Hyper-Threading Technology Architecture and MIcroarchitecture

Intel’s Hyper-Threading Technology brings the concept of simultaneous multi-threading to the Intel Architecture. Hyper-Threading Technology makes a single physical processor appear as two logical processors; the physical execution resources are shared and the architecture state is duplicated for the two logical processors. From a software or architecture perspective, this means operating systems and user programs can schedule processes or threads to logical processors as they would on multiple physical processors. From a microarchitecture perspective, this means that instructions from both logical processors will persist and execute simultaneously on shared execution resources. This paper describes the Hyper-Threading Technology architecture, and discusses the microarchitecture details of Intel's first implementation on the Intel Xeon processor family. Hyper-Threading Technology is an important addition to Intel’s enterprise product line and will be integrated into a wide variety of products. Intel is a registered trademark of Intel Corporation or its subsidiaries in the United States and other countries. Xeon is a trademark of Intel Corporation or its subsidiaries in the United States and other countries. INTRODUCTION The amazing growth of the Internet and telecommunications is powered by ever-faster systems demanding increasingly higher levels of processor performance. To keep up with this demand we cannot rely entirely on traditional approaches to processor design. Microarchitecture techniques used to achieve past processor performance improvement–superpipelining, branch prediction, super-scalar execution, out-of-order execution, caches–have made microprocessors increasingly more complex, have more transistors, and consume more power. In fact, transistor counts and power are increasing at rates greater than processor performance. Processor architects are therefore looking for ways to improve performance at a greater rate than transistor counts and power dissipation. Intel’s Hyper-Threading Technology is one solution. Processor Microarchitecture Traditional approaches to processor design have focused on higher clock speeds, instruction-level parallelism (ILP), and caches. Techniques to achieve higher clock speeds involve pipelining the microarchitecture to finer granularities, also called super-pipelining. Higher clock frequencies can greatly improve performance by increasing the number of instructions that can be executed each second. Because there will be far more instructions in-flight in a superpipelined microarchitecture, handling of events that disrupt the pipeline, e.g., cache misses, interrupts and branch mispredictions, can be costly. Intel Technology Journal Q1, 2002 Hyper-Threading Technology Architecture and Microarchitecture 2 ILP refers to techniques to increase the number of instructions executed each clock cycle. For example, a super-scalar processor has multiple parallel execution units that can process instructions simultaneously. With super-scalar execution, several instructions can be executed each clock cycle. However, with simple inorder execution, it is not enough to simply have multiple execution units. The challenge is to find enough instructions to execute. One technique is out-of-order execution where a large window of instructions is simultaneously evaluated and sent to execution units, based on instruction dependencies rather than program order. Accesses to DRAM memory are slow compared to execution speeds of the processor. One technique to reduce this latency is to add fast caches close to the processor. Caches can provide fast memory access to frequently accessed data or instructions. However, caches can only be fast when they are small. For this reason, processors often are designed with a cache hierarchy in which fast, small caches are located and operated at access latencies very close to that of the processor core, and progressively larger caches, which handle less frequently accessed data or instructions, are implemented with longer access latencies. However, there will always be times when the data needed will not be in any processor cache. Handling such cache misses requires accessing memory, and the processor is likely to quickly run out of instructions to execute before stalling on the cache miss. The vast majority of techniques to improve processor performance from one generation to the next is complex and often adds significant die-size and power costs. These techniques increase performance but not with 100% efficiency; i.e., doubling the number of execution units in a processor does not double the performance of the processor, due to limited parallelism in instruction flows. Similarly, simply doubling the clock rate does not double the performance due to the number of processor cycles lost to branch mispredictions. 0 5 10 15 20 25