach tolerates up to 3× more permanent faults than component redundancy and 2× more than any other purely coarse-grain solution. Component redundancy is preferable at low fault densities, while coarse-grain and mixed-grain reconfigurability maximize availability at medium and high fault densities, respectively.