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Datenbank Spektrum 20(3) - November 2020

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  • Zeitschriftenartikel
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Sattler, Kai-Uwe; Kemper, Alfons; Härder, Theo
  • Zeitschriftenartikel
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020)
  • Zeitschriftenartikel
    Efficient Compilation of Regular Path Queries
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Tetzel, Frank; Lehner, Wolfgang; Kasperovics, Romans
    Ad hoc code generation is a state-of-the-art processing paradigm for database execution engines. It minimizes resource consumption by generating specialized code, tailored and streamlined for the single query at hand. In this work, we apply ad hoc code generation to regular path queries (RPQs), an advanced query type in declarative graph query languages. We investigate code generation from multiple angles. We propose COAT, an embedded domain specific language (EDSL) in C++ to improve accessibility of code generation by simplifying the interaction with compiler APIs. Furthermore, we analyze and compare two back ends for COAT providing the just-in-time (JIT) compilation functionality: LLVM, a compiler framework popularly used in databases for code generation, and AsmJit, a JIT assembler with very low compilation latency. We evaluate various compilation techniques for RPQs on different synthetic graph workloads.
  • Zeitschriftenartikel
    RDMA Communciation Patterns
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Ziegler, Tobias; Leis, Viktor; Binnig, Carsten
    Remote Direct Memory Access (RDMA) is a networking protocol that provides high bandwidth and low latency accesses to a remote node’s main memory. Although there has been much work around RDMA, such as building libraries on top of RDMA or even applications leveraging RDMA, it remains a hard problem to identify the most suitable RDMA primitives and their combination for a given problem. While there have been some initial studies included in papers that aim to investigate selected performance characteristics of particular design choices, there has not been a systematic study to evaluate the communication patterns of scale-out systems. In this paper, we address this issue by systematically investigating how to efficiently use RDMA for building scale-out systems.
  • Zeitschriftenartikel
    mxkernel: A Novel System Software Stack for Data Processing on Modern Hardware
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Mühlig, Jan; Müller, Michael; Spinczyk, Olaf; Teubner, Jens
    Emerging hardware platforms are characterized by large degrees of parallelism, complex memory hierarchies, and increasing hardware heterogeneity. Their theoretical peak data processing performance can only be unleashed if the different pieces of systems software collaborate much more closely and if their traditional dependencies and interfaces are redesigned. We have developed the key concepts and a prototype implementation of a novel system software stack named mxkernel . For MxKernel, efficient large scale data processing capabilities are a primary design goal. To achieve this, heterogeneity and parallelism become first-class citizens and deep memory hierarchies are considered from the very beginning. Instead of a classical “thread” model, mxkernel provides a simpler control flow abstraction: mxtasks model closed units of work, for which mxkernel will guarantee the required execution semantics, such exclusive access to a specific object in memory. They can be a very elegant abstraction also for heterogeneity and resource sharing. Furthermore, mxtasks are annotated with metadata, such as code variants (to support heterogeneity), memory access behavior (to improve cache efficiency and support memory hierarchies), or dependencies between mxtasks (to improve scheduling and avoid synchronization cost). With precisely the required metadata available, mxkernel can provide a lightweight, yet highly efficient form of resource management, even across applications, operating system, and database. Based on the mxkernel prototype we present preliminary results from this ambitious undertaking. We argue that threads are an ill-suited control flow abstraction for our modern computer architectures and that a task-based execution model is to be favored.
  • Zeitschriftenartikel
    Designing an Event Store for a Modern Three-layer Storage Hierarchy
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Glombiewski, Nikolaus; Götze, Philipp; Körber, Michael; Morgen, Andreas; Seeger, Bernhard
    Event stores face the difficult challenge of continuously ingesting massive temporal data streams while satisfying demanding query and recovery requirements. Many of today’s systems deal with multiple hardware-based trade-offs. For instance, long-term storage solutions balance keeping data in cheap secondary media (SSDs, HDDs) and performance-oriented main-memory caches. As an alternative, in-memory systems focus on performance, while sacrificing monetary costs, and, to some degree, recovery guarantees. The advent of persistent memory (PMem) led to a multitude of novel research proposals aiming to alleviate those trade-offs in various fields. So far, however, there is no proposal for a PMem-powered specialized event store. Based on ChronicleDB, we will present several complementary approaches for a three-layer architecture featuring main memory, PMem, and secondary storage. We enhance some of ChronicleDB’s components with PMem for better insertion and query performance as well as better recovery guarantees. At the same time, the three-layer architecture aims to keep the overall dollar cost of a system low. The limitations and opportunities of a PMem-enhanced event store serve as important groundwork for comprehensive system design exploiting a modern storage hierarchy.
  • Zeitschriftenartikel
    Towards Log-Less, Fine-Granular State Machine Replication
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020) Skrzypzcak, Jan; Schintke, Florian
    State machine replication is used to increase the availability of a service such as a data management system while ensuring consistent access to it. State-of-the-art implementations are based on a command log to gain linear write access to storage and avoid repeated transmissions of large replicas. However, the command log requires non-trivial state management such as allocation and pruning to prevent unbounded growth. By introducing in-place replicated state machines that do not use command logs, the log overhead can be avoided. Instead, replicas agree on a sequence of states, and former states are directly overwritten. This method enables the consistent, fault-tolerant replication of basic data management primitives such as counters, sets, or individual locks with little to no overhead. It matches the properties of fast, byte-addressable, non-volatile memory particularly well, where it is no longer necessary to rely on sequential access for good performance. Our approach is especially well suited for small states and fine-granular distributed data management as it occurs in key-value stores, for example.
  • Zeitschriftenartikel
    (Datenbank-Spektrum: Vol. 20, No. 3, 2020)