By Using Existing Processor Resources More Efficiently, Hyperthreading Technology Improves Performance at Little Cost and Increases Chip Size by Less Than

Hyperthreading technology, which brings the concept of simultaneous multithreading to the Intel architecture, was first introduced on the Intel Xeon processor in early 2002 for the server market. In November 2002, Intel launched the technology on the Intel Pentium 4 at clock frequencies of 3.06 GHz and higher, making the technology widely available to the consumer market. This technology signals a new direction in microarchitecture development and fundamentally changes the cost-benefit tradeoffs of microarchitecture design choices. This article describes how the technology works, that is, how we make a single physical processor appear as multiple logical processors to operating systems and software. We highlight the additional structures and die area needed to implement the technology and discuss the fundamental ideas behind the technology and why we can get a 25-percent boost in performance from a technology that costs less than 5 percent in added die area. We illustrate the importance of choosing the right sharing policy for each shared resource by describing, examining, and comparing three different sharing policies: partitioned resources, threshold sharing, and full sharing. The choice of policy depends on the traffic pattern, complexity and size of the resource, potential deadlock/livelock scenarios, and other considerations. Finally, we show how this technology significantly improves performance on several relevant workloads. The die photos and descriptions in this article illustrate the technology’s first implementations on Intel’s Xeon and Pentium 4 processor families. These first implementations emphasized cost containment. Future implementations should provide even greater performance benefits.