Semantically annotated hypermedia services

Hypermedia systems’ researchers investigate the various approaches in the way documents and resources are linked, navigated and stored in a distributed environment. Unfortunately, those systems fail to provide effortlessly usable discrete services, since it is difficult both to discover and to invoke any of them. This paper proposes the usage of emerging technologies that try to augment the Web resources with semantics in order to provide Hypermedia services that can be easily discovered, and integrated by potential third party developers. In this context, we analyze the benefits for the Hypermedia community upon the adoption of Semantic Web technologies for the description of Hypermedia services, and we implement an initial corresponding ontology

The Bionic DBMS is Coming, but What Will It Look Like?

Software has always ruled database engines, and commodity processors riding Moore's Law doomed database machines of the 1980s from the start. However, today's hardware landscape is very different, and moving in directions that make database machines increasingly attractive. Stagnant clock speeds, looming dark silicon, availability of reconfigurable hardware, and the economic clout of cloud providers all align to make custom database hardware economically viable or even necessary. Dataflow workloads (business intelligence and streaming) already benefit from emerging hardware support. In this paper, we argue that control flow workloads with their corresponding latencies are another feasible target for hardware support. To make our point, we outline a transaction processing architecture that offloads much of its functionality to reconfigurable hardware. We predict a convergence to fully "bionic" database engines that implement nearly all key functionality directly in hardware and relegate software to a largely managerial role

Scalable and dynamically balanced shared-everything OLTP with physiological partitioning

Scaling the performance of shared-everything transaction processing systems to highly parallel multicore hardware remains a challenge for database system designers. Recent proposals alleviate locking and logging bottlenecks in the system, leaving page latching as the next potential problem. To tackle the page latching problem, we propose physiological partitioning (PLP). PLP applies logical-only partitioning, maintaining the desired properties of sharedeverything designs, and introduces a multi-rooted B+Tree index structure (MRBTree) that enables the partitioning of the accesses at the physical page level. Logical partitioning and MRBTrees together ensure that all accesses to a given index page come from a single thread and, hence, can be entirely latch free; an extended design makes heap page accesses thread private as well. Moreover, MRBTrees offer an infrastructure for easy repartitioning and allow us to have a lightweight dynamic load balancing mechanism (DLB) on top of PLP. Profiling a PLP prototype running on different multicore machines shows that it acquires 85 and 68%fewer contentious critical sections, respectively, than an optimized conventional design and one based on logical-only partitioning. PLP also improves performance up to almost 50 % over the existing systems, while DLB enhances the system with rapid and robust behavior in both detecting and handling load imbalance

OLTP on Hardware Islands

Modern hardware is abundantly parallel and increasingly heterogeneous. The numerous processing cores have non-uniform access latencies to the main memory and to the processor caches, which causes variability in the communication costs. Unfortunately, database systems mostly assume that all processing cores are the same and that microarchitecture differences are not significant enough to appear in critical database execution paths. As we demonstrate in this paper, however, hardware heterogeneity does appear in the critical path and conventional database architectures achieve suboptimal and even worse, unpredictable performance. We perform a detailed performance analysis of OLTP deployments in servers with multiple cores per CPU (multicore) and multiple CPUs per server (multisocket). We compare different database deployment strategies where we vary the number and size of independent database instances running on a single server, from a single shared-everything instance to fine-grained shared-nothing configurations. We quantify the impact of non-uniform hardware on various deployments by (a) examining how efficiently each deployment uses the available hardware resources and (b) measuring the impact of distributed transactions and skewed requests on different workloads. Finally, we argue in favor of shared-nothing deployments that are topology- and workload-aware and take advantage of fast on-chip communication between islands of cores on the same socket.Comment: VLDB201

Improving OLTP Scalability Using Speculative Lock Inheritance

Transaction processing workloads provide ample request level concurrency which highly parallel architectures can exploit. However, the resulting heavy utilization of core database services also causes resource contention within the database engine itself and limits scalability. Meanwhile, many database workloads consist of short transactions which access only a few database records each, often with stringent response time requirements. Performance of these short transactions is determined largely by the amount of overhead the database engine imposes for services such as logging, locking, and transaction management. This paper highlights the negative scalability impact of database locking, an effect which is especially severe for short transactions running on highly concurrent multicore hardware. We propose and evaluate Speculative Lock Inheritance, a technique where hot database locks pass directly from transaction to transaction, bypassing the lock manager bottleneck. We implement SLI in the Shore-MT storage manager and show that lock inheritance fundamentally improves scalability by decoupling the number of simultaneous requests for popular locks from the number of threads in the system, eliminating contention within the lock manager even as core counts continue to increase. We achieve this effect with only minor changes to the lock manager and without changes to consistency or other application-visible effects

Critical Sections: Re-Emerging Scalability Concerns for Database Storage Engines

Critical sections in database storage engines impact performance and scalability more as the number of hardware contexts per chip continues to grow exponentially. With enough threads in the system, some critical section will eventually become a bottleneck. While algorithmic changes are the only long-term solution, they tend to be complex and costly to develop. Meanwhile, changes in enforcement of critical sections require much less effort. We observe that, in practice, many critical sections are so short that enforcing them contributes a significant or even dominating fraction of their total cost and tuning them directly improves database system performance. The contribution of this paper is two-fold: we (a) make a thorough performance comparison of the various synchronization primitives in the database system developer’s toolbox and highlight the best ones for practical use, and (b) show that properly enforcing critical sections can delay the need to make algorithmic changes for a target number of processors

Toward Scalable Transaction Processing -- Evolution of Shore-MT

Designing scalable transaction processing systems on modern multicore hardware has been a challenge for almost a decade. The typical characteristics of transaction processing workloads lead to a high degree of unbounded communication on multicores for conventional system designs. In this tutorial, we initially present a systematic way of eliminating scalability bottlenecks of a transaction processing system, which is based on minimizing unbounded communication. Then, we show several techniques that apply the presented methodology to minimize logging, locking, latching etc. related bottlenecks of transaction processing systems. In parallel, we demonstrate the internals of the Shore-MT storage manager and how they have evolved over the years in terms of scalability on multicore hardware through such techniques. We also teach how to use Shore-MT with the various design options it offers through its sophisticated application layer Shore-Kits and simple Metadata Frontend

Data-Oriented Transaction Execution

While hardware technology has undergone major advancements over the past decade, transaction processing systems have remained largely unchanged. The number of cores on a chip grows exponentially, following Moore's Law, allowing for an ever-increasing number of transactions to execute in parallel. As the number of concurrently-executing transactions increases, contended critical sections become scalability burdens. In typical transaction processing systems the centralized lock manager is often the first contended component and scalability bottleneck. In this paper, we identify the conventional thread-to-transaction assignment policy as the primary cause of contention. Then, we design DORA, a system that decomposes each transaction to smaller actions and assigns actions to threads based on which data each action is about to access. This allows each thread to mostly access thread-local data structures, minimizing interaction with the contention-prone centralized lock manager. Built on top of a conventional storage engine, DORA's design maintains all the ACID properties. Evaluation of a prototype implementation of DORA on a multicore system demonstrates that DORA attains up to 4.6x higher throughput than the state-of-the-art storage engine when running a variety of OLTP workloads, such TPC-C, TPC-B, and Nokia’s TM1

PLP: Page Latch-free Shared-everything OLTP

Scaling the performance of shared-everything on-line transaction processing to highly-parallel multicore hardware remains a great challenge for database system designers. Developments in OLTP technology remove locking and logging from being scalability bottlenecks on such systems, leaving page latching as the next potential problem. To tackle the page latching problem, we design a system around physiological partitioning (PLP). The PLP design applies logical-only partitioning, maintaining the desired properties of shared-everything designs, and introduces a multi-rooted B+Tree index structure (MRBTree) which allows us to partition the accesses at the physical page level. That is, logical partitioning, along with MRBTrees ensure that all accesses to a given index page come from a single thread and, hence, can be entirely latch-free. We extend the design to make heap page accesses thread-private as well. The elimination of page latching allows us to simplify key code paths in the system such as B+Tree operations leading to more efficient yet easier maintainable code. The profiling of a prototype PLP system shows that it acquires 85% and 68% fewer contentious critical sections per transaction than an optimized conventional design and one based on logical-only partitioning respectively. As a result the PLP prototype improves performance by up to 40% and 18% over the two systems on two multicore machines