[Rcpp-devel] Stack imbalance warning when using Rcpp and OpenMP

Michael,

On 26 July 2011 at 17:08, Michael Braun wrote:
| I have an R list that I want to pass to C++ code through Rcpp, and then process each of the list elements with another function, in parallel through OpenMP.  Here is a simplified example in which I compute the square root of a number in each list element:
| 
| // file omp.cpp
| 
| #include <Rcpp.h>
| #include <omp.h>
| 
| double func(SEXP Y_) {
|   using namespace Rcpp;
|   double Y = as<double>(Y_);
|   double res = sqrt(Y);
|   return(res);
| }
| 
| RcppExport SEXP omp3 (SEXP R_) {
| 
|   BEGIN_RCPP
| 
|  using namespace Rcpp;
| 
|  List R = R_;
|  int n = R.size();
|  int i;
| 
|  NumericVector X(n);
|  
| #pragma omp parallel shared(R, n, X) private(i)
|  {
| #pragma omp for
|  for (i=0; i<n; i++) {
|    X(i) = func(R(i));
|  }
|  }
|  
|  return(wrap(X));
| 
|  END_RCPP
| 
| }
| 
| 
| The R code to call the omp3 C++ function is:
| 
| dyn.load("omp.so")
| 
| R <- vector("list",5)
| for(i in 1:5) R[[i]] = i  
| 
| res <- .Call("omp3",R)
| 
| dyn.unload("omp.so")
| 
| 
| When I run this program, I almost always get stack imbalance warnings like:
| 
| Warning: stack imbalance in '.Call', 33 then 32
| Warning: stack imbalance in '<-', 31 then 30
| Warning: stack imbalance in 'eval.with.vis', 27 then 26
| Warning: stack imbalance in '.Internal', 26 then 25
| Warning: stack imbalance in '<-', 20 then 19
| Warning: stack imbalance in '{', 18 then 17
| Warning: stack imbalance in 'if', 16 then 15
| Warning: stack imbalance in '{', 14 then 13
| Warning: stack imbalance in 'for', 8 then 7
| 
| I say almost because it does not happen all the time, but if I run the same script a few more times, I will invariably get a stack imbalance warning. (Also, the numbers are not always the same).
| 
| But there's more.  If I run the program a bunch of times, eventually I get:
| 
| Lost warning messages
| > Error in eval.with.vis(expr, envir, enclos) : 
|   REAL() can only be applied to a 'numeric', not a 'NULL'
| Error during wrapup: C stack usage is too close to the limit
| 
| 
| Once I get this error, I will no longer get stack imbalance warnings, but I cannot predict how many attempts it would take before the C stack usage error comes up.
| 
| And if I am really lucky, I'll get memory not mapped segfaults, but not every time.
| 
| I have run this on my Mac Pro with both g++ 4.6 and Intel C++ compiler 12.0.4.  I have also run it on a Red Hat Linux machine with g++ 4.4 (the segfaults happen much more frequently under Linux than under Mac OSX).  The stack imbalance warnings come up on both machines, with all compilers.
| 
| I also wonder if this is related to another problem with Rcpp and OpenMP that I posted back in late March.  In that case, the problem was using an old compiler; updating the compiler solved the compilation problem.   The difference between that case and this example is calling the second function with a SEXP as an argument. This example compiles, but does not run.
| 
| I know that Rcpp is rigorously tested, so the problem is probably something that I am doing wrong.  In particular, I wonder if each element of the list should be passed as a SEXP, or if there is a better way to do it.  In any case, I greatly appreciate your assistance.

I would have to agree with your conjecture here. You are simply not being
careful enough about OpenMP usage.  And R tells you that there are still
dynamically created objects floating around.

I would recommend stronger synchronisation before and particularly after your
OpenMP block.  For a working example of OpenMP with Rcpp, see the directory
examples/OpenMP/ which was added in the 0.9.5 release.

| (Also, I deeply apologize for not using the inline package with the example.  I could not figure out how to include the two different functions.)

See help(cxxfunction) and particularly the "includes=" argument. That said,
for OpenMP you need to pass -fopenmp and we currently do not have an option
for that in cxxfunction.

Hope this helps,  Dirk


| Thanks,
| 
| Michael
| 
| 
| 
| 
|