Hyperthreading and HyperTransport have absolutely nothing to do with each other.
HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency computer bus. The HyperTransport Technology Consortium is in charge of promoting and developing HyperTransport technology. The technology is used by AMD and Transmeta in x86 processors, PMC-Sierra and Broadcom in MIPS microprocessors, NVIDIA, Via, SiS, ULi/ALi, and AMD in PC chipsets, Apple Computer and HP in Desktops and notebooks, HP, Sun, IBM, and IWill in servers, Cray in supercomputers, and Cisco Systems in routers.
HyperTransport runs at 200-1400 MHz (compared to PCI at either 33 or 66 MHz). It is also a DDR or "double-data-rate" bus, meaning it sends data on both the rising and falling edges of the 1400 MHz clock signal. This allows for a maximum data rate of 2800 MTransfers/s per pair. The frequency is auto-negotiated.
HyperTransport supports an auto-negotiated bus widths, from 2 (bidirectional serial, 1 bit each way) to 32-bit (16 each way) busses are allowed. The full-sized, full-speed 32-bit bus has a transfer rate of 22,400 MByte/s, making it much faster than existing standards. Busses of various widths can be mixed together in a single application, which allows for high speed busses between main memory and the CPU, and lower speed busses to peripherals, as appropriate. The technology also has much lower latency than other solutions.
Hyper-Threading Technology (HTT) is Intel's trademark for their implementation of the simultaneous multithreading technology on the Pentium 4 microarchitecture. It is basically a more advanced form of Super-threading that first debuted on the Intel Xeon processors and was later added to Pentium 4 processors. The technology improves processor performance under certain workloads by providing useful work for execution units that would otherwise be idle, for example during a cache miss.
The advantages of Hyper-Threading are listed as improved support for multi-threaded code, allowing multiple threads to run simultaneously, improved reaction and response time, and increased number of users a server can support.
Hyper-Threading works by duplicating certain sections of the processorthose that store the architectural statebut not duplicating the main execution resources. This allows a Hyper-Threading equipped processor to pretend to be two "logical" processors to the host operating system, allowing the operating system to schedule two threads or processes simultaneously. Where execution resources in a non-Hyper-Threading capable processor are not used by the current task, and especially when the processor is stalled, a Hyper-Threading equipped processor may use those execution resources to execute the other scheduled task. (Reasons for the processor to stall include a cache miss, a branch misprediction and waiting for results of previous instructions before the current one can be executed.)
Except for its performance implications, this innovation is transparent to operating systems and programs. All that is required to take advantage of Hyper-Threading is symmetric multiprocessing (SMP) support in the operating system, as the logical processors appear as standard separate processors.
However, it is possible to optimize operating system behaviour on Hyper-Threading capable systems, example, consider an SMP system with two physical processors that are both Hyper-Threaded (for a total of four logical processors). If the operating system's process scheduler is unaware of Hyper-Threading, it would treat all four processors the same. As a result, if only two processes are eligible to run, it might choose to schedule those processes on the two logical processors that happen to belong to one of the physical processors. Thus, one CPU would be extremely busy while the other CPU would be completely idle, leading to poor overall performance. This problem can be avoided by improving the scheduler to treat logical processors different from physical processors.
According to Intel, the first implementation only used an additional 5% of the die area over the "normal" processor, yet yielded performance improvements of 15-30%.
There has been confusion between the use of HT referring to HyperTransport and the use of HT to refer to Intel's Hyper-Threading feature of their Pentium 4 based microprocessors. Hyper-Threading is known as Hyper-Threading Technology (HTT) or HT-Technology. Because of this potential for confusion, the HyperTransport Consortium always uses the written out form: "HyperTransport".