Database and System Design for Emerging Storage Technologies

Emerging storage technologies offer an alternative to disk that is durable and allows faster data access. Flash memory, made popular by mobile devices, provides block access with low latency random reads. New nonvolatile memories (NVRAM) are expected in upcoming years, presenting DRAM-like performance alongside persistent storage. Whereas both technologies accelerate data accesses due to increased raw speed, used merely as disk replacements they may fail to achieve their full potentials. Flash’s asymmetric read/write access (i.e., reads execute faster than writes opens new opportunities to optimize Flash-specific access. Similarly, NVRAM’s low latency persistent accesses allow new designs for high performance failure-resistant applications. This dissertation addresses software and hardware system design for such storage technologies. First, I investigate analytics query optimization for Flash, expecting Flash’s fast random access to require new query planning. While intuition suggests scan and join selection should shift between disk and Flash, I find that query plans chosen assuming disk are already near-optimal for Flash. Second, I examine new opportunities for durable, recoverable transaction processing with NVRAM. Existing disk-based recovery mechanisms impose large software overheads, yet updating data in-place requires frequent device synchronization that limits throughput. I introduce a new design, NVRAM Group Commit, to amortize synchronization delays over many transactions, increasing throughput at some cost to transaction latency. Finally, I propose a new framework for persistent programming and memory systems to enable high performance recoverable data structures with NVRAM, extending memory consistency with persistent semantics to introduce memory persistency.PhDComputer Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107114/1/spelley_1.pd

Delayed disengagement of attention from distractors signalling reward

Attention refers to the set of cognitive mechanisms that facilitate the prioritization of incoming sensory information. Existing research suggests that motivationally salient stimuli, such as those associated with reward, are prioritized by the attention system and that this prioritization occurs independently of an observer's goals. Specifically, studies of visual search have shown that stimuli signalling the availability of monetary reward are more likely to capture eye movements, even when participants are motivated to ignore such stimuli. In the current study we ask whether reward magnitude influences only the likelihood that stimuli will capture spatial attention, or whether reward also influences the ease with which people can disengage attention from a location when they are motivated to move their attention elsewhere. Three experiments examined the time taken to disengage from a centrally presented distractor that signalled the availability of high or low reward. We found that participants took longer to move their eyes away from a high-reward distractor, even though this came at financial cost (Experiment 1), that participants were unable to suppress a high-reward distractor consistently presented at the central location (Experiment 2), that slower responding was not due to behavioural freezing in the presence of a signal of high reward (Experiment 3), and that slower responding persisted even when rewards were no longer available (Experiment 4). These results indicate that reward modulates attentional disengagement: signals of high reward hold attention for longer, even when this is counterproductive for performance of ongoing tasks. Our findings further highlight the role of reward in the conflict between automatic and goal-directed attentional processing

Storage Management in the NVRAM Era ABSTRACT

Emerging nonvolatile memory technologies (NVRAM) offer an alternative to disk that is persistent, provides read latency similar to DRAM, and is byte-addressable. Such NVRAMs could revolutionize online transaction processing (OLTP), which today must employ sophisticated optimizations with substantial software overheads to overcome the long latency and poor random access performance of disk. Nevertheless, many candidate NVRAM technologies exhibit their own limitations, such as greater-than-DRAM latency, particularly for writes. In this paper, we reconsider OLTP durability management to optimize recovery performance and forward-processing throughput for emerging NVRAMs. First, we demonstrate that using NVRAM as a drop-in replacement for disk allows near-instantaneous recovery, but software complexity necessary for disk (i.e., Write Ahead Logging/ARIES) limits transaction throughput. Next, we consider the possibility of removing software-managed DRAM buffering. Finally, we measure the cost of ordering writes to NVRAM, which is vital for correct recovery. We consider three recovery mechanisms: NVRAM Disk-Replacement, In-Place Updates (transactions persist data in-place), and NVRAM Group Commit (transactions commit/persist atomically in batches). Whereas In-Place Updates offers the simplest design, it introduces persist synchronizations at every page update. NVRAM Group Commit minimizes persist synchronization, offering up to a 50 % throughput improvement for large synchronous persist latencies. 1