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Developed by George Woltman
Latest release 25.9 / March 27, 2009[1]
Operating system Microsoft Windows
Type Mersenne prime finder / system stability tester
License GIMPS prize terms

Prime95 is the name of the Microsoft Windows-based software application written by George Woltman that is used by GIMPS, a distributed computing project dedicated to finding new Mersenne prime numbers. As of October 2008, 12 new Mersenne prime numbers have been found by the network of participants, and, on average, a new Mersenne prime is discovered approximately every year. The Linux-based version is called MPrime.

Although most of the GIMPS software's source code is publicly available, it is technically not free software, since it has a restriction that users must abide by the project's distribution terms[2] if the software is used to discover a prime number with at least 100,000,000 decimal digits and wins the $150,000 bounty offered by the EFF.[3]

The code that is used to generate checksums is not publicly available due to security reasons.[4]


[edit] Use for stress testing

Over the years, Prime95 has become extremely popular among PC enthusiasts and overclockers as a stability testing utility. It includes a "Torture Test" mode designed specifically for testing PC subsystems for errors in order to help ensure the correct operation of Prime95 on that system, which effectively stress-tests a PC.

The stress-test in Prime can be configured to better test various components of the computer by changing the fast fourier transform (FFT) size. Three pre-set configurations are available: Small FFTs, In-Place FFTs, and Blend. Small and In-Place modes primarily test the FPU and the caches of the CPU, whereas the Blend mode tests everything, including the memory.

By selecting Custom, the user can gain further control of the configuration. For example, by selecting 8-8KB as the FFT size, the program stresses primarily the CPU. By selecting 2048-4096KB and unchecking the "Run FFTs in-place" checkbox, providing the maximum amount of RAM free in the system, the program tests the memory and the chipset. If the amount of memory to use option is set too high then system will start using the paging file and the test will not stress the memory.

On an absolutely stable system, Prime95 would run indefinitely. If an error occurs (the tray icon will become red from the default green, indicating that the test has halted), there is a chance that the system is unstable. There is an ongoing debate about terms "stable" and "Prime-stable", as Prime95 often fails before the system becomes unstable or crashes in any other application. This is because Prime 95 is designed to subject the CPU to an incredibly intense workload, and to halt when it encounters even one minor error, whereas most normal applications do not stress the CPU anywhere near as much, and will continue to operate unless they encounter a fatal error.

In the overclocking community a rule of thumb is often used to determine how long to run Prime95: test the CPU (8KB FFT) for 10 hours and the memory (4096KB FFT) for 10 hours, and if the system passes, there is a high chance that it is stable.[5] Moreover, a large proportion of system overclockers and enthusiasts favor Prime95 over other benchmarking suites because Prime95 pushes the CPU's floating point units extremely hard, causing the CPU to become extremely hot. In addition, Prime95 stresses a computer machine far more than the majority of software based torture-suites. The nature of this is because the operating system usually shuts down the floating-point unit when unused by other programs, whereas Prime95 is well-optimized to continuously and effectively thread the FPU, resulting the FPU to be deeply pipelined thereby generating significantly more heat because of elevated power consumption under the massive workload conditions. In CPUs which are not properly cooled, errors are likely to occur. Prime95 also constantly accesses main memory up to 60MB per second. This constant activity will detect memory problems that other programs do not.

Lastly, power supply units of any machine running Prime95 are subject to the consistent ramifications of such harsh conditions. Power must be maintained clean, while providing adequate voltage, particularly to the CPU, RAM, and chipsets (Mainboard Chipsets such as the Northbridge where the memory controller may or may not reside, see Athlon64 for on-die memory controllers) to provide peak performance while maintaining stability. Cray Research used programs similar to Prime95 diagnostics package for over a decade for the purpose of stability testing.[6]

[edit] Limits

Some versions of Prime95 cannot test Mersenne numbers beyond 279,300,000 βˆ’ 1.[7] This is slightly smaller than a 24 million digit number.

[edit] See also

[edit] References

  1. ^ The latest client software
  2. ^ GIMPS prize terms
  3. ^ Cooperative Computing Awards
  4. ^
  5. ^
  6. ^
  7. ^

[edit] External links

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