Would it be possible to choose your own sieve range. Right now you can only get ranges of 2e7, i think you could increase speed quite a bit by choosing a larger range like 1e8, which is 5x as large, but it will only take 3x or 4x as much time to sieve.
The sieve algorithm takes time roughly proportional to the square root of the number of k sieved at once. Sieving the 67 Riesel k together is more than 8 times more efficient than sieving each one seperately.
The time is also roughly proportional to the square root of the range of n sieved, so sieving 0M-20M is about 4.5 times more efficient than sieving 4M-5M.
Above is about srsieve, not gcwsieve, but i think the algorithms are roughly the same.
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