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From: "Wiljan Derks" <Wiljan.Derks@zonnet.nl>
Newsgroups: comp.lang.ada
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Subject: Re: Gnat 3.15p & Windows & Hyperthreading Q
Date: Tue, 29 Mar 2005 20:33:05 +0200
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Date: 2005-03-29T20:33:05+02:00
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I did some performance tests on XP profesional with hyperthreading on a 
2.8Ghz P4.
For that I used some code that checks the CPU performance.
I did this by making some compute procedure and check how often it is 
calculated.
In my program I did make multiple tasks to check the functioning of hyper 
threaded.

It turns out that hyper threading does not help at all with the total 
perfromance that is available.
My conclusions where as follows:
    * When hyperthreading is turned of, the system can do X computations.
    * When hyperthreading is turned on and both threads are loaded, each of 
them can typically do less then X/2 computations.
Thus turning on hyper threading gives basically two CPU which are both half 
speed when being used.
When one of the CPU's is free, the other is faster.
So it looks like one CPU is multiplexed in time (but very fast).

The only advantage that hyper threading has, is that the system might be 
more responsive.
Thus when accidentally locking one CPU at high priority, one can still break 
into the system (using the other CPU).

Wiljan