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Old 2019-11-04, 22:52   #1
PhilF's Avatar
Feb 2005

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Default GMP-ECM running on processors it wasn't tuned for

I have a question I thought I knew the answer to, but now I am not so sure. Perhaps someone with more knowledge about the code can chime in here.

I have 3 recent linux builds of GMP-ECM that were built on different systems with different CPUs. All are Xeons, but one is a Haswell, one is a Broadwell, and one is a Skylake (not AVX-512 though).

I find that the Skylake build runs faster on Haswells than the Haswell build does. So, no big deal I thought, go ahead and run the Skylake build on the Haswells, since it is only a performance issue.

But twice now I have had the Skylake build fail right at the beginning of execution with an "illegal instruction" fatal error, and both times it was running on a Haswell. But most of the time the code runs fine on Haswells (or at least appears to).

So my question is, does running builds that are tuned for a different CPU affect anything besides performance? I would hate to think that running a "mis-tuned" program could actually affect its ability to successfully find factors.

Last fiddled with by PhilF on 2019-11-04 at 22:55
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Old 2019-11-05, 00:36   #2
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Feb 2005

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I may have answered my own question.

I just started GMP-ECM on a Broadwell processor, and even though I used the Broadwell build, I start got an illegal instruction error.

I would still like some reassurance, though, that if I use a Skylake build on a Haswell that I can trust the results.
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Old 2019-11-05, 08:26   #3
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Dec 2003

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It would make sense getting an error if the Skylake one was built with AVX512. But I do not think GMP-ECM uses AVX512, I'm not sure if GMP does...

All the 3 processors supports AVX2/FMA3.
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