Memory tiering is an effective strategy for enhancing the memoryto-core ratio in modern compute servers.Intel's Flat Memory Mode (FLAT) represents a cutting-edge, hardware-managed memory tiering system for CXL memory, operating at cache-line (64B) granularity with yet unknown potential for database management systems.In this paper, we evaluate the performance of FLAT using OLAP workloads in SAP HANA, an in-memory database management system.Our findings demonstrate that flat memory outperforms directly attached CXL memory by leveraging data placement and exploiting locality.However, FLAT currently accepts only a 1:1 ratio between local and CXL memory.When the local memory capacity increases, FLAT operates in a mixed mode (MIXED) containing flat memory and additional dedicated memory with remaining local memory in a separate NUMA node.Our evaluation of MIXED shows that performance degradation depends on access patterns and the amount of exploited data locality when moving table data to flat memory or allocating heap memory in flat memory.To address the configuration complexity of two separate NUMA nodes in MIXED, we propose a new CONSOLIDATED mode that merges two NUMA nodes.Our evaluation shows that the consolidated mode results in negligible performance degradation, regardless of the ratio between dedicated memory and flat memory, and requires no software modifications at the application level.

CXL-based disaggregated memory systems offer options to expand the memory beyond the limits of a single server via cache-coherent memory expansion cards or memory pools. Especially, In-Memory Database Management Systems (IMDBMSs) can benefit from alleviating two critical constraints: (1) limited memory capacity in a server and (2) long restart time during failover to reload data to memory. However, the usage and effectiveness of CXL memory in enterprise-scale IMDBMSs has yet to be validated. In this work---for the first time---we investigate dynamic memory expansion employing commercial CXL memory devices for IMDBMSs. Our detailed performance analysis reveals that the performance impact of higher latency and lower memory bandwidth impact depends on the memory access patterns of data structures (cf. (1)). Additionally, we present the feasibility of CXL shared memory between servers to improve restart times during failover (cf. (2)). Our evaluation shows the effectiveness of CXL memory integrated into the SAP HANA Cloud IMDBMS. OLTP workloads have a negligible performance degradation while OLAP workloads have a wide range of performance degradation. CXL shared memory shows a 40% reduction of the restart time for TPC-H SF10 and 84% potential reduction for TPC-H SF100.

The separation and independent scalability of compute and memory is one of the crucial aspects for modern in-memory database systems (IMDBMSs) in the cloud. The new, cache-coherent memory interconnect Compute Express Link (CXL) promises elastic memory capacity through memory pooling. In this work, we adapt the well-known IMDBMS, SAP HANA, for memory pools by features of table data placement and operational heap memory allocation on far memory, and study the impact of the limited bandwidth and higher latency of CXL. Our results show negligible performance degradation for TPC-C. For the analytical workloads of TPC-H, a notable impact on query processing is observed due to the limited bandwidth and long latency of our early CXL implementation. However, our emulation shows it would be acceptably smaller with the improved CXL memory devices.

Limited memory volume is always a performance bottleneck in an in-memory database management system (IMDBMS) as the data size keeps increasing. To overcome the physical memory limitation, heterogeneous and disaggregated computing platforms are proposed, such as Gen-Z, CCIX, OpenCAPI, and CXL. In this work, we introduce flexible CXL memory expansion using a CXL type 3 prototype and evaluate its performance in an IMDBMS. Our evaluation shows that CXL memory devices interfaced with PCIe Gen5 are appropriate for memory expansion with nearly no throughput degradation in OLTP workloads and less than 8% throughput degradation in OLAP workloads. Thus, CXL memory is a good candidate for memory expansion with lower TCO in IMDBMSs.

The significant overhead needed to transfer the data between CPUs and memory devices is one of the hottest issues in many areas of computing, such as database management systems. Disaggregated computing on the memory devices is being highlighted as one promising approach. In this work, we introduce a new near-memory acceleration scheme for in-memory database operations, called Acceleration DIMM (AxDIMM). It behaves like a normal DIMM through the standard DIMM-compatible interface, but has embedded computing units for data-intensive operations. With the minimized data transfer overhead, it reduces CPU resource consumption, relieves the memory bandwidth bottleneck, and boosts energy efficiency. We implement scan operations, one of the most data-intensive database operations, within AxDIMM and compare its performance with SIMD (Single Instruction Multiple Data) implementation on CPU. Our investigation shows that the acceleration achieves 6.8x more throughput than the SIMD implementation.

Troubleshooting imperative programs at runtime is very challenging because the final optimized plan is quite different from the original design time model. In this demonstration, we present ProcAnalyzer, an expressive and intuitive tool for troubleshooting issues related to performance, code quality, and security. We propose end-to-end graph (E2EGraph) that provides a holistic view of design time, compile time, and runtime behavior so that end users and engine developers easily find the correlations between design time and runtime. ProcAnalyzer provides suggestions and visualization to find problematic statements through the E2EGraph.

Hybrid transactional and analytical processing (HTAP) systems like SAP HANA make it much simpler to manage both operational load and analytical queries without ETL, separate data warehouses, et al. To represent both transactional and analytical business logic in a single database system, stored procedures are often used to express analytical queries using control flow logic and DMLs. Optimizing these complex procedures requires a fair knowledge of imperative programming languages as well as the declarative query language. Therefore, unified optimization techniques considering both program and query optimization techniques are essential for achieving optimal query performance. In this paper, we propose a novel unified optimization technique for efficient iterative query processing. We present a notion of query motion that allows the movement of SQL queries in and out of a loop. Additionally, we exploit a new cost model that measures the quality of the execution plan with consideration for queries and loop iterations. We describe our experimental evaluation that demonstrates the benefit of our technique using both a standard decision support benchmark and real-world workloads. An extensive evaluation shows that our unified optimization technique enumerates plans that achieve performance improvements of up to an order of magnitude faster than plans generated by the existing loop-invariant code motion technique.

Detecting major system anomalies with observed multivariate time series requires not only the characteristics of each time series but also the status of the entire time series dynamics. Therefore, we propose a method that can detect substantial anomalies by generating a transfer network and an influence network from a multivariate time series. To form a transfer network, each vertex represents a single time series. Each edge indicates the strength of the information flow between each pair of time series using transfer entropy. With the transfer network, we exploit the random walk approach to calculate the affinity score between two vertices and create an influence network that reflects both the direct and indirect influences. In our experiment, we show the efficacy of the proposed method using simple synthetic time series networks and the real data set such as world stock indices and key performance indicators of the SAP HANA in-memory database system.

Detecting performance anomalies and finding their root causes are tedious tasks requiring much manual work. Functionality enhancements in DBMS development as in most software development often introduce performance problems in addition to bugs. To detect the problems as soon as they are introduced, which often happens during the early phases of a development cycle, we adopt performance regression testing early in the process. In this paper, we describe a framework that we developed to manage performance anomalies after establishing a set of conditions for a problem to be considered an anomaly. The framework uses Statistical Process Control (SPC) charts to detect performance anomalies and differential profiling to identify their root causes. By automating the tasks within the framework we were able to remove most of the manual overhead in detecting anomalies and reduce the analysis time for identifying the root causes by about 90 percent in most cases. The tools developed and deployed based on the framework allow us continuous, automated daily monitoring of performance in addition to the usual functionality monitoring in our DBMS development.