I've decided to take the doublecheck to x=30000. This will cover y<x for all of t those x. This could be about 6 months of work, but I don't know for certain yet because I need to do a lot of sieving.
To help me with that, I have made changes to xyyxsieve (code is committed, but exe is not on sourceforge yet) to reduce the memory usage of the program. In the previous version, a range of 1000 x can take 8 GB of memory (ouch). The changes have reduced that memory requirement by a factor of 10. Another good result of that change is a boost in speed by about 30%.
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