Overclocking is usually practiced by PC enthusiasts in order to increase the performance of their computers. Some hardware enthusiasts purchase low-end computer components which they then overclock, thereby attaining performance of a high-end system, while others will overclock high-end components, attaining levels of performance that surpass that of even the newest generation of computer hardware.
Users who choose to overclock their components usually focus their efforts on processors, video cards, motherboard chipsets, and Random Access Memory (RAM).
Considerations for overclocking
Overclocking allows one to boost a computer system's performance by increasing clock frequencies. There are several methods of overclocking, and no two components will overclock by exactly the same amount. One important consideration when overclocking a component is to ensure that it is supplied with adequate power for operation at its new speed; however, applying too much voltage could permanently damage a component. Improper settings carry the potential of destroying components, and, in extreme cases, even causing them to catch fire[verification needed]. Since good voltage regulators and tight tolerances are required for extreme overclocking, only more expensive motherboards—with advanced settings that computer enthusiasts are likely to use—have built-in overclocking capabilities. Motherboards of lower quality such as those typically found in OEM systems typically lack such said features, limiting overclocking primarily to the do-it-yourself community or purchasers of 'top of the line' systems from top tier companies.
There are two considerations related to cooling and overclocking. The first is related to the increased heat production of overclocked components, and the second is related to transistor performance.
Due to the excessive heat produced by overclocked components, an effective cooling system is critical to avoid damaging the hardware. Because most stock cooling systems are designed for the amount of heat produced during non-overclocked use, overclockers typically turn to more effective cooling solutions, often employing heavy duty heatsinks and more powerful fans. Water cooling is often used as well, and when properly implemented provides much more effective cooling than heatsink and fan combinations.
Due to changes in MOSFET device characteristics, circuits slow down at high temperatures. Since overclockers aim to operate circuits at higher frequencies, it is critical to keep delay from increasing to the point where data cannot propagate completely within a clock cycle. Wire resistance also increases slightly at higher temperatures; this is a small additional factor contributing to decreased circuit performance.
In order to ensure that overclocked components stay at a desirable temperature, various cooling methods such as forced convection (a fan blowing across a surface), liquid cooling (liquid coolant carrying waste heat to a radiator, similar to an automobile engine cooling system), liquid nitrogen, dry ice, phase change cooling (as used in refrigerators), and submersion (placing the entire computer in an inert fluid) are often applied. Liquid nitrogen (and other coolants which are consumed as they are used) is usually used only as an extreme measure in record-setting attempts or one-off experiments (destroying the cooled hardware in some cases) rather than for cooling an everyday system. Such extreme methods are generally intolerable in the long term, as they may require refilling reservoirs of coolant or be noisy. What's more, silicon-based MOSFETs will cease to function ("freeze out") below temperatures of roughly 100 K, so using extremely cold coolants may cause devices to cease functioning. Of the aforementioned methods, air cooling, liquid cooling, and phase change cooling are the most popular due to their efficiency, availability, and affordability.
In 2003, Tom's Hardware Guide experimented with a Pentium 4 3.4 GHz HT processor, using liquid nitrogen and forced convection for cooling. They managed to achieve a clock frequency of over 5 GHz, which is a considerable increase over the original clock speed, and much faster than any processor in production at the time. These tests are of interest to enthusiasts as illustrations of what is possible when great amounts of heat can be removed from a system, and provide upper bounds on the performance achievable with less-drastic cooling solutions.
Stability and functional correctness
An overclocked component is by definition operating outside of the manufacturer's recommended operating conditions, and as such may operate incorrectly, leading to system instability. An unstable overclocked system, while fast, can be frustrating to use. Another risk is silent data corruption—errors that are initially undetected. Overclockers generally claim that testing can ensure an overclocked system is stable and functioning correctly.
Before overclocking a system, it is important to understand that the key to ensuring system stability is proper testing, and it is generally impossible for anyone but the processor manufacturer to thoroughly test a processor's functionality. A particular "stress test" can verify only the functionality of the specific instruction sequence used in combination with the data and may still not detect faults in those operations (for example, an arithmetic operation may produce the correct result but incorrect flags; if the flags are not checked, the error will go undetected). Achieving good fault coverage requires immense engineering effort, and despite the resources dedicated to validation by manufacturers, mistakes can still be made . To further complicate matters, in process technologies such as silicon on insulator, devices display hysteresis—a circuit's performance is affected by the events of the past, so without carefully targeted tests it is possible for a particular sequence of state changes to work at overclocked speeds in one situation but not another even if the voltage and temperature are the same. Often, an overclocked system which passes stress tests will "inexplicably" experience instabilities in programs not used as stress tests .
Many overclockers, however, are satisfied with perceived stability; while their system may operate incorrectly, the errors may not be overtly apparent to the user. In overclocking circles, "stress tests" or "torture tests" are used to check for correct operation of a component. These workloads are selected as they put a very high load on the component of interest (e.g. a graphically-intensive application for testing video cards, or a processor-intensive application for testing processors). Popular stress tests include Prime95, Super PI, SiSoftware Sandra, and Memtest86. The hope is that any functional-correctness issues with the overclocked component will show up during these tests, and if no errors are detected during the test, the component is then deemed "stable". As previously discussed, fault coverage is important in stability testing, so the tests are often run for long periods of time (hours or even days).
Factors allowing overclocking
Overclockability arises in part due to the economics of the manufacturing processes of CPUs. In most cases, CPUs with different rated clock speeds are manufactured via exactly the same process. The clock speed that the CPU is rated for is the speed at which the CPU has passed the manufacturer's functionality tests when operating in worst-case conditions (for example, the highest allowed temperature and lowest allowed supply voltage). Manufacturers must also leave additional margin for reasons discussed below.
When a manufacturer rates a chip for a certain speed, it must ensure that the chip functions properly at that speed over the entire range of allowed operating conditions. When overclocking a system, the operating conditions are usually tightly controlled, making the manufacturer's margin available as free headroom. Other system components are generally desiged with margins for similar reasons; overclocked systems absorb this designed headroom and operate at lower tolerances. Pentium architect Bob Colwell calls overclocking an "uncontrolled experiment in better-than-worst-case system operation". 
Some of what appears to be spare margin is actually required for proper operation of a processor throughout its lifetime. As semiconductor devices age, various effects such as hot carrier injection, negative bias thermal instability and electromigration reduce circuit performance. When overclocking a new chip it is possible to take advantage of this margin, but as the chip ages this can result in situations where a processor that has operated correctly at overclocked speeds for years spontaneously fails to operate at those same speeds later. If the overclocker is not actively testing for system stability when these effects become significant, errors encountered are likely to be blamed on sources other than the overclocking.
Measuring effects of overclocking
Many de facto benchmarks are used to evaluate performance. The benchmarks can themselves become a kind of 'sport', in which users compete for the highest scores. As discussed above, stability and functional correctness may be compromised when overclocking, and meaningful benchmark results depend on correct execution of the benchmark. Because of this, benchmark scores may be qualified with stability and correctness notes (e.g. an overclocker may report a score, noting that the benchmark only runs to completion 1 in 5 times, or that signs of incorrect execution such as display corruption are visible while running the benchmark).
Given only benchmark scores it may be difficult to judge the difference overclocking makes to the computing experience. For example, some benchmarks test only one aspect of the system, such as memory bandwidth, without taking into consideration how higher speeds in this aspect will improve the system performance as a whole. Apart from demanding applications such as video encoding, high-demand databases and scientific computing, memory bandwidth is typically not a bottleneck, so a great increase in memory bandwidth may be unnoticeable to a user depending on the applications they prefer to use. Other benchmarks, such as 3DMark attempt to replicate game conditions, but because some tests involve non-deterministic physics, such as ragdoll motion, the scene is slightly different each time and small differences in test score are overcome by the noise floor.
The extent to which a particular part will overclock is highly variable. Processors from different vendors, production batches, steppings, and invididual units will all overclock to varying degrees.
Manufacturer and vendor overclocking
Commercial system builders or component resellers sometimes overclock to sell items at higher profit margins. The retailer makes more money by buying lower-value components, overclocking them, and selling them at prices appropriate to a non-overclocked system at the new speed. In some cases an overclocked component is functionally identical to a non-overclocked one of the new speed, however, if an overclocked system is marketed as a non-overclocked system (it is generally assumed that unless a system is specifically marked as overclocked, it is not overclocked), it is considered fraudulent.
Overclocking is sometimes offered as a legitimate service or feature for consumers, in which a manufacturer or retailer tests the overclocking capibility of processors, memory, video cards, and other hardware products. Several video card manufactures now offer factory overclocked versions of their graphics accelerators, complete with a warranty, which offers an attractive solution for enthusiasts seeking an improved performance without sacrificing common warranty protections. Such factory overclocked products often demand a marginal price premium over reference-clocked components, but the performance increase and cost savings can sometimes outweigh the price increases associated with similar, albeit higher-performance offerings from the next product tier.
Naturally, manufacturers would prefer enthusiasts pay additional money for profitable high-end products, in addition to concerns of less reliable components and shortened product life spans impacting brand image. It is speculated that such concerns are often motivating factors for manufacturers to implement overclocking prevention mechanisms such as CPU locking. These measures are sometimes marketed as a consumer protection benefit, which typically generates a mixed reception from overclocking enthusiasts.
The user can, in many cases, purchase a slower, cheaper component and overclock it to the speed of a more expensive component.
Faster performance in games, applications, and system tasks at no additional expense.
Some systems have "bottlenecks", where small overclocking of a component can help realize the full potential of another component to a greater percentage than the limiting hardware is overclocked. For instance, many motherboards with AMD Athlon 64 processors limit the speed of four units of RAM to 333 MHz. However, the memory speed is computed by dividing the processor speed (which is a base number times a CPU multiplier, for instance 1.8 GHz is most likely 9x200 MHz) by a fixed integer such that, at stock speeds, the RAM would run at a clock rate near 333 MHz. Manipulating elements of how the processor speed is set (usually lowering the multiplier), one can often overclock the processor a small amount, around 100-200 MHz (less than 10%), and gain a RAM clock rate of 400 MHz (20% increase), realizing the full potential of the RAM.
Overclocking can be an engaging hobby in itself and supports several dedicated online communities.
Many of the disadvantages of overclocking can be mitigated or reduced in severity by skilled overclockers. However, novice overclockers may make mistakes while overclocking which can introduce avoidable drawbacks, and potentially result in damage to the overclocked components.
These disadvantages are unavoidable by both novices and veterans.
The lifespan of a processor is negatively affected by higher operation frequencies, increased voltages and heat. However, overclockers argue that with the rapid obsolescence of processors coupled with the long life of solid state microprocessors (10 years or more), the overclocked component will likely be replaced before its eventual failure. Also, since many overclockers are enthusiasts, they often upgrade components more often than the general population, offering futher mitigation of this disadvantage.
Increased clock speeds and voltages result in higher power consumption and therefore higher power bills.
While overclocked systems may be tested for stability before usage, stability problems may surface after prolonged usage due to new workloads or untested portions of the processor core. Aging effects previously discussed may also result in stability problems after a long period of time.
High-performance fans used for extra cooling can produce large amounts of noise. Popular models of fans used by overclockers can produce 50 decibels or more. Some people do not mind the extra noise, and it is common for overclockers to have computers that are much louder than stock machines. Noise can be reduced by utilising strategically placed larger fans which deliver more performance with less noise in the place of smaller and noiser fans, or by the use of alternate cooling methods, such as liquid and phase-change cooling.
Without adequate cooling, the excess heat produced by an overclocked processing unit increases the ambient air temperature of an interior case; consequently, other components may be slightly affected.
Overclocking will not necessarily save money. Non-trivial speed increases often require premium cooling equipment to avoid unacceptably high temperatures. It can also become an expensive pastime. Most people who consider themselves overclockers spend significantly more on computer equipment than the average person.
Overclocking has a risky potential to end in component failure ("heat death"). Most warranties do not cover defunct units that result from overclocking activities .
Disadvantages of Overclocking by Novices and Improper Overclocking
Increasing the operation frequency of a component will increase its thermal output in a linear fashion, while an increase in voltage causes a quadratic increase. Overly aggressive voltage settings or improper cooling may cause chip temperatures to rise so quickly that irreversible damage is caused to the chip causing immediate failure or significantly reducing its lifetime.
With the advent of ever wider ranges of voltage options on motherboards, the risk of fire or burns is not insignificant[verification needed]. The chip itself, or power regulating ICs may overheat and capacitors may burst.
More common than hardware failure is functional incorrectness. Although the hardware is not permanently damaged, this is inconvenient and can lead to instability and data loss. In rare[verification needed], extreme cases entire filesystem failure may occur, causing the loss of all data.
With poor placement of fans, turbulence and vortexes may be created in the computer case, resulting in reduced cooling effectiveness and increased noise. In addition, improper fan mounting may cause rattling or vibration.
Improper installation of exotic cooling solutions like liquid or phase-change cooling may result in failure of the cooling system, which may result in water damage or damage to the processor due to the sudden loss of cooling.
Products sold specifically for overclocking are sometimes just decoration ("rice"). Novice buyers should be aware of the marketing hype surrounding some products. Examples include heat spreaders and heatsinks designed for chips which do not generate enough heat to benefit from these devices.
The utility of overclocking is limited for a few reasons:
Personal computers are mostly used for tasks which are not computationally demanding, or which are performance-limited by bottlenecks outside of the local machine. For example, web browsing does not require a very fast computer, and the limiting factor will almost certainly be the speed of the internet connection. Other general office tasks such as word processing and sending email are more dependent on the efficiency of the user than on the speed of the hardware. In these situations any speed increases through overclocking are unlikely to be noticeable.
It is generally accepted that, even for computationally-heavy tasks, speed increases of less than ten percent are difficult to discern. For example, when playing video games, it is difficult to discern an increase from 60 to 66 frames per second without the aid of an on-screen frame counter. Generally, gains of a few percent are sought for prestige rather than real-world computational benefit.
Overclocking graphics cards
Graphics cards can also be overclocked, with utilities such as nVidia's Coolbits, or the PEG Link Mode on ASUS motherboards. Overclocking a video card usually shows a much better result in gaming than overclocking a processor or memory. Just like overclocking a processor, sufficient cooling is a must.
Along with the higher clock frequencies come higher temperatures, coupled with the fact that most video cards are sold with coolers designed only to support standard stock temperatures many graphics cards overheat and burn out when overclocked too much.
Prior to irreversible damage to the graphics card, in game distortions known as artifacts become visible and serve as a good warning sign. Two such discriminated "warning bells" are widely understood: green-flashing, random triangles appearing on the screen in 99% of cases correspond to overheating problems on the GPU (Graphics Processing Unit) itself, while white, flashing dots appearing randomly (usually in groups) on the screen mean that the card's RAM (memory) is overheating. It is common to run into one of those problems when overclocking graphics cards. Showing both symptoms at the same time usually means an over-overclocked card (one which is drastically overheating), or poor quality components used to produce the card (in which case the card is not overclockable by any lengths).
Some overclockers may also make use of a hardware voltage modification where a potentiometer is applied to the video card to give the GPU more voltage and much better overclocking ability. Voltage mods are very risky and usually result in a dead video card. It is also worth mentioning that adding physical elements to the video card immediately voids the warranty (even if the component has been designed and manufactured with overclocking in mind, and has the appropriate section in its warranty). Also, any and all manual hardware modifications which require more than swapping whole components around (soldering a potentiometer to a graphics card is a good example of such action) should be performed by people who feel comfortable with doing it, and preferably have some technical education in the area. If you're looking to do such a modification but lack the knowledge or expertise, find someone who can help instead of attempting it yourself. Manual modifications are very sensitive in nature, and the process is prone to errors.
The difference between "Flashing" and "Unlocking" a video card
Flashing and Unlocking are ways to gain performance out of a video card, without overclocking it per se.
Flashing refers to using the BIOS of another card, based on the same core and design specs, to "override" the original BIOS, thus effectively making it a higher model card; however, 'flashing' can be difficult, and sometimes a bad flash can be irreversible. Sometimes stand-alone software to modify the BIOS files can be found (GeForce 6/7 series are well regarded in this aspect). It is not necessary to acquire a BIOS file from a better model video card (although it should be said that the card which BIOS is to be used should be compatible, i.e. the same model base, design and/or manufacture process, revisions etc.). For example, video cards with 3D accelerators (99% of today's market) have two voltage and speed settings - one for 2D and one for 3D - but were designed to operate with three voltage stages, the third being somewhere in the middle of the aforementioned two, serving as a fallback when the card overheats or as a middle-stage when going from 2D to 3D operation mode. Therefore, it could be wise to set this middle-stage prior to "serious" overclocking, specifically because of this fallback ability - the card can drop down to this speed, reducing by a few (or sometimes a few dozen, depending on the setting) percent of its efficiency and cool down, without dropping out of 3D mode (and afterwards return to the desired full-speed clock and voltage settings).
Some cards also have certain abilities not directly connected with overclocking. For example, nVidia's GeForce 6600GT (AGP flavor) features a temperature monitor (used internally by the card), which is invisible to the user in the 'vanilla' version of the card's BIOS. Modifying the BIOS (taking it out, reprogramming the values and flashing it back in) can allow a 'Temperature' tab to become visible in the card driver's advanced menu.
Unlocking refers to enabling extra pipelines and/or pixel shaders. This is commonly done on the 6800LE, the 6800GS and 6800 (AGP models only). While these models have either 8 or 12 pipes enabled, they share the same 16x6 GPU core as a 6800GT or Ultra, but may not have passed inspection when all their pipelines and shaders were unlocked.
Generally, cards in the same 'family' share the same basic design, even though they run at different speeds and may have different features, effectively varying their performance (as observed with GeForce 6 series of cards, ranging from LS to 'vanilla' to GT to Ultra). Why? Because creating a completely new design costs more than producing the same card and disabling some features, underclocking it, and offering it as a 'budget' model. Besides that, the manufacturing process is not perfect; some cards come off the bench performing worse than others of the same design (or sometimes with defects), and can be designated as 'lower cost, slower' versions (i.e. the defective processing pipelines are disabled, the card's speed is reduced, and from an otherwise incapable GeForce 6800 we can get a 6800LE).
It is important to remember that while pipeline unlocking sounds very promising, there is absolutely no way of determining if these 'unlocked' pipelines will operate without errors, or at all (this information is solely at the manufacturer's discretion). In a worst-case scenario, the card may not start up ever again, resulting in a 'dead' piece of equipment. It IS possible to revert to the card's previous settings, but it involves manual BIOS flashing using special tools and an identical but original BIOS chip.