[Rcpp-devel] When calling same Rcpp function several times different results are returned

[Danas Zuokas]

I have tried this with no luck.

2015-07-14 14:34 GMT+03:00 Romain Francois <romain at r-enthusiasts.com>:

> Or just use a std::vector<double> for sg. That should do it.
>
> Romain
>
> Envoyé de mon iPhone
>
> > Le 14 juil. 2015 à 13:21, JJ Allaire <jj.allaire at gmail.com> a écrit :
> >
> > The examples in the RcppParallel documentation assume that access to
> > vectors and matrixes are *aligned* (i.e. fall into neat buckets
> > whereby reading and writing doesn't overlap between worker instances).
> > Your example appears to access arbitrary elements of sg (depending on
> > what's passed in gi) which probably creates overlapping reads/writes.
> > You should also study the documentation for join carefully.
> >
> > There's nothing incorrect about RcppParallel's behavior here, rather
> > you need to think more carefully about the access patterns of your
> > data and how they might conflict. You may need to introduce locking to
> > overcome the conflicts, which in turn could kill the performance
> > benefit you gain from parallelism. No easy answers here :-\
> >
> >> On Tue, Jul 14, 2015 at 7:15 AM, Danas Zuokas <danas.zuokas at gmail.com>
> wrote:
> >> Yes it is the same question on SO and I did consider RHertel's comments.
> >> But this problem (sums by group id) is not parallelFor it is
> parallelReduce:
> >> I split vector, calculate sums and then aggregate those sums.
> >> Please correct me if I am wrong.
> >>
> >> 2015-07-14 13:54 GMT+03:00 Dirk Eddelbuettel <edd at debian.org>:
> >>>
> >>>
> >>> On 14 July 2015 at 09:25, Danas Zuokas wrote:
> >>> | I have written parallel implementation of sums in groups using
> >>> RcppParallel.
> >>>
> >>> Isn't this the same question as
> >>>
> >>>
> http://stackoverflow.com/questions/31318419/when-calling-same-rcpp-function-several-times-different-results-are-returned
> >>>
> >>> You got some excellent comments there by SO user 'RHertel'. Did you
> >>> consider those?
> >>>
> >>> Dirk
> >>>
> >>> | // [[Rcpp::depends(RcppParallel)]]
> >>> | #include <Rcpp.h>
> >>> | #include <RcppParallel.h>
> >>> | using namespace Rcpp;
> >>> | using namespace RcppParallel;
> >>> |
> >>> | struct SumsG: public Worker
> >>> | {
> >>> |   const RVector<double> v;
> >>> |   const RVector<int> gi;
> >>> |
> >>> |   RVector<double> sg;
> >>> |
> >>> |   SumsG(const NumericVector v, const IntegerVector gi, NumericVector
> >>> sg): v(v), gi(gi), sg(sg) {}
> >>> |   SumsG(const SumsG& p, Split): v(p.v), gi(p.gi), sg(p.sg) {}
> >>> |
> >>> |   void operator()(std::size_t begin, std::size_t end) {
> >>> |     for (std::size_t i = begin; i < end; i++) {
> >>> |       sg[gi[i]] += v[i];
> >>> |     }
> >>> |   }
> >>> |
> >>> |   void join(const SumsG& p) {
> >>> |     for(std::size_t i = 0; i < sg.length(); i++) {
> >>> |       sg[i] += p.sg[i];
> >>> |     }
> >>> |   }
> >>> | };
> >>> |
> >>> | // [[Rcpp::export]]
> >>> | List sumsingroups(NumericVector v, IntegerVector gi, int ni) {
> >>> |   NumericVector sg(ni);
> >>> |   SumsG p(v, gi, sg);
> >>> |   parallelReduce(0, v.length(), p);
> >>> |
> >>> |   return List::create(_["sg"] = p.sg);
> >>> | }
> >>> |
> >>> | It compiles using Rcpp::sourceCpp. Now when I call it from R
> >>> sumsingroups(1:10,
> >>> | rep(0:1, each = 5), 2) several times I get the right answer (15 40)
> and
> >>> then
> >>> | something different (usually some multiplicative of the right
> answer).
> >>> Running
> >>> |
> >>> |
> >>> | res <- sumsingroups(1:10, rep(0:1, each = 5), 2)
> >>> | for(i in 1:1000) {
> >>> |     tmp <- sumsingroups(1:10, rep(0:1, each = 5), 2)
> >>> |     if(res[[1]][1] != tmp[[1]][1]) break
> >>> |     Sys.sleep(0.1)
> >>> | }
> >>> |
> >>> | breaks at random iteration returning
> >>> |
> >>> | $sg
> >>> | [1]  60 160
> >>> |
> >>> | or
> >>> |
> >>> | $sg
> >>> | [1] 30 80
> >>> |
> >>> | I am new to Rcpp and RcppParallel and do not know what could cause
> such
> >>> | behavior.
> >>> |
> >>> | Things that did not help:
> >>> |
> >>> |  1. Added for (std::size_t i = 0; i < sg.length(); i++) sg[i] = 0; to
> >>> both of
> >>> |     constructors.
> >>> |  2. Changed names so that they are different in Worker definition
> and in
> >>> |     function implementation.
> >>> |
> >>> | _______________________________________________
> >>> | Rcpp-devel mailing list
> >>> | Rcpp-devel at lists.r-forge.r-project.org
> >>> |
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
> >>>
> >>> --
> >>> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
> >>
> >>
> >>
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