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Old 2019-05-11, 18:16   #12
kriesel's Avatar
Mar 2017
US midwest

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Default Why don't we save interim residues on the primenet server?

This is often asked in the context of wanting to be able to continue a run abandoned before completion by someone. It's not unusual for someone to quit participating when their assigned exponent(s) are anywhere from 2 to 98% complete in a primality test.

Full length residues saved to the primenet server at some interval, perhaps every 20 million iterations, are sometimes proposed as a means of minimizing the lost throughput from abandoned uncompleted tests. The combined output of GIMPS would represent a considerable load on the server's resources to implement this, and require additional considerable expenditure to support, which is not in the Mersenne Research Inc. budget. For users with slow internet connections, the individual load could also be considerable as a fraction of available bandwidth. Transfer times could stall the application and reduce total throughput.
Detailed analysis and discussion at

However, it is feasible to save smaller interim residues, such as 64-bit or 2048-bit. And this is currently being done. Recent versions of prime95 automatically save 64-bit residues at 500,000 iterations and at every multiple of 5,000,000. The 2048-bit are generated at the end of PRP tests, possibly only type 1 and type 5 PRP tests, per posts 606-609 of
The stored interim 64-bit residues from different runs can be compared to see if runs are matching along the way or when one or another diverges.

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Last fiddled with by kriesel on 2020-02-20 at 20:35
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