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.

在过去的几十年中，对基于磁盘的传统在线事务处理（OLTP）系统的微体系结构行为进行了广泛的研究。结果表明，传统的OLTP系统大多未充分利用可用的微体系结构资源。另一方面，内存中OLTP系统处理主内存中的所有数据，因此可以省略缓冲池。此外，由于它们通常是从头开始设计的，因此它们通常采用更轻量级的并发控制机制，注重缓存的数据结构和更简洁的代码库。因此，与基于磁盘的数据库系统相比，在针对内存处理进行了优化的平台上运行OLTP时，我们预计微体系结构行为会出现显着差异。尤其是，我们期望内存系统比基于磁盘的系统更好地利用微体系结构功能，例如指令和数据缓存。本文通过分析并与运行OLTP工作负载时基于磁盘的系统的行为进行对比，从而阐明了内存数据库系统的微体系结构行为。结果表明，尽管进行了所有设计更改，但内存中的OLTP仍表现出与基于磁盘的OLTP非常相似的微体系结构行为：一半以上的执行时间流向了内存停顿，其中指令高速缓存未命中或长时延数据未命中来自最后一级缓存（LLC）的数据是整个执行时间的主要因素。即使完全设计的内存系统可以消除指令高速缓存未命中的情况，指令停顿的减少放大了LLC数据丢失的影响。结果，只有30％的CPU周期用于引退指令，而70％的CPU周期则浪费在传统的基于磁盘的内存和新一代的内存OLTP中。