question: Why does 5GHZ seem to be the ceiling for conventional desktop overclocking?
comment: CPU speed is limited by time of transmission and heat. The electrical signal has to be transmitted through microscopic wires in a chip. Though the theoretical limit is the speed of light, the realistic limit is determined by the material that conducts the electrical charge. Additionally the gates -- the circuits -- have to switch from 0 to 1 -- open to closed -- and that can only happen so quickly. So there are physical, engineering limits on how fast those things can take place. It is possible to make those things happen faster -- that's basically what overclocking is -- but when you do that it generates additional heat. Without appropriate cooling, the heat will eventually burn out the microprocessor. Because of the size of microprocessors there's only so much cooling you can bring to the table, so as you try to speed up the chip you reach a point where the additional heat outstrips any attempts at cooling, and you burn the chip up. The combination of transmission speed and heat dissipation create the limits on CPUs.
passages: BULLET::::- eXtended Frequency Range (XFR), an automated overclocking feature which boosts clock speeds beyond the advertised turbo frequency.
   Athlon XP-Ms were popular with desktop overclockers, as well as underclockers. The lower voltage requirement and higher heat rating selected CPUs that were essentially "cherry picked" from the manufacturing line. Being some of the best cores "off the line", these CPUs typically overclocked more reliably than their desktop-headed counterparts. Also, the fact that they were not locked to a single multiplier was a significant simplification in the overclocking process. Some "Barton" core Athlon XP-Ms have been successfully overclocked as high as 3.1 GHz.
   Between 2001 and 2003, Intel and AMD made few changes to the designs of their processors. Most performance increases were created by raising the processor's clock speed rather than improving the microprocessor's core. Around mid-2004, Intel encountered serious problems in increasing their Pentium 4's clock speed beyond 3.4 GHz because of the enormous amount of heat generated by the already hot Prescott core processor when working at higher clock speeds. In response, Intel started exploring ways to improve the performance of its microprocessors in ways other than raising the clock speeds of the processors such as increasing the sizes of the processors' caches, using a P6 microarchitecture descendant in Pentium M CPUs and beyond, and using multiple processing cores in its processors.
   Section::::Considerations.:Factors allowing overclocking.
   Manufacturers of modern processors typically charge premium prices for processors that operate at higher clock rates, a practice called binning. For a given CPU, the clock rates are determined at the end of the manufacturing process through actual testing of each processor. Chip manufacturers publish a "maximum clock rate" specification, and they test chips before selling them to make sure they meet that specification, even when executing the most complicated instructions with the data patterns that take the longest to settle (testing at the temperature and voltage that runs the lowest performance). Processors successfully tested for compliance with a given set of standards may be labeled with a higher clock rate, e.g., 3.50 GHz, while those that fail the standards of the higher clock rate yet pass the standards of a lesser clock rate may be labeled with the lesser clock rate, e.g., 3.3 GHz, and sold at a lower price.
   Underclocking and undervolting would be attempted on a desktop system to have it operate silently (such as for a home entertainment center) while potentially offering higher performance than currently offered by low-voltage processor offerings. This would use a "standard-voltage" part and attempt to run with lower voltages (while attempting to keep the desktop speeds) to meet an acceptable performance/noise target for the build. This was also attractive as using a "standard voltage" processor in a "low voltage" application avoided paying the traditional price premium for an officially certified low voltage version. However again like overclocking there is no guarantee of success, and the builder's time researching given system/processor combinations and especially the time and tedium of performing many iterations of stability testing need to be considered. The usefulness of underclocking (again like overclocking) is determined by what processor offerings, prices, and availability are at the specific time of the build.
   Intel's CPU throttling technology, SpeedStep, is used in its mobile and desktop CPU lines.
   When the workload on the processor calls for faster performance, the processor's clock will try to increase the operating frequency in regular increments as required to meet demand. The increased clock rate is limited by the processor's power, current, and thermal limits, the number of cores currently in use, and the maximum frequency of the active cores. Frequency increases occur in increments of 133 MHz for Nehalem processors and 100 MHz for Sandy Bridge, Ivy Bridge, Haswell and Skylake processors. When any electrical or thermal limits are exceeded, the operating frequency automatically decreases in decrements of 133 or 100 MHz until the processor is again operating within its design limits. Turbo Boost 2.0 was introduced in 2011 with the Sandy Bridge microarchitecture, while Intel Turbo Boost Max 3.0 was introduced in 2016 with the Broadwell-E microarchitecture.
   Over the decade that followed, few commercial CPU products exceeded 4 GHz, with the majority of performance improvements now coming from incrementally improved microarchitectures, better systems integration, and higher compute density—this largely in the form of packing a larger numbers of independent cores onto the same die, often at the "expense" of peak operating frequency (Intel's 24-core Xeon E7-8890 from June 2016 has a base operating frequency of just 2.2 GHz, so as to operate within the constraints of a single-socket 165 W power consumption and cooling budget).
   Engineers also continue to find new ways to design CPUs so that they complete more instructions per clock cycle, thus achieving a lower CPI (cycles or clock cycles per instruction) count, although they may run at the same or a lower clock rate as older CPUs. This is achieved through architectural techniques such as instruction pipelining and out-of-order execution which attempts to exploit instruction level parallelism in the code.
  
  IBM is working on 100Ghz CPU. In 2010, IBM demonstrated a graphene based transistor that can execute 100 billion cycles per second.
  
  Section::::Comparing.
   As of 2015, Xeon 3600 and 5600 series Westmere-EP processors have become somewhat sought after as an upgrade route for older X58 motherboards. In many cases, proper BIOS support has allowed users to fit these 6-core processors onto boards originally designed for 2 or 4-core processors. In fully threaded workloads, total system performance will increase by a value equal to the number of cores added (i.e. moving from a 4-core 2.6 GHz to a 6-core 2.6 GHz system would enable 50% more total processing power). With proper BIOS support and the correct supporting components, many users have reported substantial overclocking potential, often as high as 4.4 GHz while staying within Intel's maximum allowed voltages (no higher than 1.35v for the core or the uncore).
   , the Guinness World Record for the highest CPU clock rate is an overclocked, 8.723 GHz AMD Piledriver-based FX-8370 chip. It surpassed the previous record achieved in 2011, an 8.429 GHz AMD FX-8150 Bulldozer-based chip.
  
  , the highest clock rate on a production processor is the IBM zEC12, clocked at 5.5 GHz, which was released in August 2012.
  
  Section::::Research.
   Section::::Overview.:Enthusiast culture.
   Overclocking offers several draws for overclocking enthusiasts. Overclocking allows testing of components at speeds not currently offered by the manufacturer, or at speeds only officially offered on specialized, higher-priced versions of the product. A general trend in the computing industry is that new technologies tend to debut in the high-end market first, then later trickle down to the performance and mainstream market. If the high-end part only differs by an increased clock speed, an enthusiast can attempt to overclock a mainstream part to simulate the high-end offering. This can give insight on how over-the-horizon technologies will perform before they are officially available on the mainstream market, which can be especially helpful for other users considering if they should plan ahead to purchase or upgrade to the new feature when it is officially released.
   where "P" is power consumption, "C" is the capacitance being switched per clock cycle, "V" is voltage, and "F" is the processor frequency (cycles per second). Increases in frequency thus increase the amount of power used in a processor. Increasing processor power consumption led ultimately to Intel's May 2004 cancellation of its Tejas and Jayhawk processors, which is generally cited as the end of frequency scaling as the dominant computer architecture paradigm. 
   Dynamic frequency scaling (automatic underclocking) is very common on laptop computers and has become common on desktop computers as well. In laptops, the processor is usually underclocked automatically whenever the computer is operating on batteries. Most modern notebook and desktop processors (utilizing power-saving schemes like AMD's Cool'n'Quiet and PowerNow!) will underclock themselves automatically under a light processing load, when the machine BIOS and the operating system support it. Intel has also used this method on numerous processors through a feature called SpeedStep. SpeedStep first appeared on chips like the Core 2 Duo and selective Pentium models, later becoming a standard in mid to high-end Core i3, i5, and i7 models.
   This chart shows some differences between the T5 and T4 processor chips.
  
  The SPARC T5 also introduces a new power management feature that consists of hardware support in the processor, and the software that allows system administrator to use the feature. Users select the policy how the system responds to over-temperature and over-current events. The dynamic voltage and frequency scaling (aka DVFS) policy can be set to maintain peak frequency, or to trade off between performance and power consumption.
  
  Section::::SPARC T5 in systems.
   Following increased computer speeds, performance measurement was changed to Millions of Whetstone Instructions Per Second (MWIPS). The original results on numerous minicomputers, mainframes and supercomputers are available in Whetstone Benchmark History and Results. This also shows original system costs and year of manufacture. For comparison purposes, a summary of results on PCs is also provided, including a table showing the relative efficiency of various programming languages. Detailed results on PCs are also available, showing speeds of the different test functions.
   Dynamic frequency scaling reduces the number of instructions a processor can issue in a given amount of time, thus reducing performance. Hence, it is generally used when the workload is not CPU-bound.
   The speed gained by overclocking depends largely upon the applications and workloads being run on the system, and what components are being overclocked by the user; benchmarks for different purposes are published.
  
  Section::::Overview.:Underclocking.
   BULLET::::- Reduce CPU supply voltage ("undervolting"). Many of today's CPUs can run stably at their stock speed, or even with a slight overclock, at a reduced voltage, which reduces heat output. Power consumption is approximately proportional to "V·f", that is, it varies linearly with the clock frequency and quadratically with the voltage. This means that even a small reduction in voltage can have a large effect in power consumption. Undervolting and underclocking can also be used with chipsets and GPUs.
  
  BULLET::::- Enable Cool'n'Quiet for AMD CPUs or SpeedStep (also known as EIST) on Intel CPUs.
   BULLET::::- Higher performance in games, en-/decoding, video editing and system tasks at no additional direct monetary expense, but with increased electrical consumption and thermal output.
   Frequency scaling
  
  In computer architecture, frequency scaling (also known as frequency ramping) is the technique of increasing a processor's frequency so as to enhance the performance of the system containing the processor in question. Frequency ramping was the dominant force in commodity processor performance increases from the mid-1980s until roughly the end of 2004. 
  
  The effect of processor frequency on computer speed can be seen by looking at the equation for computer program runtime: 
   In 1992, both Hewlett-Packard and Digital Equipment Corporation broke the difficult 100 MHz limit with RISC techniques in the PA-7100 and AXP 21064 DEC Alpha respectively. In 1995, Intel's P5 Pentium chip ran at 100 MHz (100 million cycles per second). On March 6, 2000, AMD reached the 1 GHz milestone a few months ahead of Intel. In 2002, an Intel Pentium 4 model was introduced as the first CPU with a clock rate of 3 GHz (three billion cycles per second corresponding to ~3.3×10seconds or 0.33 nanoseconds per cycle). Since then, the clock rate of production processors has increased much more slowly, with performance improvements coming from other design changes.
   The clock rate alone is generally considered to be an inaccurate measure of performance when comparing different CPUs families. Software benchmarks are more useful. Clock rates can sometimes be misleading since the amount of work different CPUs can do in one cycle varies. For example, superscalar processors can execute more than one instruction per cycle (on average), yet it is not uncommon for them to do "less" in a clock cycle. In addition, subscalar CPUs or use of parallelism can also affect the performance of the computer regardless of clock rate.
  
  Section::::See also.
  
  BULLET::::- Crystal oscillator frequencies
  