UCLA

The reliability of memory subsystems is worsening rapidly and needs to be considered as one of the primary design objectives when designing today's computer systems. From on-chip embedded memories in Internet-of-Things (IoT) devices and on-chip caches to off-chip main memories, they have become the limiting factor in the reliability of these computing systems. Today's applications demand large capacity of on-chip or off-chip memory or both. With aggressive technology scaling, coupled with the increase in the total area devoted to memory in a chip, memories are becoming particularly sensitive to manufacturing process variation, environmental operating conditions, and aging-induced wearout. However, the challenge with memory reliability is that the resiliency techniques need to be effective but with minimal overhead. Today's typical error correcting schemes do not take into consideration the data value that they are protecting and are purely based on positional errors. This increases their overheads and makes them too expensive, especially for on-chip memories. Also, the drive for denser off-chip main memories is worsening their reliability. But strengthening today's error correction techniques will result in non-negligible increase in overheads. Hence, this dissertation proposes Lightweight Opportunistic Memory Resilience. We exploit the following three aspects to make memories more reliable with low overheads: (1) Underlying memory fault models, (2) Data value behavior of commonly used applications, and (3) The architecture of the memory itself. We opportunistically exploit these three aspects to provide stronger protection against memory errors. We design novel error detecting and correcting codes and develop several other architectural fault tolerance techniques at minimal overheads compared to the conventional reliability techniques used in today's memories.

In part 1 of this dissertation, we address the reliability concerns in lightweight on-chip caches or embedded memories like scratchpads in IoT devices. These memories are becoming larger in size, but needs to be low power. Using standard error correcting codes or traditional row/column sparing to recover from faults are too expensive for them. Here, we leverage the fact that manufacturing defects and aging-induced hard faults usually only affect only a few bits in a memory. These bits, however, inhibit how low of a voltage these chips can be operated at. Traditional software fails even when a small number of bits in a memory are faulty. For the first time, we provide two solutions, FaultLink and SAME-Infer, which help deal with these weak faulty cells in the memory by generating a custom-tailored fault-aware application binary image for each chip. Next, we designed Software-Defined Error Localization Code (SDELC) and Parity++ as lightweight runtime error recovery techniques that leverage the insight that data values have locality in them and certain ranges of data values occur more frequently than others. Conventional ECC is too expensive for these lightweight memories. SDELC uses novel ultra-lightweight error-localizing codes to localize the error to a chunk in the data. It then heuristically recovers from the localized error by exploiting side information about the application's memory contents. Parity++ is a novel unequal message protection scheme that preferentially provides stronger error protection to certain ''special messages". This protection scheme provides Single Error Detection (SED) for all messages and Single Error Correction (SEC) for a subset of special messages. Both these novel codes utilize data value behavior to provide single error correction at 2.5x-4x lower overhead than a conventional hamming single error correcting code.

In part 2 of this dissertation, we focus on off-chip main memory technologies. We primarily leverage the details of the memory architecture itself and their dominant fault mechanisms to effectively design reliability schemes. The need for larger main memory capacity in today's workstation or server environments is driving the use of non-volatile memories (NVM) or techniques to enable high density DRAMs. Due to aggressive scaling, the single-bit error rate in DRAMs is steadily increasing and DRAM manufacturers are adopting on-die error correction coding (ECC) schemes, along with within memory controller ECC, to correct single-bit errors in the memory. In COMET we have shown that today’s standard on-die ECCs can lead to silent data corruption if not designed correctly. We propose a collaborative on-die and in-controller error correction scheme that prevents double-bit error induced silent data corruption and corrects 99.9997% of these double-bit errors at absolutely no additional storage, latency, and area overheads. Not just DRAMs, reliability is a major concern in most of the emerging NVM technologies. In Compression with Multi-ECC (CME), we propose a new opportunistic compression-based ECC protection scheme for magnetic memory-based main memories. CME compresses every memory line and uses the saved bits to add stronger protection. In some of these NVMs, error rates increase as we try to improve read/write latencies. In PCM-Duplicate, we propose an enhanced PCM architecture that reduces PCM read latency by more than 3x and makes it comparable to that of DRAM. We then use ECC to tolerate the additional errors that arise because of the proposed optimizations.

Overall, we have developed a complementary suite of novel methods for tolerating faults and correcting errors in different levels of the memory hierarchy. We exploit the memory architecture and fault mechanisms as well as the application data behavior to tune the proposed solutions to the particular memory characteristics; lightweight solutions for low-cost embedded memories and latency-critical on-chip caches while stronger protection for off-chip main memory subsystems. With memory reliability being a major bottleneck in today’s systems, these novel solutions are expected to alleviate this problem, help cope with the unique outcomes of hardware variability in memory systems and provide improved reliability at minimal cost.