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Why are modern CPUs “underclocked”?
When I was searching around for a desktop awhile back I came across a lot of discussions where techies talked about taking an e.g. 2.67GHz processor and 'overclocking' it so that it ran at 4GHz. If a CPU is capable of such speed at all, why doesn't it come that way out of the box?
First of all, not all CPUs are capable of overclocking. Many have fixed or range-limited multipliers. This is intended by the industry, hardware vendors are happy to sell CPUs and peripheral hardware with more freedom for higher prices. Real 'overclockers' seem to pay anything as long as it enables them to double the factory defaults ...
Secondly it's a cooling and efficiency problem. Energy consumption and frequency don't scale linearly, nor does the actual performance (especially considering that, with faster CPUs, other system components quickly become bottlenecks ...).
With overclocked CPUs, there is also a strong variance in durability and lifetime even within a manufacturing series. The frequency at which they're sold is a frequency at which all units of a series are known to work stable, regardless of possible differences in detail. One CPU of a series may fail quickly as you overclock it while another may work stable up to 4+ Ghz.
CPU Binning is relevant here:
http://en.wikipedia.org/wiki/Product_binning
Semiconductor manufacturing is an imprecise process, with some estimates as low as 30% for yields. Defects in manufacturing are not always fatal, however. In many cases, it is possible to salvage a part by trading off performance characteristics, such as by reducing its clock frequency or by disabling non-critical parts that are defective. Rather than simply discarding these products, their performance level can be marked down accordingly and sold at a lower price, fulfilling the needs of lower-end market segments.
This practice occurs throughout the semiconductor industry, including central processing units, computer memory, and graphics processors as well.
As well as the tolerances and MTBF reasons posted, there is another one as well.
(Please bear with me as I have not kept up with hardware for a very long time.)
The cost for intel to make a fabrication plant that can create a specific chip is a very large fixed cost. The cost for them to make a single processor once they have built the plant is very, very small.
There is an economic advantage of making the same die for a series of chips, and then locking the chips at different multipliers for product differentiation and pricing. This way, the chips all come out of the same plant. Instead of having a unique plant for each single speed of chip. If you want to buy a low-end chip, the economic way for intel to do it for you is often to sell you a mid-end chip which is set up to run at a lower frequency.
You will see this in other markets as well, when the manufacturing process requires a high initial fixed cost and a very low marginal cost. Every major brand aluminum bicycle, for example, is actually made in the same factory, by the same robots.
Because in many cases, over-clocking results in a reduced life (in terms of time), and a lot more heat.
Some processors are sold as over-clockable - Like AMD's Black Edition (which has an unlocked multiplier), and Intels Extreme Edition.
It is the difference between recommended speed and possible speed.
The manufacturers can't make a processor to max out at the exact speed the processor is created for; it's created with ability above that, but you don't know what the upper range is until you cross it.
Not to mention the extra heat that may be produced that the system is not built to handle, thus the need for extra cooling systems when overclocking too far.
There are reasons for this :
No one says the computations are guaranteed to happen correctly at overclocked speeds. :-)
It's accuracy vs. speed... it's a risk people take, sometimes by testing the CPU to see when it starts producing wrong results.
Overclocking produces more heat and makes the computer less stable. In order to overclock, you have to have an upgraded fan and/or water cooling system.
The price differentiation aspect (which fianchetto describe pretty well) has recently shifted somewhat from clock speed to number of cores. Except for a few very cheap processors Intel has their entire lineup in the 2.8 to 3.6 GHz range, it's approximately the same for AMD.
There is no doubt that AMD is currently pushing their top 4 and 6 core models to the limit of what they can reasonably sell as stable.
What Intel is doing with their Sandy Bridge CPUs is more noteworthy, the current top model of that line is a 4 core clocked at 3.4 GHz, but overclocking results are in the 4.4 to 4.8 GHz range with stock cooler at stock voltage, for a top model that is an extremely high overclock. This suggests that they could release a 4 GHz model with very little effort.
However, if you ask Intel marketing Sandy Bridge is not their top architecture, as that spot is taken by the older 6 core Gulftown. The problem is that Sandy Bridge is just so much better that it's hard to justify the 2 extra cores more than making up for the difference, but according to marketing strategy they have to. So in order to keep that claim somewhat credible they handicap Sandy Bridge artificially.
If Intel weren't as far ahead of AMD as they are they would push their hardware harder, but since AMD doesn't have anything to match the Sandy Bridges as they are Intel let the quirks of marketing decide what to sell.
Apart of technical reasons, which other mentioned here, it's also the question of marketing strategy. Particularly the market segmentation. If you want to charge premium prices from the premium market, then products for that market must have some differentiation with low-end market. In case of CPUs it's achieved by disabling cores, disabling part of cache's and lowering the speed in the products for the low-end market.
