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Prime95 – What’s it all about?

As overclockers, we all use Prime95 to test the stability of our system. But what is Prime95 and what are prime numbers all about? If you’re a computer dink (like me) you should be interested in not only Prime95, but prime numbers too! If you’re not, read on anyway. You’ll learn something about them and Prime95.

Introduction

Prime95 is a program developed by the GIMPS team. In case you don’t know, GIMPS stands for the Great Internet Mersenne Prime Search (more on Mersenne primes later). The purpose of Prime95 is twofold: (1) test a prospective machine to see if it is stable enough to execute the Prime95 code and (2) test for Mersenne primes.

Before we discuss Prime95, let’s take a little side trip.

Prime Numbers

As computer dinks/nerds we’ve all probably heard of prime numbers. If not, then you’re missing out big time! I realize that mathematics is not everyone’s favorite subject, but math is the foundation of computers. Computer science and computational theory are math heavy subjects and if you want to be a computer scientist then you will have to learn the math.

Ok, what is a prime number? A prime number is any number that has only 2 factors – itself and 1. More formally, a prime number, p, is a positive integer (p > 1) that has no positive integer divisors other than 1 and p. Examples of primes are 2, 3, 5, 7, and 2^25,964951-1. By the way, all primes are odd (except for 2), but not all odd numbers are prime.

Prime numbers lend themselves to many proofs (groan) and have been the subject of study since the time of Pythagoras. Euclid and Eratosthenes not only studied primes, but made great contributions to the subject. Many of the theorems developed and proven by Euclid are foundational to computer security, especially cryptography. Encryption keys, for example, are based on prime numbers. Eratosthenes developed an algorithm called the Sieve of Eratosthenes to find prime numbers. You’ve no doubt seen this before if you’ve studied algorithms or taken programming courses.

So what about Prime95? Prime95 searches for a special class of primes called Mersennes. A Mersenne prime is a prime of the form 2^p-1 where p is a prime exponent. Not all numbers in the set {2^p - 1 | p > 1} are prime. Prime95 uses a set of algorithms to test each exponent to see if it will make up a Mersenne prime or not. The mathematics of the algorithms are relatively straightforward, if you have the right background! They are all based on proofs (groan) developed over the centuries by great mathematicians.

Prime95

First of all, download the latest version of Prime95 from the GIMPS site at http://www.mersenne.org/freesoft.htm. Installation is quick and easy. Open the self-extracting exe file and follow the instructions. Once you’ve installed Prime95, open the program. You should see nothing but a menu across the top and white space in the window. The menu selections are Test, Advanced, Options, and Help. For stress testing, we’re only interested in the Options menu.

Open the options menu and you’ll see several entries, including Torture Test, which is the one we want. Select Torture Test and a dialogue box (Run a Torture Test) will pop up.

This dialogue box allows you to pick a pre-configured test or create a custom one of your own. If you’re in a hurry select Small FFTs, In-Place Large FFTs, or Blend and let it run. Otherwise, read on.

Small FFTs

FFT stands for Fast Fourier Transform and is an algorithm to compute discrete Fourier transforms for a sequence of n complex numbers (groan, more math – I’m outta here). The Small FFT test loads neatly into your L2 cache so it tests primarily your CPU. Ram will generally only be affected whenever more data has to be fetched into the L2 cache. The small FFT test is good if you’re just looking to give your CPU a good burn in or test your cooling sub-system out. This test will heat up your CPU, so watch the temps.

In-Place Large FFTs

Due to the nature of in-place FFTs, they do a lot of moving data and swapping to and from memory during the computations. So this test gives your CPU a work out along with some of your RAM. It will also heat up your CPU quickly so watch your temps closely.

Blend

Blend is my favorite since it works out everything. Blend will use small and large in-place FFTs so everything (CPU and RAM) gets a nice work out. Unless you use something like memtest to check out your memory, I recommend the Blend setting. Blend, like the other test, does indeed give your system a work out and the temps rise rapidly. Not to be repetitive, but keep an eye on your temps lest the toasty aroma of a burned CPU begins to fill your nostrils.

Custom

Use the custom setting if you want to key in on a certain area like small FFTs or if you want to run a certain range of FFTs for specific amount of time. You can also control the amount of memory to use, but I would recommend giving it the max unless you want to limit for a specific reason.

System Stability and Testing

Ok, so how do we know we have a stable overclock in our rig? Just because your systems posts and boots into Windows does not signify stability. The system has to be stressed to see how the overclocked components respond. It is also very important to know exactly what temps we can expect from our rigs when they are stressed under a full load. Many of us realize that temperature caused from load on a CPU does not slowly creep up. CPUs generally go from their idle to load temps in less than a minute. If we don’t know where the upper limit is, then disaster can occur quite rapidly.

Some die-hard perfectionists insist you should run Prime95 (Blend) for one hour after tweaking your CPU or FSB to test stability. I would recommend starting Prime95 use once you reach the 10% overclock limit. In other words, once you’ve gotten to a point where your MHz is 10% over stock, then start running Prime95 after each tweak. I would suggest 30 minutes to see if there are any “immediate” problems. Watch your temps closely during this time. If Prime95 bombs within 30 minutes, you most likely have a stability problem and need to back your settings off one notch. Then run Prime95 again after you tweak.

Once you think you have a stable overclock and are ready to do some testing, set up your Prime95 test and start it up. I recommend the Blend setting. Let it run for at least 24 hours (yes, 24 hours) before you declare victory. If Prime95 is still running after that, you most likely have a stable system.

Conclusion

No doubt you will experience different errors while testing. Here are some FAQs that will help you along.

http://www.mersenne.org/faq.htm
http://mersenne.org/ips/faq.html

Mr. Mumbles

i think about 10 hours would be sufficient to see if you are stable

Kipper

1. Not all prime numbers are odd, 2 is an even number (i hope im right on this one)
2. i think 6 hours is more than enough to show stablity

blair

man awesome, now i understand this prime95 thing whole lot better now :) i've tested for 2 hours once and got no errors... i must pull a 24-hour test out and do that :) when i go away for a day i'll let it run :) thanks hea[s for that Answer as well really good breakdown

t4ct1c47

Personally I run the small FTT test first for 6 hours, to totally pillage my CPU and then the blend test for another 6 hours so the whole system gets some abuse.

Different people have different preferences though and this FAQ is still pretty handy.

BRISKbaby

Just so you know... I had an error at 16 hours before. Pretty frustrating. Good FAQ, REP+.

mattlb0619

very nice info.. !

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