Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Possible Duplicate:
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?

share|improve this question

migrated from stackoverflow.com Apr 25 '11 at 22:08

This question came from our site for professional and enthusiast programmers.

marked as duplicate by Sathya Apr 26 '11 at 14:26

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Look up manufacturing tolerances and the statistics of normal variation. Nothing is average. Especially over worse case temperature and voltage ranges. –  hotpaw2 Apr 25 '11 at 22:10
21  
My car has a top speed of 150mph. Why do I have to drive 65? –  Shinrai Apr 25 '11 at 22:14
20  
@Shinrai that seems to me a poor analogy. –  Mr.Wizard Apr 26 '11 at 8:18
3  
as an aside: how exactly do you "overclock" something by default ? the factory set the default to begin with. –  Sirex Apr 26 '11 at 13:16
    
@Mr.Wizard - Looks like we're tied, haha. I agree that it's not the best analogy, it's more of "This is what I read when I see this" –  Shinrai Apr 26 '11 at 14:52

10 Answers 10

up vote 25 down vote accepted

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.

share|improve this answer
3  
I'd like to hit the cooling and efficiency topic here. I know people who run large compute clusters, and in summer months, sometimes their cooling doesn't keep up. They worked out a way to underclock their servers in software, so that when their systems can't handle the heat, they can drastically scale back the power being used and thus the heat being generated, rather than having to shut down machines that are running jobs that, even with modern clusters with multiple cores, can take several months to run. Amping up the processors to get extra cycles doesn't make sense there. –  VarLogRant Apr 26 '11 at 4:52
1  
Similarly, I'm on a netbook right now, for the uses I give it, it doesn't need all the proc speed, and running cooler would be running better, so again, ramping the clock down, not up, makes more sense. –  VarLogRant Apr 26 '11 at 4:53
3  
@VarLogRant you've just described Dynamic Frequency Scaling, (en.wikipedia.org/wiki/Dynamic_frequency_scaling) which many modern CPUs, especially for mobile devices, do. Power consumption is effectively a function of clock speed (since CMOS circuitry consumes very little power when in a static state) and so for mobile chips this is an invaluable tool to conserve power. –  SplinterReality Apr 26 '11 at 6:17
    
+1 for..exactly. –  Garet Claborn Apr 26 '11 at 6:55

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.

share|improve this answer
    
You don't have to have an upgraded cooling system. But it sure helps. –  Wuffers Apr 25 '11 at 22:48

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.

share|improve this answer

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.

share|improve this answer

There are reasons for this :

  1. You can't have the same code CPUs overclocked at the same speed. Ex: E2400 with different steppings will have different potential of overclocking.
  2. Your CPU won't be stable at the overclocked speed.
  3. CPUs are produced for many countries with different weather, temp. So at original, they are pretty sure it's stable.
  4. Somehow, it is interesting for overclocker and geeks who want to get stronger PC with better knowledge.
share|improve this answer

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.

share|improve this answer
    
Same thing happens with engines, too. When car engines are meticulously crafted to their design ("blueprinting"), they perform far better than the same design made on the production line with a far more generous tolerance. Unlike car engines, CPU's generally come out much closer to their ideal design, and yield more capable processors that can withstand overclocking. –  Toybuilder Apr 26 '11 at 1:33
    
I am curious about the bicycle note. Can you give more information? –  Mr.Wizard Apr 26 '11 at 8:21

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.

share|improve this answer

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.

share|improve this answer
    
I've read that Intel does this for the "Core" series of processors. They manufacture the Core processors as two cores on a single die. If one of the chips tests bad, they disable it, and sell the die as a Core Solo. Two viable Core Duo chips are sealed in the same enclosure to make a Core Extreme. In this way, Intel is salvaging their otherwise defective stock, and ensuring that the defect rate for Core Extreme chips is effectively zero. –  SplinterReality Apr 26 '11 at 6:13
    
@charles I'm not sure that is true for latest Intel CPUs, however. The Core 2 series was not true multi-core, but multiple chips on the same die.. whereas Core i3, i5, i7 etc are all true multi-core designs. See extremetech.com/article2/0,2845,2049688,00.asp –  Jeff Atwood Apr 26 '11 at 6:16
    
@Jeff Ahh, so my knowledge is a little dated. Thanks for the update, I'm sure other readers will find that tidbit useful as well. –  SplinterReality Apr 26 '11 at 6:20
1  
@Jeff - sure about the Core 2 series? I thought it was the Core processors (without the 2) that were basically two P4s glued together, and in practice were normally both slower and hotter than a single-core P4. The "2" in "Core 2" doesn't mean dual core - that's what the "Duo" means in "Core 2 Duo". –  Steve314 Apr 26 '11 at 12:36
    
@Jeff - BTW - probably worth mentioning the economics aspect of binning. When testing your chips and downrating some of them, you probably don't end up downrating enough of them to satisfy the cheap low-end processor market, and of course you don't want a glut in the high-end market driving the price down. –  Steve314 Apr 26 '11 at 12:43

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.

share|improve this answer
    
Very interesting information –  Chris Marisic Apr 26 '11 at 12:46

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.

share|improve this answer

protected by nhinkle Apr 26 '11 at 7:32

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.