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  Hyper-Threading On The Desktop 
  Nov 14, 2002, 02:00am EST 
 

Introduction


By: Sander Sassen

The introduction of the new Pentium 4 at 3.06GHz is not only a milestone in processor clockspeed, but also marks the introduction of Hyper-Threading on the desktop. Whereas the 266MHz increase in clockspeed over the previous top-of-the-line Pentium 4 enables the 3.06GHz to take the performance crown, Hyper-Threading is what makes all the difference here, especially in a multitasking environment.

Hyper-Threading has been a standard feature on Intelís Xeon processors that are based on the same NetBurst architecture as the desktop Pentium 4. The Hyper-Threading feature in both the Xeon and Pentium 4 processor basically makes two logical processors available in one physical processor. So we have one physical processor that is able to execute two threads simultaneously, which can be independently halted, interrupted or altered during execution. This is also referred to as SMT, Simultaneous Multi Threading.

Pentium 4 Without Heatspreader


Fig 1. A closer look at the 3.06GHz Pentium 4, with the heat spreader removed.

The difference between a Hyper-Threading processor, whether Xeon or Pentium 4, and a conventional SMP, Symmetric Multi Processing, configuration is that the latter uses two physical, single-threaded processors. The logical processors in a Hyper-Threading processor share certain processor resources such as the execution engine, the floating point unit, the on-board cache and naturally the system bus.

This has advantages and disadvantages, some of the advantages being a shorter data path between the two logical processors and high-speed communication between the logical processors. The disadvantages are that both logical processors need to compete for use of the shared resources; the execution engine, the floating point unit, the cache, etc. But in essence a Hyper-Threading processor offers true-multitasking abilities in a single physical processor unlike previous, conventional CPUs.



1. Introduction
2. SMT and SMP
3. Scheduling and Caching
4. Proformance or Performance
5. Hardware and Software
6. Benchmarks
7. Benchmarks: Multitasking
8. Benchmarks: Multitasking Cont.
9. Summary
10. Appendix

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