An analysis of software interface issues for SMT processors

Simultaneous Multithreading (SMT) has gradually progressed from a research concept to commercial processor technology. This thesis explores three software interface issues on SMT that are important to its real-world applicability. These issues are: operating system performance on SMT, the impact of spinning on SMT, and register file limitations to scaling SMT. We investigate these issues with a new, detailed simulation infrastructure capable of modeling all operating system activity.; First, we present an analysis of operating system execution on SMT. Many of the applications most amenable to multithreading technologies, such as the Apache web server, spend a significant fraction of their time in kernel code. We compare Apache's user- and kernel-mode behavior to a multiprogrammed SPECInt workload. Overall, our results demonstrate the micro-architectural impact of an OS-intensive workload on an SMT processor. The synergy between the SMT processor and Web and OS software produces a greater throughput gain over superscalar execution than seen on any previously examined workloads, including commercial databases.; Second, we study the cost of synchronization on SMT. Spinning can exact a large performance cost on SMT, because all threads share execution resources. We quantify the impact of spinning on SMT and the performance benefit of replacing spinning with SMT-lock-based code. We observe that spinning's degradation of performance ranges widely between more than 3x on multiprogrammed workloads to a negligible amount on the Apache workload.; Finally, we explore architectural register sharing on SMT. A significant impediment to the construction of SMTs larger than two or four contexts is register file size. We introduce and evaluate mini-threads, a simple extension to SMT that increases thread-level parallelism without the commensurate increase in register hardware. A mini-threaded SMT CPU adds additional per-thread state to each hardware context; an application executing in a context can create mini-threads that will utilize its own per-thread state, but share the context's architectural register set. Our results quantify the factors affecting performance in detail and demonstrate that mini-threads can improve performance significantly, particularly on small-scale, space-sensitive CPU designs.