Editing Overclocking (section)

=== Stability and functional correctness ===

{{See also| Stress testing#Hardware }}

As an overclocked component operates outside of the manufacturer's recommended operating conditions, it may function incorrectly, leading to system instability. Another risk is [[Reliability, availability and serviceability (computer hardware)|silent data corruption]] by undetected errors. Such failures might never be correctly diagnosed and may instead be incorrectly attributed to software bugs in applications, [[device drivers]], or the operating system. Overclocked use may permanently damage components enough to cause them to misbehave (even under normal operating conditions) without becoming totally unusable.

A large-scale 2011 field study of hardware faults causing a system crash for consumer PCs and laptops showed a four to 20 times increase (depending on CPU manufacturer) in system crashes due to CPU failure for overclocked computers over an eight-month period.<ref>{{cite conference|url=http://research.microsoft.com/pubs/144888/eurosys84-nightingale.pdf|title=Cycles, cells and platters: an empirical analysis of hardware failures on a million consumer PCs.|conference=Proceedings of the sixth conference on Computer systems (EuroSys '11).|pages=343–356|year=2011|access-date=2012-12-05|archive-date=2012-11-14|archive-url=https://web.archive.org/web/20121114111006/http://research.microsoft.com/pubs/144888/eurosys84-nightingale.pdf|url-status=live}}</ref>

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.<ref>{{cite journal | citeseerx = 10.1.1.62.9086 | title = Coverage Metrics for Functional Validation of Hardware Designs | publisher = IEEE Design & Test of Computers | year = 2001 | first1 = Serdar |last1=Tasiran |first2=Kurt |last2=Keutzer}}</ref> Achieving good [[fault coverage]] requires immense engineering effort; even with all of the resources dedicated to validation by manufacturers, faulty components and even design faults are not always detected.

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 [[status register|flags]]; if the flags are not checked, the error will go undetected.

To further complicate matters, in process technologies such as [[silicon on insulator]] (SOI), 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 rates 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.<ref>{{cite web | url = http://blogs.msdn.com/oldnewthing/archive/2005/04/12/407562.aspx | first = Raymond | last = Chen | title = The Old New Thing: There's an awful lot of overclocking out there | date = April 12, 2005 | access-date = 2007-03-17 | archive-date = 2007-03-08 | archive-url = https://web.archive.org/web/20070308074036/http://blogs.msdn.com/oldnewthing/archive/2005/04/12/407562.aspx | url-status = dead }}</ref>

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]], [[Superpi]], OCCT, [[AIDA64]], [[Linpack]] (via the LinX and IntelBurnTest [[GUI]]s), SiSoftware Sandra, [[BOINC]], Intel Thermal Analysis Tool and [[Memtest86]]. The hope is that any functional-correctness issues with the overclocked component will manifest themselves during these tests, and if no errors are detected during the test, then the component is deemed "stable". Since fault coverage is important in [[Software testing|stability testing]], the tests are often run for long periods of time, hours or even days. An overclocked computer is sometimes described using the number of hours and the stability program used, such as "prime 12 hours stable".