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Old 2019-11-14, 13:01   #478
Just call me Henry
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Sep 2007
Cambridge (GMT)

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Originally Posted by EdH View Post
In playing with my Colab instance of GMP-ECM with GPU enabled (K80 GPU), I have made some observations:

The K80 shows 832 cores. If I run threads up to half of the cores, vs. over half the cores, the time for the run just about doubles. It is pretty close to constant within those halves. For example, running stage 1 for 5+3,1185L, it takes just about 22 seconds to complete <417 curves at 11e4. >416 curves takes about 41 seconds. Of note, I am using multiples of 32 cores.

Running a single curve without the GPU takes about .4 second. Obviously, running more curves on a single CPU core would sequentially add time - 416 curves by CPU would take about 166 seconds.

This appears to indicate that while the GPU sounds impressive with all its cores, a quad core CPU would keep up with the K80 GPU in stage 1 runs.

Am I missing something here?
The gpu doesn't care what size of number you give it while the cpu does. A quad core cpu probably wouldn't keep up so well on a 1000 bit number(I believe your test case had 720 bits).
I seem to recall that it was possible to compile versions of gpu-ecm that ran faster with a limit of ~256 or ~512 bits. I am not sure whether ~768 worked. I think it needed to be a power of 2.
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