Prime95
From Overclocking Wiki
Prime95 is the Microsoft Windows-based client software for the Great Internet Mersenne Prime Search (GIMPS) distributed computing project. MPrime is the GNU-licensed version of the same software for FreeBSD and Linux. Other versions are available for OS X, Strong ARM machines, and OS/2.
Prime95 is free, and can be used without enlisting in the prime search project.
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Use by project participants
Prime95 works as a part of GIMPS by requesting work from the server, completing the work, and reporting the results back. Generaly, the work offered by the server involves offering likely Mersenne prime number candidates and a list of possible divisors.
The client program will try to factor the number using the divisors presented. If it finds the prime is factorable, it will report the results back to the server. If no factors are found, this information is also reported back to the server. Once a work unit is completed, the client requests more work from the server and continues; subsequent work might involve a different prime candidate and almost certainly involves different divisors.
The client may also be asked to verify results. If other clients have reported that a certain number is prime or composite, the servers may request other clients perform the calculations again to double-check the results.
Users who find a large prime number gain some noterierty in mathematical circles, though they really did nothing more than contribute processing time to the project.
Use by overclockers
Overclocking a system inherently makes it unstable as the components in the system are operated beyond their design limits. An incorrectly overclocked system won't even boot, while a system that's aggresively overclocked might not quite be stable.
To test their systems for stability, overclockers will often run computationally-intensive programs such as Prime95 to assure that the system will work at the higher, out-of-spec settings. The goal is to discover stability problems as soon as possible while testing so new settings can be investigated and tested. Leaving the testing to normal use might not reveal stability problems, including data corruption or crashes, that result in the loss of work.
Prime95 is a good candidate for this stress testing application because it does computationally intensive work that stresses the proessor and the processor's interface to memory. Further, the program tests its own results for correctness and immediately reports any discrepancies--something that few desktop applications do.
To run Prime95 as a stress test, you can:
- Start the Prime95 program
- Choose "Torture Test..." from the "Options" menu. The resulting "Run a Torture Test" dialog box is shown here:
- Mark the "Custom" radio button, which will enable the controls in the "Torture test settings" group box.
- Set the "Min FFT size" and "Max FFT size" values. These values indicate how much data the program will touch; sizes less than your processor's L2 cache memory size mean you're only truly exercising the processor and not the memory subsystem. Larger sizes mean you'll be executing the memory subsystem, as well.
- Set the "Time to run FFT size (in minutes) control to the desired value. The program runs the minimum FFT size you've specified for the provided duration, then stops. The program then doubles the FFT size; if it's greater than the maximum size, it stops running the Torture Test. Going from 8 kilobytes to 4096 kilobytes requires nine doubles, so running each test for 15 minutes will take 135 minutes, or two hours, fifteen minutes total.
The STRESS.TXT file that ships with the Prime95 package includes advice about what to do if you find an error. Any problem you find is most likely a problem with your machine. A smart way to proceed is to do your build, then run Prime95 at stock settings to see if there's anything wrong with your components. Once you think you've got conservative overclocking settings in place, run Prime95 with FFT sizes large enough to exercise your memory and processor overnight -- for eight to ten hours. If you find any errors at all, you can be very certain that your overclock is too aggressive, particularly if a run at stock settings worked without errors.
If you find the FFT size settings confusing, feel free to use the "Blend" option in the "Run a Torture Test" dialog. This will provide adequate testing if run for at least eight hours.
Running as a benchmark
Prime95 also provides the ability to run a short suite of computations and report the times for the work as a benchmark. To do this in the Windows version:
- Start Prime95
- Use the "Benchmark" command in the "Options" menu
Before starting the benchmark, you should stop any application which might be using the CPU. For the most accurate results, this includes any services. Note that even a web browser, while idle, might be playing animations or other media and using CPU time. While the benchmark is running, you should make sure you don't move the mouse or use any other programs.
The program will write a RESULTS.TXT file that you can reference. You can also compare your results to machines with the same CPU you're using at the benchmark page at mersenne.org.
Overclocking will imrprove this benchmark score, as will a similar processor with a faster stock clock speed. Note, though, that larger FFT lengths overflow the processor's cache memory. As such, a processor with more cache will score better than processors with slightly faster clock speeds through the longer FFT runs. This further demonstrates that the larger FFT sizes will stress the memory subsystem.
Version
The current version of Prime95 for 32-bit Windows is 25.9. When comparing benchmark results, it's imperative to make sure you're running the same version as the published results you're examining.
Multi core systems
When attempting to stress test a multi core system using Prime95 is necessary to use the current beta version 25.x which is freely available for download here. Otherwise, the current stable version will only stress one of your cores requiring you to as as many instances of the program as you have cores. For example, on a Core 2 Duo (with 2 cores) you would need to run two copies of Prime95 at the same time. The Orthos tool is available to automate the task of starting multiple processes and setting their affinity to the available machines. Orthos uses the Prime95 engine so results are similar. One word of caution about Orthos is that it can only stress two cores per instance. If you have a quad core chip, you'll have to run two instances of it and manually set the cpu affinity (through the task manager) which can be annoying.
