Non-volatile memory technologies are among the most promising technologies for implementing the main memories and caches in future microprocessors and replacing the traditional DRAM and SRAM technologies. However, one of the most challenging design issues of the non-volatile memory technologies is the limited write. In this article, we first propose to exploit the narrow-width values to improve the lifetime of non-volatile last-level caches with word-level write variation reduction. Leading zeros masking scheme is proposed to reduce the write stress to the upper half of the narrow-width data. To balance the write variations between the upper half and the lower half of the narrow-width data, two swapping schemes, the swap on write (SW) and swap on replacement (SRepl), are proposed. Two existing optimization schemes, the multiple dirty bit (MDB) and read before write (RBW), are adopted with our word-level swapping design. To further reduce the write variation on the partition level, we propose to exploit the cache partitioning design to improve the lifetime. Based on the observation that different applications demonstrate different cache access (write) behaviors, we propose to partition the last-level cache for different applications and balance the write variations by partition swapping. Both software-based and hardware-based partitioning and swapping schemes are proposed and evaluated for different situations. Our experimental results show that both our word- and partition-level designs can improve the lifetime of the non-volatile caches effectively with low performance and energy overheads.

Due to the high cell density, low leakage power consumption, and less vulnerability to soft errors, non-volatile memory technologies are among the most promising alternatives for replacing the traditional DRAM and SRAM technologies used in implementing main memory and caches in the modern microprocessor. However, one of the difficulties is the limited write endurance of most non-volatile memory technologies. In this paper, we propose to exploit the narrow-width values to improve the lifetime of non-volatile last level caches. Leading zeros masking scheme is first proposed to reduce the write stress to the upper half of the narrow-width data. To balance the write variations between the upper half and the lower half of the narrow-width data, two swap schemes, the swap on write (SW) and swap on replacement (SRepl), are proposed. To further reduce the write stress to non-volatile caches, we adopt two optimization schemes, the multiple dirty bit (MDB) and read before write (RBW), to improve their lifetime. Our experimental results show that by combining all our proposed schemes, the lifetime of non-volatile caches can be improved by 245% on average.

The negative bias temperature instability (NBTI) in CMOS devices is one of most prominent sources of aging mechanisms, which can induce severe threats to the reliability of modern processors at deep submicron semiconductor technologies. Due to the unbalanced duty cycle ratio of the SRAM cells, the data cache suffers a heavy NBTI stress and this will further exacerbate the aging effect in the data cache. In this paper, an aging-aware design is proposed to combat the NBTI-induced aging in the data cache. First, the detailed lifetime behaviors of the cachelines in the data cache are studied. Then, different schemes are proposed to mitigate the negative aging effects by balancing the duty cycle ratio of the SRAM cells in the cachelines according to their different lifetime phases. By applying our proposed idle-time-based cacheline invalidation, early write-back, and bit-flipping schemes, the duty cycle ratio of the data cache can be well balanced. By adopting the drowsy scheme for invalidated cachelines, our design can also reduce the power consumption significantly, which will further optimize the thermal behavior and aging effect of data caches.

The degradation of CMOS devices over the lifetime can cause the severe threat to the system performance and reliability at deep submicron semiconductor technologies. The negative bias temperature instability (NBTI) is among the most important sources of the aging mechanisms. Applying the traditional guardbanding technique to address the decreased speed of devices is too costly. Due to presence of the narrow-width values, integer register files in high-performance microprocessors suffer a very high NBTI stress. In this paper, we propose an aging-aware register file (AARF) design to combat the NBTI-induced aging in integer register files. The proposed AARF design can mitigate the negative aging effects by balancing the duty cycle ratio of the internal bits in register files. By gating the leading bits of the narrow-width values during the register accesses, our AARF can also achieve a significantly power reduction, which will further reduce the temperature and NBTI degradation of integer register files. Our experimental results show that AARF can effectively reduce the NBTI stress with a 36.9% power saving for integer register files.

Protecting on-chip cache memories against soft errors has become an increasing challenge in designing new generation reliable microprocessors. Previous efforts have mainly focused on improving the reliability of the cache data arrays. Due to its crucial importance to the correctness of cache accesses, the tag array also demands high reliability against soft errors. Exploiting the address locality of memory accesses, we propose to duplicate most recently accessed tag entries in a small tag replication buffer (TRB) thus to protect the information integrity of the tag array in the data cache. Experimental results show that our proposed TRB scheme achieves a high 90% access-with-replica (AWR) rate with low performance (~0%), energy (16.3%), and area (19.9%) overheads. We also conduct a detailed design space exploration for the TRB design and propose a selective TRB scheme that achieves a higher AWR rate (97.4%) for the dirty cachelines with negligible overheads. To provide a comprehensive evaluation of the tag-array reliability, we further conduct an architectural vulnerability factor (AVF) analysis for the tag array in the data cache and propose a refined metric, detected-without-replica-AVF (DOR-AVF), which combines the AVF and AWR analysis. Based on our DOR-AVF analysis, a selective TRB scheme with early write-back (S-TRB-EWB) is proposed, which achieves a zero DOR-AVF and 100% AWR rate at a negligible performance overhead. Results from statistical fault/error injection experiment also confirm the effectiveness of our TRB schemes and the achieved reliability of the cache tag array that recovers 100% of detected errors.

Localized heating-up creates thermal hotspots across the chip, with the integer register file ranked as the hottest unit in high-performance microprocessors. In this paper, we perform a detailed study on the thermal behavior of a low-power value-aware register file (VARF) that is subjected to internal fine-grain hotspots. To further optimize its thermal behavior, we propose and evaluate three thermal-aware control schemes, thermal sensor (TS), access counter (AC), and register-id (ID) based, to balance the access activity and thus the temperature across different partitions in the VARF. The simulation results using SPEC CINT2000 benchmarks show that the register-id controlled VARF (ID-VARF) scheme achieves optimized thermal behavior at minimum cost as compared to the other schemes. We further evaluate the performance impact of the thermal-aware VARF design with the dynamic thermal management (DTM). The experimental results show that the ID-VARF can improve the performance by 26.1% and 7.2% over the conventional register file and the original VARF design, respectively.

Soft errors induced by energetic particle strikes in on-chip cache memories have become an increasing challenge in designing new generation reliable microprocessors. Previous efforts have exploited information redundancy via parity/ECC codings or cacheline duplication for information integrity in on-chip cache memories. Due to various performance, area/size, and energy constraints in various target systems, many existing unoptimized protection schemes may eventually prove significantly inadequate and ineffective. In this paper, we propose a new framework for conducting comprehensive studies and characterization on the reliability behavior of cache memories, in order to provide insight into cache vulnerability to soft errors as well as design guidance to architects for highly efficient reliable on-chip cache memory design. Our work is based on the development of new lifetime models for data and tag arrays residing in both the data and instruction caches. Those models facilitate the characterization of cache vulnerability of stored items at various lifetime phases. We then exemplify this design methodology by proposing reliability schemes targeting at specific vulnerable phases. Benchmarking is carried out to showcase the effectiveness of our approach.

Protecting the register value and its data buses is crucial to reliable computing in high-performance microprocessors due to the increasing susceptibility of CMOS circuitry to soft errors induced by high-energy particle strikes. 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. In this paper, we propose to exploit narrow-width register values, which present the majority of the generated values, for making a duplicate of the value within the same data item; this in-register duplication(IRD)eliminates the requirement for additional copy registers. The data path pipeline is augmented to efficiently incorporate parity encoding and parity checking such that error recovery is seamlessly supported in IRD and the parity checking is overlapped with the execution stage to avoid increasing the critical path. A detailed architectural vulnerability factor (AVF) analysis shows that IRD significantly reduces the AVF from 8.4% in a conventional unprotected register file to 0.1% in an IRD register file. Our experimental evaluation using the SPEC CINT2000 benchmark suite 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.

Soft-error induced reliability problems have become a major challenge in designing new generation microprocessors. Due to the on-chip caches' dominant share in die area and transistor budget, protecting them against soft errors is of paramount importance. Recent research has 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 overdesigned for the changing error rates. In this paper, we explore the design of a self-adaptive reliable data cache that dynamically adapts its employed reliability schemes to the changing operating environments thus to maintain a target reliability. The proposed data cache is implemented with three levels of error protection schemes, a monitoring mechanism, and a control component that decides whether to upgrade, downgrade, or keep the current protection level based on the feedback from the monitor. Our experimental evaluation using a set of SPEC CPU2000 benchmarks shows that our self-adaptive data cache achieves similar reliability to a cache protected by the most reliable scheme, while simultaneously minimizing the performance and power overheads.

Protecting the register value and its data buses is crucial to reliable computing in high-performance microprocessors due to the increasing susceptibility of CMOS circuitry to soft errors induced by high-energy particle strikes. 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. In this paper, we propose to exploit narrow-width register values, which present the majority of the generated values, for duplicating a copy of the value within the same data item, called in-register duplication (IRD), eliminating the requirement of additional copy registers. The datapath pipeline is augmented to efficiently incorporate parity encoding and parity checking such that error recovery is seamlessly supported in IRD and the parity checking is overlapped with the execution stage to avoid increasing the critical path. Our experimental evaluation using the SPEC CINT2000 benchmark suite shows that IRD provides superior read-with-duplicate (RWD) and error detection/recovery rates under heavy error injection as compared to previous reliability schemes