Prime95 version 27.7 / 27.9

[antimaths]

what are the limits for p in 2^p - 1 = Mersenne prime? What is the highest number that I can test?

[James Heinrich]

Originally Posted by antimaths

what are the limits for p in 2^p - 1 = Mersenne prime?

From the source, /gwnum/gwnum.h:

/* Some mis-named #defines that describe the maximum Mersenne number */
/* exponent that the gwnum routines can process. */

#define MAX_PRIME         79300000L    /* Maximum number of x87 bits */
#define MAX_PRIME_SSE2   595800000L    /* SSE2 bit limit */
#define MAX_PRIME_AVX    595800000L    /* AVX bit limit */
#define MAX_FFTLEN         4194304L    /* 4M FFT max for x87 */
#define MAX_FFTLEN_SSE2   33554432L    /* 32M FFT max for SSE2 */
#define MAX_FFTLEN_AVX    33554432L    /* 32M FFT max for AVX */

Originally Posted by antimaths

What is the highest number that I can test?

Assuming you have a CPU capable of SSE2/AVX, M595,799,999, but since that's already got 2 known factors M595,799,947 would make more sense. Of course, it takes 16THz-days to do a single L-L test on that exponent (anywhere from 2-5+ years on a modern CPU).

[Jeff Gilchrist]

Hi everyone. I'm seeing some strange behaviour, at least v26 used to work fine with this. If I have a worktodo.ini file that looks like this:

Pminus1=1,2,8700011,1,150000,"3"
Pminus1=1,2,8700031,1,150000,"3"

v27.7 using an AVX core will process the exponent with P-1, it ignores the "3" at the end telling Prime95 it has a factor of 3 and reports:

P-1 found a factor in stage #2, B1=150000, B2=15000000, E=12.
UID: ANONYMOUS, 2^8700011+1 has a factor: 3

I'm pretty sure this used to work in v26. The other thing that happens is that the worktodo.ini file gets re-written and I lose the "3" on everything:

Pminus1=1,2,8700031,1,150000,0

What am I doing wrong. Did things change in v27.7, how do I complete what I used to be able to do? The readme.txt file still shows that a comma separated list of known factors is between quotes.

[James Heinrich]

Originally Posted by Jeff Gilchrist

What am I doing wrong. Did things change in v27.7, how do I complete what I used to be able to do? The readme.txt file still shows that a comma separated list of known factors is between quotes.

The readme.txt version appears to be wrong:

Pminus1=k,b,n,c[,"comma-separated-list-of-known-factors"]

The correct version appears in whatsnew.txt under v26.5:

Pminus1=k,b,n,c,B1,B2[,how_far_factored][,B2_start][,"factors"]

[Dubslow]

I was looking through the AVX yjmp tables for the FFT lengths you use, and I didn't realize until just now that there are two yjmp tables, each with slightly different information. The first is "yjmptable DD 0", and the second is "yjmptablep DD 0"; is the second for P-1 or something?

The major difference I noticed is that some lengths appear to be missing from the second; the first has 2000K, 2016K, 2016K, lots of 2M, 2240K, 2240K, lots of 2304K, while the second skips 2000K, 2016K, and 2240K completely, and has a lot fewer entries under 2304K. I didn't check the other lengths, though I'd guess there are similar differences throughout the tables. Why?

Edit: It seems from a perfunctory glance the the lengths missing from the second table are those which are not 7-smooth. What would that change?

[Prime95]

One table for k*b^n-c calculations, the other for k*b^n+c calculations.

[Jwb52z]

I don't know whether I should be worried or not, but it ran a few iterations just now to check the roundoff error before starting a new P-1 on my machine and the roundoff error it was usin as a base was 0.24275. It said if it was higher than that, a bigger FFT would be used. All the 1,000 iterations had a roundoff error of over 0.25xxx. Should I be worried alot? The exponent in question was 55288637.

[Dubslow]

No, that's perfectly normal.

That just means that the FFT selection was risky, so that's why Prime95 ran the test; when the test finished, it decided that the small/aggressive length was too risky.

That's the so called "soft crossover" -- between all FFT lengths is a range of a few thousand exponents which try the smaller FFT, but which might need use the larger FFT depending on hardware.

Without such a test, George would have to use a "hard crossover" which is a clear line between two lengths, but that line would have to be pretty low to guarantee it would work for all hardware, which means some hardware might not use the most efficient length.

(PS Thanks George )

[Jwb52z]

Thank you for the fast response. I think that, and I know i've not been doing this long even including all the time put together leaving out the times I temporarily quit, this may be the first time i saw that happen, or at least caught it being on my screen.

[choong]

after two minutes i start prime 95 blend test, my cpu usage still stay at 100% but my multiplyer start to go up and down.... just wanna ask is that normal.

for your info i have a overcloked 2500k ...cpu voltage was set at 1.245v ..

[Dubslow]

Yes, that's normal. Almost all CPUs have a function called Turbo Boost, where if your CPU hasn't been doing anything but then jumps to 100% usage, it will increase the multiplier past the usual point; after a few minutes at 100%, there's too much heat, and Turbo Boost deactivates and the multiplier drops back to the base line.

For a non-overclocked 2500K, the baseline multiplier is 33, so speed = 3.3 GHz. This may or may not be your baseline, depending on how you did the overclock.