Overclocking is the process of forcing a computer component to run at a higher clock rate (more clock cycles per second) than it was designed for or was designated by the manufacturer, usually practiced by personal computer enthusiasts in order to emulate an increase in the performance of their computers. Some of them purchase low-end computer components which they then overclock to higher speeds, or overclock high-end components to attain levels of performance beyond the default factory settings. Others overclock outdated components to keep pace with new system requirements, rather than purchasing new hardware products as expected by the computer industry.
Users who overclock their components mainly focus their efforts on processors, video cards, motherboard chipsets, and Random Access Memory (RAM). It is done through manipulating the CPU multiplier and the motherboard's front side bus (FSB) speed until a maximum stable operating frequency is reached. While the idea is simple, variation in the electrical and physical characteristics of computing systems complicates the process. CPU multipliers, bus dividers, voltages, thermal loads, cooling techniques and several other factors can affect it.
There are several considerations when overclocking. The first consideration is to ensure that the component is supplied with adequate power to operate at the new speed. However, supplying the power with improper settings or applying excessive voltage can permanently damage a component. Since tight tolerances are required for overclocking, only more expensive motherboards—with advanced settings that computer enthusiasts are likely to use—have built-in overclocking capabilities. Motherboards with fewer settings, such as those found in Original Equipment Manufacturer (OEM) systems, lack such features in order to eliminate the possibility of misconfiguration and cut down on the support costs and warranty claims to the manufacturer.
All electronic circuits produce heat generated by the movement of electrons. As clock frequencies in digital circuits increase, the heat generated by overclocked components also increases. Due to increased heat produced by overclocked components, an effective cooling system is necessary to avoid damaging the hardware. In addition, digital circuits slow down at high temperatures due to changes in metal–oxide–semiconductor field-effect transistor (MOSFET) device characteristics. Wire resistance also increases slightly at higher temperatures, contributing to decreased circuit performance.
Because most stock cooling systems are designed for the amount of power produced during non-overclocked use, overclockers typically turn to more effective cooling solutions, such as powerful fans or heavy duty heatsinks. Size, shape, and material all influence the ability of a heatsink to dissipate heat. Efficient heatsinks are often made entirely of copper, which has high thermal conductivity, but is expensive. Aluminium is more widely used; it has poorer thermal conductivity, but is significantly cheaper than copper. Iron and steel are not often used in heatsinks due to their poor thermal conductivity. Many heatsinks combine two or more materials to achieve a balance between performance and cost.
Water cooling carrying waste heat to a radiator. Automobile engines and nuclear reactors use the same method, and both with water. (for cars ethylene glycol is often added to prevent freezing, and reactors can use heavy water) Water cooling is extremely powerful, especially in comparison with air cooling.
Thermoelectric cooling devices, also known as Peltier devices, are recently popular with the onset of high Thermal Design Power (TDP) processors made by Intel and AMD. Thermoelectric cooling devices create temperature differences between two plates by running an electric current through the plates. This method of cooling is highly effective but has a drawback that it leads to a lot of excess heat. For this reason, it is often necessary to supplement thermoelectric cooling devices with a convection-based heatsink or a water cooling system.
Other cooling methods are forced convection and phase change cooling which is used in refrigerators. Liquid nitrogen and dry ice are used as coolants in extreme measures, such as record-setting attempts or one-off experiments rather than cooling an everyday system. These extreme methods are generally impractical in the long term, as they require refilling reservoirs of vaporizing coolant and condensation is formed on components due to difference between component temperature and air temperature. Moreover, silicon-based junction gate field-effect transistors (JFET) will degrade below temperatures of roughly and eventually cease to function or "freeze out" at , so using extremely cold coolants may cause devices to fail.
Submersion cooling, used for Cray-2 supercomputer, involves sinking a part of computer system directly into a chilled liquid substance that is thermally conductive but sufficiently low in electrical conductivity. The advantage of this technique is that no condensation can form on components. A good submersion liquid is Fluorinert made by 3M, which is expensive and requires permits to purchase it. Another option is mineral oil, but any impurities like water or scenting agents might cause it to conduct electricity.
As an overclocked component operates outside of the manufacturer's recommended operating conditions, it may function incorrectly, leading to system instability. An unstable overclocked system, while it may work fast, can be frustrating to use. Another risk is silent data corruption—errors that are initially undetected. Such failures might never be correctly diagnosed and may instead be incorrectly attributed to software bugs in applications or the operating system.
In general, overclockers claim that testing can ensure that an overclocked system is stable and functioning correctly. Although software tools are available for testing hardware stability, it is generally impossible for any private individual to thoroughly test the functionality of a processor. Achieving good fault coverage requires immense engineering effort considering that even with all of the resources dedicated to validation by manufacturers, faults can still sometimes pass through undetected.
A particular "stress test" can verify only the functionality of the specific instruction sequence used in combination with the data and may 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.
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 experiences instabilities in other programs.
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 different math-intensive applications for testing general CPUs). Popular stress tests include Prime95, Super PI, SiSoftware Sandra, BOINC, Intel Thermal Analysis Tool 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". Since fault coverage is important in stability testing, the tests are often run for long periods of time, hours or even days.
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 at or below 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. Sometimes manufacturers have an excess of similarly high-performing parts and cannot sell them all at the flagship price, so some are marked as medium-speed chips to be sold for medium prices. The performance of a given CPU stepping usually does not vary as widely as the marketing clock levels.
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.
Overclocking is sometimes offered as a legitimate service or feature for consumers, in which a manufacturer or retailer tests the overclocking capability 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 negative reception from overclocking enthusiasts.
Graphics cards can also be overclocked, with utilities such as NVIDIA's Coolbits, RivaTuner, 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.
Sometimes, it is possible to see that a graphics card is pushed beyond its limits before any permanent damage is done by observing on-screen distortions known as artifacts. Two such discriminated "warning bells" are widely understood: green-flashing, random triangles appearing on the screen usually correspond to overheating problems on the GPU (Graphics Processing Unit) itself, while white, flashing dots appearing randomly (usually in groups) on the screen often 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 that the card is severely pushed beyond its heat/speed/voltage limits. If seen at normal speed, voltage and temperature, they may indicate faults with the card itself.
Some overclockers use a hardware voltage modification where a potentiometer is applied to the video card to manually adjust the voltage. This results in much greater flexibility, as overclocking software for graphics cards is rarely able to freely adjust the voltage. Voltage mods are very risky and may result in a dead video card, especially if the voltage modification ("voltmod") is applied by an inexperienced individual. A pencil volt mod refers to changing a resistor's value on the graphics card by drawing across it with a graphite pencil. This results in a change of GPU voltage. 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).
Flashing and Unlocking are two popular ways to gain performance out of a video card, without technically overclocking.
Flashing refers to using the firmware of another card, based on the same core and design specs, to "override" the original firmware, 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 firmware files can be found, i.e. NiBiTor, (GeForce 6/7 series are well regarded in this aspect). It is not necessary to acquire a firmware file from a better model video card (although it should be said that the card in which firmware 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 (the vast majority 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. The 6800LE, the 6800GS and 6800 (AGP models only) and Radeon X800 Pro VIVO were some of the first cards to benefit from unlocking. 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. In more recent generations, both ATI and Nvidia have laser cut pipelines to prevent this practice..
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 firmware flashing using special tools and an identical but original firmware chip.