For the second time this week a P-1 bug has been found and fixed. Version
22.12 can be downloaded from http://www.mersenne.org/freesoft.htm
The bug affects P-1 stage 2 in "low memory" situations. The bug has
been present since version 20.
From the whatsnew.txt file:
A bug was fixed that caused some factors to be missed in stage 2 of P-1
when the available memory did not let the program allocate 12 temporary
variables. If testing a number in the 16 millions using an FFT size of
768K, then each temporary takes 768K * 8 bytes or 6MB. If your memory
setting was less than 72MB (6MB * 12 temporaries) then you were affected
by the bug. Actual the program allocates some fixed tables so anything
less than about 75MB triggered the bug.
More details:
If you are double-checking in the 8 millions, the bug would appear if you
have less than 37MB or so allocated to prime95.
This bug did not cause all factors to be missed, in fact it could cause some
factors to be found that ordinarily would not be found. Specifically, in
this low-memory situation prime95 steps through the range between B1 and B2 in
multiples of 210. When processing a prime p, the code actually included the
prime "mirrored" around the previous multiple of 210. For example, if
210000+1 is prime, then 210000-1 was improperly included. If 210000+3
is prime, then 210000-3 was included.
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