20150305, 17:24  #1 
"Victor de Hollander"
Aug 2011
the Netherlands
2^{3}×3×7^{2} Posts 
1M9M GPU TF vs. CPU P1/ECM (KWh/factor)
There are some people (including myself) that are doing TF, P1 and/or ECM in the 1M9M region. Currently the 2M range is TFed to 65bits. The goal is to find factors for exponents without known factors.
The lower the exponent the more effort (GHzd) it takes to TF to the same bitlevel. The lower exponents are 'cheaper' to do P1/ECM on, due to the the smaller FFT sizes. Which poses the question: at what point makes using P1/ECM using a CPU makes more sense than TF on a GPU. I know comparing CPUs and GPUs is a bit like comparing apples to oranges, but the idea is to spend electricity wisely (read: lowest KWh/factor). Of course there are other uses of GPU resources that are more helpful to GIMPS (DCTF, LLTF), but let's take them out of the equation for the moment. Power and GHzdd approximation CPU: Intel i5 2500k 3033 GHzdd of P1/ECM and uses 135W. GPU: AMD 280X that can TF ~600 GHzdd (<69bits) in these low ranges and uses 250W. Power/GHzd: (nice numbers for easier calc) CPU 135W * 24h / 32.4GHzdd = 100Wh/GHzd GPU: 250W * 24h / 600GHzdd = 10Wh/GHzd Assuming TF results in 1 factor in 200 runs (due to some P1/ECM already done). Rng  65>66bit  effort/factor  Wh/factor 2M  3.74 GHzd  748 GHzd  7,480 4M  1.87 GHzd  347 GHzd  3,470 6M  1.25 GHzd  250 GHzd  2,500 8M  0.93 GHzd  186 GHzd  1,860 I've been doing some P1 (B1=10e6 B2=200e6) in the 1.51.7M range and so far found 83 factor in ~1300 attempts, which works out to about 1/16 (remember nice numbers ;) ). Expanding that to the higher ranges: Rng  P1 GHzd  effort/factor  Wh/factor 2M  1.94 GHzd  31.04 GHzd  3,104 4M  3.68 GHzd  58.88 GHzd  5,888 6M  5.23 GHzd  83.68 GHzd  8,368 8M  7.71 GHzd  123.36 GHzd  12,336 With ECM I ran 2300 curves (B1=5e4 B2=5e6) in the 1.51.7M range and found 2 factors. The experts will probably kill me for saying this: but let's assume 1 factor in 1000 curves. Rng  ECM GHzd  effort/factor  Wh/factor 2M  0.0845 GHzd  84.5 GHzd  8,450 4M  0.180 GHzd  180 GHzd  18,000 6M  0.270 GHzd  270 GHzd  27,000 8M  0.397 GHzd  397 GHzd  39,700 GPU TF until Wh/F_{TF GPU} of the next bitlevel > Wh/F_{CPU P1/ECM} ?????? That would imply: 2M no futher GPU TF 4M to 66bits 6M to 67bits 8M to 68bits Is there something fundamentally wrong with my assumptions, or is GPU TF just still quite efficient in the >4M region? Disclaimer: Just to be very clear, this endeavour is purely for FUN! Nothing scientific to be gained here. 
20150305, 18:40  #2  
Aug 2002
Buenos Aires, Argentina
5·269 Posts 
Quote:
Last fiddled with by alpertron on 20150305 at 18:40 

20150305, 22:26  #3  
Sep 2002
Oeiras, Portugal
1,451 Posts 
Quote:
I am curently devoting my (scarce) resources to ECM in that range and TF there and in the 1  2M range. 

20150306, 00:17  #4  
"Victor de Hollander"
Aug 2011
the Netherlands
2^{3}·3·7^{2} Posts 
Quote:
With my current hardware (if I didn't make any big mistakes) would seem to be: Rng  bits 2M 65 3M 65 4M 66 5M 67 6M 67 7M 68 8M 68 9M 69 I'm not obliging people to use these values. Different CPUs, GPUs could get different values. It could also be the case that there is more processing power of one of the two available. Software could become more efficient in the future, etc.... And people are of course free to use their resources as they please. 

20150307, 09:29  #5 
Feb 2010
Sweden
173 Posts 
I am also interested in the 2M range, but sometimes I go 1M, <1M and rarely 3M. It is only for fun, so don't take my opinion seriously. But, lets talk about 2M. I have 10x less powerful card ( ~29GHd/d) and less efficient than yours (in the tables at www.mersenne.ca it says that mine is 1.005 GHd/W and yours is 1.498 GHd/W). Even with these numbers I usually go in 2M to TF68, before I succumb to Pminus1 or ECM2. Here a powerful enough reason is, that it is not fun any more to go TF70 (or not as effective in finding factors). Usually for the same time I will complete ~250 ECM curves at (B1=5e4 B2=5e6) at the CPU. That way I would test the exponent to ~TF75. The reason I do crazy Pminus1 (or it was crazy with my previous CPU, now seems like fine), after ECM is only for full exhaustion of all viable options in reasonable time. Rarely Pminus1 is of any help after ECM, but sometimes there are surprises. So seeing your GPU, I would say go to TF68 in >2M, clearly at least 1 bitlevel up (TF66). For 1M, I guess TF66 would be fine. Than go ECM, it is still more efficient in finding factors up to 3.5M range. For the rest of you, who would say that I am an idiot, please read my reason again  JFF.

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