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Here's where we start to see the real benefits Hyper-Threading can offer. The upper two bars on the graph show the scores obtained by running a single 3DMark2001SE loop as the only process. As 3DMark2001SE is not mutli-threaded, there is naturally no performance difference whatsoever between the two.
But look at what happens when we run the 3DMark2001SE loop with the SETI@Home client running in the background. SETI@Home is very processor-intensive, but only demands the use of specific execution resources. On a processor without Hyper-Threading, the remaining execution resources are essentially wasted. But with Hyper-Threading enabled, 3DMark2001SE is able to make use of the previously unused execution resources, and as a result, its performance improves very drastically. In short, while there's no difference between the two systems with only one application running, Hyper-Threading gives no less than a 40% improvement with two running. Impressive.
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How about this? We measured the time necessary to compress a 1GB file (using ZIP format). Our compression program is not multi-threaded, so when run by itself, we expect Hyper-Threading to yield no performance increase. Now look at what happens when we run the same compression test while playing an MP3, and running a fairly demanding Visualization Plug-In. The demanding Plug-In grinds the non-Hyper-Threading system to a hault, almost doubling the compression time, but the Hyper-Threaded system is able to maintain much more reasonable performance.
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But how about games? We fired up Serious Sam: The Second Encounter, and benchmarked it on our Hyper-Threading and non-Hyper-Threading systems, and, since it's not multi-threaded, observed no real difference. Then we decided to experiment by running the same benchmark, while running the exact same 1GB ZIP compression test we used above. The Hyper-Threading Pentium 4's increased efficiency allows it to perform some 10% faster than the other when running these two processes simultaneously.