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Old 2011-09-03, 12:11   #7
R. Gerbicz
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"Robert Gerbicz"
Oct 2005

112×13 Posts

Originally Posted by fivemack View Post
There is a tiny problem in the downloaded code when I use some compilers: the prototype for func() doesn't match the definition of func() because you've changed the first two parameters from mpz_t to lli. Trivial to fix.

Is it asymptotically better to run this on several cores using a small interval on each, or on one core using the largest interval that fits in memory? I have quite a large machine and can allocate 32GB to the process if that would help.

I am a little surprised that when I do

echo -e "100000000000\n100010000000\n10000\n1\n" | time ./a.out

I get

Testing p=100000000073...100001026973, p==5 mod 12, time=1 sec.

and then no further output for at least a thousand seconds; is there something in the implementation to make 5 mod 12 unusually quick? I tried 10^9 .. 10^9+10^8 and it worked fine, so I don't think it's hanging.
Thanks I will correct, in one of the first version of my code func used mpz_t type for n1,n2.

In fact when the code prints "Testing ..." it is still testing those primes, so they are not checked, but found all those type of primes from that interval and computed the product of these primes. The ratio of speed for different types are: 3:2:1 for large primes, so p==1 mod 3 is the fastest and p==11 mod 12 is the slowest.

It is better to run the largest interval on one core that fits in memory. That was the reason I've used only one core for my search. (running t cores and on each of them testing interval/t primes yield the same speed with 1 core and interval primes.)
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