Micro-architectural Analysis of In-memory OLTP

Micro-architectural behavior of traditional disk-based online transaction processing (OLTP) systems has been investigated extensively over the past couple of decades. Results show that traditional OLTP mostly under-utilize the available micro-architectural resources. In-memory OLTP systems, on the other hand, process all the data in mainmemory, and therefore, can omit the buffer pool. In addition, they usually adopt more lightweight concurrency control mechanisms, cache-conscious data structures, and cleaner codebases since they are usually designed from scratch. Hence, we expect significant differences in micro-architectural behavior when running OLTP on platforms optimized for inmemory processing as opposed to disk-based database systems. In particular, we expect that in-memory systems exploit micro architectural features such as instruction and data caches significantly better than disk-based systems. This paper sheds light on the micro-architectural behavior of in-memory database systems by analyzing and contrasting it to the behavior of disk-based systems when running OLTP workloads. The results show that despite all the design changes, in-memory OLTP exhibits very similar microarchitectural behavior to disk-based OLTP systems: more than half of the execution time goes to memory stalls where L1 instruction misses and the long-latency data misses from the last-level cache are the dominant factors in the overall stall time. Even though aggressive compilation optimizations can almost eliminate instruction misses, the reduction in instruction stalls amplifies the impact of last-level cache data misses. As a result, the number of instructions retired per cycle barely reaches one on machines that are able to retire up to four for both traditional disk-based and new generation in-memory OLTP.

Micro-architectural analysis of in-memory OLTP: Revisited

Anastasia Ailamaki, Danica Porobic, Utku Sirin

Micro-architectural behavior of traditional disk-based online transaction processing (OLTP) systems has been investigated extensively over the past couple of decades. Results show that traditional OLTP systems mostly under-utilize the available micro-architectural resources. In-memory OLTP systems, on the other hand, process all the data in main-memory and, therefore, can omit the buffer pool. Furthermore, they usually adopt more lightweight concurrency control mechanisms, cache-conscious data structures, and cleaner codebases since they are usually designed from scratch. Hence, we expect significant differences in micro-architectural behavior when running OLTP on platforms optimized for in-memory processing as opposed to disk-based database systems. In particular, we expect that in-memory systems exploit micro-architectural features such as instruction and data caches significantly better than disk-based systems. This paper sheds light on the micro-architectural behavior of in-memory database systems by analyzing and contrasting it to the behavior of disk-based systems when running OLTP workloads. The results show that, despite all the design changes, in-memory OLTP exhibits very similar micro-architectural behavior to disk-based OLTP: more than half of the execution time goes to memory stalls where instruction cache misses or the long-latency data misses from the last-level cache (LLC) are the dominant factors in the overall execution time. Even though ground-up designed in-memory systems can eliminate the instruction cache misses, the reduction in instruction stalls amplifies the impact of LLC data misses. As a result, only 30% of the CPU cycles are used to retire instructions, and 70% of the CPU cycles are wasted to stalls for both traditional disk-based and new generation in-memory OLTP.

2021

Micro-architectural Analysis of Database Workloads

Utku Sirin

Database workloads have significantly evolved in the past twenty years. Traditional database systems that are mainly used to serve Online Transactional Processing (OLTP) workloads evolved into specialized database systems that are optimized for particular types of workloads. Data warehousing applications have led to Online Analytical Processing (OLAP) workloads and real-time analytical processing applications have led to Hybrid Transactional and Analytical Processing (HTAP) workloads. Similarly, modern hardware has significantly evolved in the past twenty years. Unicore, simple processors with megabytes of main memory have evolved into multi-core, power-limited processors with hundreds of gigabytes of main memory. Furthermore, the processors have complex micro-architectural features such as Single Instruction Multiple Data (SIMD) instructions and complex branch predictors. Advancements in processor technology have led to further evolution of database systems with novel system architectures and query processing paradigms. We present the micro-architectural behavior of modern database workloads on a modern processor for various categories of workloads and generations of database systems. We examine three main categories of database workloads and study them separately: OLTP, OLAP, and HTAP. We show that OLTP systems spend most of their execution time waiting for instruction-cache or data-cache misses, where the data-cache misses are due to the random data-accesses during the index lookup operation. While using an efficient index structure can significantly reduce the number of data-cache misses, the main micro-architectural bottleneck remains the data-cache misses due to the costly random data-accesses. Hence, OLTP systems should adopt techniques that mitigate the random data-accesses. OLAP systems spend most of their execution time in data-cache misses, where the data-cache misses are due to high pressure on the memory bandwidth for sequential-scan-heavy queries, and are due to random data-accesses for join-intensive queries. OLAP systems that follow tuple-at-a-time execution models efficiently use the CPU cycles. However, they require executing a significantly larger number of instructions hence are significantly slower than the systems that follow vector-at-a-time and compiled execution models. Therefore, OLAP systems should use efficient execution models and adopt techniques that mitigate data-cache misses. HTAP systems combine OLTP and OLAP systems into a single, unified system, where the OLTP and OLAP systems run on the same hardware and on the same data. Running on the same hardware results in hardware-level interference, where the OLTP throughput significantly drops due to the OLAP side sharing the hardware resources. Running on the same data results in increased OLAP query execution time due to the need for the OLAP side to process the fresh tuples generated by the OLTP side. Therefore, HTAP systems should adopt techniques that mitigate the hardware-level interference and should make sure that the OLAP side uses enough resources to minimize the increased query execution time.

EPFL2021

ALLARM: Optimizing Sparse Directories for Thread-Local Data

Amitabha Roy

Large-scale cache-coherent systems often impose unnecessary overhead on data that is thread-private for the whole of its lifetime. These include resources devoted to tracking the coherence state of the data, as well as unnecessary coherence messages sent out over the interconnect. In this paper we show how the memory allocation strategy for non-uniform memory access (NUMA) systems can be exploited to remove any coherence-related traffic for thread-local data, as well removing the need to track those cache lines in sparse directories. Our strategy is to allocate directory state only on a miss from a node in a different affinity domain from the directory. We call this ALLocAte on Remote Miss, or ALLARM. Our solution is entirely backward compatible with existing operating systems and software, and provides a means to scale cache coherence into the many-core era. On a mix of SPLASH2 and Parsec workloads, ALLARM is able to improve performance by 13% on average while reducing dynamic energy consumption by 9% in the on-chip network and 15% in the directory controller. This is achieved through a 46% reduction in the number of sparse directory entries evicted.

2014

Micro-architectural Analysis of Database Workloads

Utku Sirin

EPFL2021