▷ Prime95: what it is and what it is for

What is Prime95 and how it works

Prime95 is a free software application written by George Woltman that uses GIMPS, a distributed computing project dedicated to finding new prime numbers for Mersenne. More specifically, Prime95 refers to the Windows and macOS versions of the software.

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Although most of the GIMPS software source code is publicly available, it is technically not free software, as users must comply with the project's distribution terms, if the software is used to discover a prime number with at least 100, 000, 000 decimal digits and Earn the $ 150, 000 reward offered by the EFF.

As such, a user who uses Prime95 to discover a qualifying prime number will not be able to claim the prize directly. A free software package would not have this restriction. The code that is used to generate checksums is not publicly available for security reasons. P rime95 does not currently have GPU support, although Woltman has indicated that it is in development. However, there are third-party programs, such as CUDALucas, that make use of the processing power of GPUs.

MPrime is the version of Prime95's Linux command line interface, which runs in a text terminal or in a terminal emulator window as a remote shell client. It is identical to Prime95 in functionality, except it lacks a graphical user interface.

The preferred tool for overclockers

Over the years, Prime95 has become extremely popular with PC enthusiasts and overclockers as a stability test utility. It includes a "Torture Test" mode specifically designed to test errors in the PC subsystems, in order to guarantee the correct operation in that system. The stress test function in Prime95 can be configured to better test various PC components by resizing the Fourier fast transform (FFT). There are three presets available: Small FFT and Local FFT, and Mix. Small and in-place modes primarily test the FPU and CPU caches, while mixed mode checks everything, including memory.

By selecting the custom mode, the user can gain more control over the settings. For example, when selecting 8-8 kB as the FFT size, the program mainly emphasizes the CPU. By selecting 2048-4096 kB and unchecking the “Run FFT in Place” checkbox, which provides the maximum amount of free RAM in the system, the program tests the memory and chipset. If the amount of memory to use this option is set too high, then the 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, at which point the stress test would end, this would indicate that the system may be unstable. There is an ongoing debate on the terms "stable" and "Prime-stable", as Prime95 often fails before the system becomes unstable or crashes in any other application. This is because Prime95 is designed to subject the CPU to an incredibly heavy workload and to stop when it encounters even a minor error, while most normal applications do not push the CPU as much as possible, and will continue working unless they find a fatal error.

In the overclocking community, a golden rule is often used to determine how long Prime95 should run: test the CPU (8 kB FFT) for 10 hours and memory (4096 kB FFT) for 10 hours, and whether the system happens, there is a high probability that it is stable. Twenty-four hours of testing is recommended to be on the safe side, as errors may appear after 16 hours or more of testing. Also, a large proportion of overclockers and system enthusiasts favor Prime95 over other benchmarking suites, because Prime95 pushes the CPU's floating point drives extremely hard, causing the CPU to become extremely hot.

Plus, Prime95 places a lot more emphasis on a PC than most software-based torture suites. The nature of this is because the operating system generally turns off the floating point drive when it is not used by other programs, while Prime95 is well optimized to thread the FPU continuously and effectively, which makes it highly channeled, Thus generating significantly more heat due to high energy consumption under conditions of massive workload. Prime95 also constantly accesses main memory up to 60MB per second. This constant activity will detect memory problems that other programs will not detect.

Finally, the power supply units of any machine running Prime95 are subject to the constant ramifications of such difficult conditions. Power must be kept clean while providing the proper voltage, particularly to the CPU, RAM, and chipsets, where the memory controller may or may not reside, to provide maximum performance while maintaining stability. Cray Research used programs similar to Prime95 for more than a decade for the purpose of stability testing.

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