Hyper-threading is Intel's copyrighted name for what Tullsen calls "simultaneous multi-threading," SMT for short. "Threads" are streams of instructions (e.g., programs) that a processor executes. "Many software applications already break their code up into separate threads, but even those that don't can take advantage of it, as long as the system has multiple programs to be run," said Tullsen. "An SMT processor executes instructions from these multiple threads/programs at once, as if they all came from a single thread. The central processing unit (CPU) duplicates the architectural state on each processor, while sharing one set of processor execution resources." Essentially, SMT makes one processor appear as two to the operating system, and Intel claims an average improvement of roughly 40% in CPU resource utilization providing greater throughput and improved performance.
Earlier this year, Intel became the first company to use hyper-threading, when it began building the technology into its high-end Intel® Xeon™ microprocessors for servers. But in September, Intel announced that it will begin shipments in the October-December quarter -- ahead of schedule -- of a Pentium 4 processor running at 3.0 gigahertz (3 billion clicks per second) enabled with hyper-threading. It is the first time the technology will be available on a desktop computer, and it is already supported under Microsoft's Windows XP operating system.
The idea of hardware multi-threading (in less aggressive forms) has been around for years (in research, and in a few high-end, exotic architectures such as the Cray MTA). But, Tullsen explains, "the SMT execution model, and the research we did, demonstrated that: (1) you could achieve the performance of multi-threading without sacrificing single-thread performance; and (2) it could be done with minimal changes to the way we were already building modern processors. Both of these were critical to convincing the mainstream processor community that this idea made sense for them. Now we're showing them that they can even get better single-thread performance with an SMT processor than a processor dedicated to single-thread execution."
For now, Intel is the only semiconductor company building the technology into its chips. But Tullsen predicts that it's only a matter of time before the technology spreads from high-end chips to all microprocessors. And as co-director of the Processor Architecture and Compilation Lab at UCSD, Tullsen is already working on SMT-related improvements, including operating systems, compilers, and the next generation of multithreaded processor architectures.
For a biographical sketch and description of Professor Tullsen's expertise, go to