TITLE: How many cpus needed to check a 100 million digit prime number efficiently?
QUESTION [6 upvotes]: If I had access to potentially large number of CPUs and wanted to quickly check 100 million digit numbers for primality using a map-reduce architecture, how many CPUs would be necessary? Each of the mapped compute instances would perform efficient checks against the number in question with an assigned range of divisors (e.g. Instance 1: checks divisors 2-1000, Instance 2: checks divisors 1001-2000, ... etc.).
Definitions: 

quickly means checking a single divisor against the 100 million digit number within hours.
efficient division means only checking the odd numbers up to the square root. Lower divisors would be only the known prime numbers to speed up computation.
1 CPU is the equivalent CPU capacity of a 1.0-1.2 GHz 2007 Opteron or 2007 Xeon processor.

Yes, I know there are better algorithms like AKS but I need to be able to divide the work among the mapped instances. If there is a better way to divide and conquer I am all ears.
The better question to ask would probably be: what is the mathematical relationship between the number of CPUs and the amount of time it takes to verify a number of a given magnitude of digits?
I'm asking this because I am trying to figure out the number of Map Reduce instances I would need to buy on Amazon AWS to make the computation feasible (a couple months/less than a year).

REPLY [2 votes]: I don't mean to thread on anyone's toes so please forgive me if I do.
I'm not a Mathematician but I am running four instances of Cudalucas on  GTX 690 Gpu's for a few days now and its Eta is saying 190 days to test for four separate 100 million digit mersenne primes. ok its a long time to wait ( and probably another 190 days to double check it) to be told no its not a prime, but a lot shorter that the calculations above.
Regards
Rob.