20141023, 21:24  #1 
Oct 2014
1 Posts 
CPU time for 100M digit prime test
Hi all,
Apologies if I just couldn't find this in some FAQ  but I can't find it. How long would it take to test a 100M digit mersenne prime on a modern computer? How many exponents could I test at the same time on an intel i7? Are there any estimates for the likelyhood of a 100M digit mersenne number being prime? I'm thinking of joining the 100M digit prime hunt but am surprised by the lack of information. But then again maybe I'm just looking in the wrong places :) 
20141023, 21:33  #2 
"Kieren"
Jul 2011
In My Own Galaxy!
2^{2}·2,539 Posts 
Uncwilly, or someone he knows of, is most likely to have your answers

20141023, 23:08  #4  
6809 > 6502
"""""""""""""""""""
Aug 2003
101×103 Posts
9382_{10} Posts 
Quote:
The likelihood is very, very low. Some would suggest that you do a 'shakedown' run of 2 or 3 double checks (per core) first. That way you will know that the machine is stable. Running all of the cores on a single number might be best for exponents that high at the moment. The longer (clock/wall time) that the test takes, the greater the likelihood of random errors. Running all cores on a single exponent will result in few exponents per year, but I think that might be off set by the fewer errors. Make sure you are using the 64 bit version of Prime95, if your OS is 64 bit too. 

20141023, 23:16  #5  
May 2013
East. Always East.
11×157 Posts 
Quote:
For example, look at a CPU benchmark page for the i74770K. M25,525,000 takes 6 milliseconds per iteration and M50,275,000 takes 12 milliseconds per iteration, and there's twice as many iterations, so you can see the "quadrupling". 100M digits happens around M333,000,000. This exponent is roughly eight times the size of M41,280,000 which means the iterations are roughly eight times longer (so 80 ms). 333,000,000 iterations at 80 ms/iteration gives 309 days. You can run as many tests as you have physical cores on your CPU. The i74770K has four physical cores (also four logical cores, but they don't help) so you can choose between 4 tests or 1 test using all four cores, which is faster but makes less efficient use of the CPU. 

20141023, 23:37  #6 
"Kieren"
Jul 2011
In My Own Galaxy!
10011110101100_{2} Posts 
There is a commonly expressed opinion that such lengthy assignments should be run with ECC RAM.

20141023, 23:40  #7 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
5·1,217 Posts 

20141024, 05:47  #8 
Aug 2010
Republic of Belarus
10110010_{2} Posts 
See these thread http://www.mersenneforum.org/showthread.php?t=13185

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