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I don’t think the parallelism really helps with this; you’ll still never find the second of two twin primes for instance.

Are there any easy ways to mitigate this though? What about say adding 2^64 at each step instead of 2?

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That's an interesting idea. I don't know enough to say if using a large offset would be enough to mitigate the downsides of +=2, i would have to read more about it. In actual use you can go directly to using random candidates for each test as a few extra seconds to generate 2 primes would be worth the benefits. In hindsight my simplistic rng() can also probably be better optimized (batch load random bits and cache) to make it faster as the slowdown primarily comes from repeated filesystem access.

I guess so long as your increment is constant there are primes you’re never going to find. If a prime happens to be 2^64 from another prime you’ll still never see it with my approach.

Probably the most reasonable thing to do would be to use a Mersenne twister or other PRNG to generate a stream of random bytes. Would be plenty fast and should hopefully have no relevant pattern. No reason you should need real randomness from the OS after the initial seed.

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