[Rcpp-devel] Rcpp function results are different on different systems

[Sokol Serguei]

Le 19/08/2021 à 17:04, Naeem Khoshnevis a écrit :
>
> Thank you so much, everyone, for responding to this email.
>
>
> Dirk,
>
>   * I didn't think about testing _equality_ of doubles because the
>     numbers are significantly different (e.g., instead of 0.5, chooses
>     1.5). However, that is a valid point, and I should be aware of that.
>   * You are right about the serial runs. Whenever I deactivate OpenMP,
>     the results are correct.
>
>
> Serguei,
>
>   * Thanks for the comments. Yes, I agree. It seems outer is a better
>     option. We have started with outer. However, outer builds the
>     entire matrix of differences first, then finds the minimum index.
>     In our application, it requires 200 GB of memory to build that matrix.
>
This problem was hardly foreseeable from examples in hand. But I still 
think that a pure R solution can be a runner:

    ac=a*sc
    bc=b*sc
    out=vapply(seq_along(bc), function(i) 
which.min(abs(ac-bc[i])+cd[i]), integer(1))

Best,
Serguei.

>   * Rcpp does the job with around 10 MB. That is why I switched to
>     Rcpp. Please let me know your thoughts.
>
>
>       Iñaki,
>
>   * Thanks for your suggestion. Yes, the problem is shared values, and
>     it resolved the issue. I really appreciate it.
>
>
> Best regards,
> Naeem
>
> On Thu, Aug 19, 2021 at 4:56 AM Iñaki Ucar <iucar at fedoraproject.org 
> <mailto:iucar at fedoraproject.org>> wrote:
>
>     On Thu, 19 Aug 2021 at 04:53, Dirk Eddelbuettel <edd at debian.org
>     <mailto:edd at debian.org>> wrote:
>     >
>     >
>     > Naeem,
>     >
>     > I would simplify, simplify, simplify -- as 'Rcpp FAQ 7.31'
>     reminds us all,
>     > testing _equality_ of doubles is challenging anyway.
>     >
>     > Besides, it may make sense to would ascertain first you get what
>     you want in
>     > _purely serial modes_ and then move to OpenMP.
>
>     Exactly. Serial execution should be fine. I.e., if you set the number
>     of threads to 1, then all platforms will return the same result.
>     However, you have defined a number of variables outside the parallel
>     region, and then you modify them inside the parallel region. OpenMP
>     takes those variables as shared by default, which leads to the
>     unexpected results you are getting. You need to tell OpenMP that those
>     variables are threadprivate. Or you could just define them inside the
>     parallel region, so that OpenMP knows that they are private without
>     additional hints.
>
>     -- 
>     Iñaki Úcar
>
>
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