The impact of architectural trends on operating system performance

Abstract

Computer systems are rapidly changing. Over the next few years, we will see wide-scale deployment of dynamically-scheduled processors that can issue multiple instructions every clock cycle, execute instructions out of order, and overlap computation and cache misses. We also expect clock-rates to increase, caches to grow, and multiprocessors to replace uniprocessors. Using SimOS, a complete machine simulation environment, this paper explores the impact of the above architectural trends on operating system performance. We present results based on the execution of large and realistic workloads (program development, transaction processing, and engineering compute-server) running on the IRIX 5.3 operating system from Silicon Graphics Inc.Looking at uniprocessor trends, we find that disk I/O is the first-order bottleneck for workloads such as program development and transaction processing. Its importance continues to grow over time. Ignoring I/O, we find that the memory system is the key bottleneck, stalling the CPU for over 50% of the execution time. Surprisingly, however, our results show that this stall fraction is unlikely to increase on future machines due to increased cache sizes and new latency hiding techniques in processors. We also find that the benefits of these architectural trends spread broadly across a majority of the important services provided by the operating system. We find the situation to be much worse for multiprocessors. Most operating systems services consume 30-70% more time than their uniprocessor counterparts. A large fraction of the stalls are due to coherence misses caused by communication between processors. Because larger caches do not reduce coherence misses, the performance gap between uniprocessor and multiprocessor performance will increase unless operating system developers focus on kernel restructuring to reduce unnecessary communication. The paper presents a detailed decomposition of execution time (e.g., instruction execution time, memory stall time separately for instructions and data, synchronization time) for important kernel services in the three workloads. IntroductionUsers of modern computer systems expect the operating system to manage system resources and provide useful services with minimal overhead. In reality, however, modern operating systems are large and complex programs with memory and CPU requirements that dwarf many of the application programs that run on them. Consequently, complaints from users and application developers about operating system overheads have become commonplace.The operating system developer's response to these complaints has been an attempt to tune the system to reduce the overheads. The key to this task is to identify the performance problems and to direct the tuning effort to correct them; a modern operating system is far too large to aggressively optimize each component, and misplaced optimizations can increase the complexity of the system without improving end-user performance. The optimization task is further complicated by the fac...