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I keep reading since years about magical overclockability of modern CPUs. It is said that CPUs (with open multiplier) which are shipped to customers with (let's say) preset 3GHz, can be overclocked "without problems" to (let's say) 4GHz.

But I can't believe that CPU manufacturers are fooling themselves. I am thinking: should a CPU always support 1GHz higher clock rate without problems, why not sell such CPU with 1GHz more preset?

Would you say, that my assumption is correct, that in fact there are lot of problems with overclocking, and for that reason, modern CPUs are preset to lower frequencies (compared to those they could handle)?

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If you were manufacturing and selling CPUs for a profit, would you take risks overclocking them by default and having to give out free processors under warranty,? Or would you use a known safe clock setting the CPU can live with for years. –  Moab Jan 19 '11 at 17:00
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7 Answers

I quote from OverClocking Risks:

Usually processors in the lower range are produced with the same manufacturing process as the CPUs sold in the mid to high range. The higher rated CPUs are factory overclocked and tested, then sold for a premium. Many users will buy the cheaper processor and over clock it to nearly the same speeds of the premium CPUs. This is a great deal if and only if you are lucky enough to get a CPU that just missed the premium cut during initial factory testing. Sometimes, you will be stuck with a CPU that only over clocks slightly above its rating.

In other words, the processor that you have was factory-tested and found to be incapable of sustaining in the long-term much higher clock-rates than what the specs say. If it could, it would have been sold as another model and with a different price.

If you are really lucky, your CPU was one that just missed by a hair a higher rating, and therefore can be overclocked with safety. However, the only way to find out is to overclock in increments until you start getting strange crashes. Of course, while exploring the limits of your CPU this way, you take the risk of burning it out or of destroying your disk etc. It is for you to decide.

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To add on to what harrymc said, usually you don't find a lot of underclocked procs in a generation until the manufacturing process has been tested and true. At that point the next generation is on it's way out and the current gen has already dropped in price (i.e. the geek tax has already been paid). Once the manufacturing has matured, the vendors then arbitrarily select x% as certified at a given rating although most of the chips are practically the same. –  Mike Brown Jan 19 '11 at 23:44
    
Also there was the famous story where Intel segmented a family of chips by whether they had a math coprocessor or not. Rather than stamp two separate die, the coprocessor was severed from the lower level proc –  Mike Brown Jan 19 '11 at 23:48
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As harrymc pointed out, the manufacturing process for a range of processors is same. Sometimes, it is difficult for manufacturers to calculate how many processors will fall into each category in a range and that can cause problems. I remember that several years ago (Athlon XP era, if I remember correctly) AMD did some miscalculations and got large amount of top of the line processors. That was a problem, since such processors are much more expensive than mid level processors. So they took those processors, underclocked them and sold them as cheaper processors, in order to cover manufacturing costs.

Another interesting point somewhat related to this are AMD processors with strange number of cores. For example some 3 core processors were actually 4 core processors with one core faulty. Some people managed to unlock the extra core and get some performance.

Another point is processor testing. Usually manufacturers put processors under much higher stress than expected when the processor is in normal use. This way they can be sure that processors will produce correct output. While it is true that processor bugs usually affect only a small number of people, they can be very embarrassing for manufacturers. Another problem is that most users assume that processors will provide correct output when they are working and there's very little protection against processor bugs available, so it is very important that processors work correctly.

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Also, while overclocking enthusiasts may have awesome liquid cooling systems and the like which help keep their system humming along at insane clock speeds, the average CPU consumer is likely to have something a little more modest. –  fennec Jan 19 '11 at 22:37
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shipped to customers with (let's say) preset 3GHz, can be overclocked "without problems" to (let's say) 4GHz.

The key phrase here is "without problems". This is the part that just isn't true. A certain significant (large) percentage of those processors certainly will have problems, even if its not until a year or so down the road when the cooling fan starts to wear down and there's dust clogging airflow through the heat sink.

Another fallacy here is the assumption that performance is the primary virtue of a cpu. For large business, which makes up a huge portion of their customer base, stability, endurance, and electrical consumption all factor in as part of their TCO (total cost of ownership) equation. We're now to the point where most processors on the market will choose to deliberately underclock themselves to save on energy use and heat output where appropriate, and we have chips like the Atom and Neo that are intended to underperform other processors on purpose to gain the advantage of low cost and higher battery life.

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. I am thinking: should a CPU always support 1GHz higher clock rate without problems,

That's not correct.

CPUs. It is said that CPUs (with open multiplier) which are shipped to customers with (let's say) preset 3GHz, can be overclocked "without problems" to (let's say) 4GHz.

Gross simplification. Can they be overclocked ? Sure. Will be they be shipped factory overclocked ? Unlikely.

For the CPUs to run faster than their rated clock speed ( hence "overclocking") - you'll have to do certain changes. Changes such as replacing stock cooling with aftermarket cooling for better heat dissipation. CPU's are "locked" to run at frequencies which are determined after running through a set of tests without causing any problems.

that my assumption is correct, that in fact there are lot of problems with overclocking

Maybe, may be not, depends - depends on a lot of factors - ambient temparture, cooling systems, yield of the silicon, motherboard support, stable power supply being amongst the factors

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Overclocking generates more heat which will cause the CPU to fail if it is excessive. Underclocking makes the CPU more resiliant to cooling problems. Most modern CPUs also have adaptive speed setting so that they can reduce the heat they generate.

I have and HP laptop that died due to poor heatsinking of the graphics chip. The excessive heat appearently causes the soldering to break.

My server runs almost constantly at its lowest CPU setting. This still leaves most of the CPU cylcles for BOINC processes.

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See this...nvidiasettlement.com –  Moab Jan 19 '11 at 17:01
    
@Mohab thanks for the link. –  BillThor Jan 19 '11 at 17:38
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One simple thing that most people are forgetting, not all overclocking simply relies on the CPU itself, you need to keep in mind additional resources like your Chipset (voltage, etc), your memory (timings, voltage, etc).

But yes most people on places like Newegg, do always say

I overclocked this thing by 1.2 GHZ! WOZERS!

However they fail to always mention that it is running hotter, and that they have upped the voltage on all other components.

So case in point, just because some CPU's are clockable 1GH higher, does not standard make for manufacturers like Intel. Oh and like mentioned already, if they failed to be that 1ghz higher they would be doing a HELL of a lot of RMA's

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One of the reasons why Intel is not clocking their processors higher right now is that they don't have to. On upper range they do not have any competition, which means they don't have to push the limits to make money. Which means, they can sell processors slower than they'd really be able to reach and thus lower the warranty returns. And then they can sell more in 2 years, because the then-new models will be faster in relation to current top models than if the current technology was pushed to the limits.

So, while not every i7 can hit 4GHz without added voltage/cooling, some of them could, and if Intel really had to, they'd hand-pick the ones that can and sell them as a separate, higher model in their range. This is called "binning".

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