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Megahertz Myth
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The megahertz myth, or less commonly the gigahertz myth, refers to the error of using clock rate to compare the performance of different microprocessors. While clock rates are a valid way of comparing the performance of different speeds of the same model and type of processor, other factors such as pipelines and instruction sets can greatly affect the performance when considering different processors. For example, one processor may take one clock cycle to add two numbers and another clock cycle to multiply by a third number, whereas another processor may do the same calculation in one clock cycle.
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Encyclopedia
The megahertz myth, or less commonly the gigahertz myth, refers to the error of using clock rate to compare the performance of different microprocessors. While clock rates are a valid way of comparing the performance of different speeds of the same model and type of processor, other factors such as pipelines and instruction sets can greatly affect the performance when considering different processors. For example, one processor may take one clock cycle to add two numbers and another clock cycle to multiply by a third number, whereas another processor may do the same calculation in one clock cycle. Comparisons between different types of processors are difficult because performance varies depending on the type of task.
A benchmark is a better way of measuring and comparing computer performance.
Basis of the myth
The myth arose because the clock speed was commonly taken as a simple measure of processor performance, and was promoted in advertising and by enthusiasts without taking into account other factors. The term came into widespread use (or was even originally coined) in the context of comparing PowerPC-based Apple Macintosh computers with Intel-based PCs. Marketing based on the myth led to the clock speed being given higher priority than actual performance, and led to AMD introducing model numbers giving a notional clock speed based on comparative performance to overcome a perceived deficiency in their actual clock speed.
For example, a processor at twice the clock speed may only accomplish half the number of instructions per cycle, thereby offering no more performance than the slower-clocked alternative.
Historical development
Background
The x86 CISC based CPU architecture which Intel introduced in 1978 was used as the standard for the DOS based IBM PC, and developments of it still continue to dominate the Microsoft Windows market. An IBM RISC based architecture was used for the PowerPC CPU which was released in 1992. In 1994 Apple Computer introduced Mac computers using these PPC CPUs, but IBM's intention to produce its own desktop computers using these chips was thwarted by delays in Windows NT and a falling out with Microsoft. Initially this architecture met hopes for performance, and different ranges of PPC CPUs were developed, often delivering different performances at the same clock speed. Similarly, at this time the Intel 80486 was selling alongside the Pentium which delivered almost twice the performance of the 80486 at the same clock speed.
Rise of the myth
Computer advertising emphasised processor megahertz, and by late 1997 rapidly increasing clock speeds enabled the Pentium II to surpass the PowerPC in performance. Apple then introduced Macs using the PowerPC G3 which they claimed outperformed Pentium IIs while consuming less power. They illustrated this with commercials showing a Pentium II on a snail, and a "Toasted Bunny"-suited character parodying Intel's commercials. Intel continued to promote their higher clock speed, and the Mac press frequently used the "megahertz myth" term to emphasise claims that Macs had the advantage in certain real world uses, particularly in laptops.
In November 2000 Intel's heavily advertised advances in clock speed reached an extreme with the release of the Pentium 4 which sacrificed per-cycle performance and used a deep instruction pipeline to gain higher clock speeds, ignoring problems that this introduced heat production and power consumption.
Comparisions between PowerPC and Pentium had become a staple of Apple presentations. At the New York Macworld Expo Keynote on July 18, 2001, Steve Jobs described an 867 MHz G4 as completing a task in 45 seconds while a 1.7 GHz Pentium 4 took 82 seconds for the same task, saying that "the name that we've given it is the megahertz myth". He then introduced senior hardware VP Jon Rubinstein who gave a tutorial describing how shorter pipelines gave better performance at half the clock speed.
AMD also produced x86 designs which competed with Intel on performance rather than price. In January 2002 the Pentium 4 gained a lead in sales and AMD marketing responded by giving their processors numeric suffixes approximating the clock rate that an AMD Thunderbird (and by inference a Pentium processor) would need to give matching performance, openly undermining the "megahertz myth".
The myth becomes counterproductive
The Pentium 4 was unsuitable for laptops due to its heat dissipation and power requirements, and in March 2003 Intel overcame these difficulties with the Pentium M, which proved capable of matching the Pentium 4 on performance at significantly lower clock rates. In 2004, problems of overheating led Intel to abandon further development of its NetBurst microarchitecture. Instead, Intel focused its future plans on the Pentium M architecture.
As a result of abandoning the NetBurst microarchitecture, Intel had to repair the marketing confusion as a result of its promotion of clock frequencies with the Pentium 4. Their next generation of chips, the Intel Core 2, had clock speeds from 1.86 GHz to 3.00 GHz, with the Intel Core 2 Duo dual core chips improving multi-tasking and offering a further increase in performance over the single core version. While the Core line was a breakthrough in terms of performance-per-watt, its low clock speed when compared to late generation Pentium 4s (rated at upwards of 3.8 GHz) seemed likely to cause some marketing confusion. Intel was trying to sell consumers processors with lower gigahertz ratings, having spent the better part of the previous five years implying that slower clock speed denotes inferiority.
The change from simple speed ratings could also cause problems for third party manufacturers setting system requirements. For example, Panasonic listed a Pentium 4-based machine running at 3 GHz as the minimum system requirement for their Blu-ray Disc drives, but a 1.83 GHz Core 2 Duo was significantly faster than the 3 GHz Pentium 4. For some consumers reading specifications on the side of a box, such statements can be very confusing.
External links
- Keynote at Macworld 2001
- 2004 news article
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