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Old 2005-03-19, 20:49   #1
James Heinrich
 
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"James Heinrich"
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Default memory usage in P-1 stage 1

We all know the memory settings specified in Prime95 apply to P-1, stage2. However, it's not clear from the readme (or anywhere else) that memory usage may exceed those values, and exceed the default/oft-quoted "8MB" value. For example, running P-1 stage 1 of a top-range exponent (M79299821, in my case) uses about 90MB. LL testing seems to use about 45MB for the same exponent. I'm not sure how much memory trial factoring takes up.

I realize this is because of the abnormally-large exponents, but maybe the readme should mention this?
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Old 2005-03-19, 21:23   #2
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trial facting takes 1.4 megs of mem when prime95 is in taskbar mode.
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Old 2005-03-22, 05:24   #3
ewmayer
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P-1 stage should need no more memory than an LL test.
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Old 2005-03-22, 18:26   #4
James Heinrich
 
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Quote:
Originally Posted by ewmayer
P-1 stage should need no more memory than an LL test.
"should" and "does" don't seem to coincide in this case... P-1,stage1 takes twice the memory that the L-L of the exponent does.
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Old 2005-03-22, 19:06   #5
ewmayer
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Quote:
Originally Posted by James Heinrich
"should" and "does" don't seem to coincide in this case... P-1,stage1 takes twice the memory that the L-L of the exponent does.
Hm, that almost sounds like stage 1 is using a right-to-left binary powering, which needs 2x the memory of a left-to-right, and roughly 50% more runtime. IMO the only reasons one would ever use right-to-left:

1) The product of all the stage 1 prime powers (needed by the LR powering) is too large to be easily computed/stored - but that needs a very large stage 1 bound;

2) If stage 2 fails to find a factor, one wants to go back and, starting with the original stage 1 residue, do a deeper stage 1 continuation.

George?
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Old 2005-03-22, 20:05   #6
Prime95
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Maybe we are confusing two terms. Prime95 is concerned with the working set, but perhaps James is reporting the total allocated memory.

P-1 allocates more memory than an LL test because it computes the left-to-right powering exponent. However, only one cache line of this memory will be in prime95's working set during stage 1 - so the two should have the same working set.

Otherwise, I'm at a loss to explain the difference.
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