Hyper-Threading: The Most Useful CPU Innovation

HT stands for Hyper-Threading, and does not stand for hyper transport (something entirely different by AMD)

Hyper-Threading (HT) is a technology developed by Intel that makes a single processor show up as two different processors to the Operating System. In reality there is only one core, but the unused processor cycles represent a ‘logical processor.’ In an HT CPU, a single process (not HT optimized) can at most use up 99% of the resources, thus always leaving a 1% gap for all other applications. Its not as small as it sounds, since, the OS sees TWO CPUs, it thinks that you have a 100% used CPU and a 0% used CPU, no matter what the number really is (Windows will show 50%). In this way, other applications that only need a bit of CPU are not too adversely affected.

If you are running an HT optimized thread, like Adobe Photoshop CS2, there are certain commands that need to be done as soon as possible on the same priority.

For example, I tell you to find out if 1+1 is equal to 2+1. You need to evaluate 1+1 and 2+1, one after the other normally, then execute a compare. A HT optimized app will tell one ‘CPU’ to do 1+1, and the other to do ‘2+1’, then compare.

The scenario above is useless, because each of those commands only takes picoseconds to evaluate. But if they were more and more complex expressions, then functions, then algorithms, you can see the benefit.

Most apps are not HT optimized. Therefore, you can think of HT optimized apps as working in the same way as two non-HT optimized CPU-intensive apps running simultaneously.

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