Understanding Processor Performance – The MHz Myth

This content was originally featured on Amdmb.com and has been converted to PC Perspective’s website. Some color changes and flaws may appear.

Most of this information was taken from the AMD White Paper entitled: “Understanding Processor Performance” available on AMD’s website.

When we think of a specific processors performance, what comes to mind exactly? In its most basic form, performance is given as the time it takes to perform a given task, be that rendering 3D images or typing a paper. Obviously then, if you increase the performance of processor, then you decrease the execution time of the above task. Specifically, for the x86 instruction set that current Intel and AMD processors follow, the performance is defined as: The work done by the processor in each clock cycle (represented as instructions per clock – IPC) times the number of clock cycles (represented by frequency) or:

Performance = IPC X Frequency

In past generations of processors, including those in the 286, 386 and 486, the architecture of the Intel, AMD and others (remember TI??) were exactly the same and meant that their IPC was equal. Performance thus began to be perceived as not relying on IPC at all and that frequency was all that mattered. Performance was perceived as being:

Performance = Frequency

It wasn’t until the fifth generation of the x86 processors that this relationship between frequency and performance failed. The AMD-K5 processor and the Intel Pentium processor took two different approaches to their processor architecture but still remained 100% compatible with the x86 instruction set. Now, for the first time, the IPC was different for competing processors and meant results for similar frequency chips on benchmarks and applications were different. Although at this time AMD was indeed behind their Intel competitors in terms of performance, this has changed in recent years. Either way, from the fifth generation on, performance was defined as:

Performance = IPC X Frequency

As anyone competent is simple arithmetic will know now, the total performance can no longer be determined by frequency alone and that the IPC of any given CPU can have a dramatic effect on the performance.

In fact, with the seventh generation processors, the AMD Athlon and the Intel Pentium 4, the architectural differences and IPCs have diverged dramatically, in favor of AMD in terms of pure IPC rating. Below is a graph (provided by AMD) of the SPECint 2000 results:

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