R help - Apr 2008 - Computing time when calling C functions - why does an extra function call induce such an overhead?

Dear list,
 
I am a little puzzled by computing time in connection with calling C functions.
With the function mysolve1 given below I solve Ax=B, where the actual matrix
operation takes place in mysolve2. Doing this 5000 times takes 3.51 secs.
However, if I move the actual matrix inversion part into mysolve1 (by
uncommenting the two commented lines and skip the call to mysolve2) then the
computations take only 0.03 secs. Can anyone tell me why there is such a
"time-overhead" in introducing the extra function call to mysolve2?
 
I am on windows XP using R 2.6.1
 
Best regards
S?ren
 

SEXP mysolve1(SEXP Ain, SEXP Bin, SEXP tolin)
{
  int *Adims, *Bdims;
  double tol = asReal(tolin);
  SEXP A, B;
  PROTECT(A = duplicate(Ain)); 
  PROTECT(B = duplicate(Bin)); 
  Adims = INTEGER(coerceVector(getAttrib(A, R_DimSymbol), INTSXP));
  Bdims = INTEGER(coerceVector(getAttrib(B, R_DimSymbol), INTSXP));
  int nrA, ncB;
  double *Ap, *Bp;
  nrA     = Adims[0];
  ncB     = Bdims[1];
  Ap      = REAL(A);
  Bp      = REAL(B);
/*   int info, *ipiv  = (int *) R_alloc(nrA, sizeof(int)); */
/*   F77_CALL(dgesv)(&nrA, &ncB, Ap, &nrA, ipiv, Bp, &nrA,
&info); */
  mysolve2(Ap, &nrA, Bp, &ncB, &tol); 
  UNPROTECT(2); 
  return B;
}
 
void mysolve2(double *A, int *nrA, double *B, int *ncB, double *tolin)
{
  int info;
  int *ipiv  = (int *) R_alloc((int) nrA, sizeof(int));    
  F77_CALL(dgesv)(nrA, ncB, A, nrA, ipiv, B, nrA, &info);
}

I've not seen any reposnse to this, probably because

-- this is not the list for C programming questions  (use R-devel)
-- there is no example provided for the times quoted.
-- the code is not compilable.

But I got a compiler warning on

   int *ipiv  = (int *) R_alloc((int) nrA, sizeof(int));

foo.c:8: warning: cast from pointer to integer of different size

(it should be *nrA) and that's most likely your error.  (BTW, using 
liberal casts like this is just stopping your tools helping you!)

The overhead of C function call is measured in nanosecs on modern systems, 
especially to functions in the same compilation unit.

On Thu, 10 Apr 2008, S?ren H?jsgaard wrote:
> Dear list,
>
> I am a little puzzled by computing time in connection with calling C 
> functions. With the function mysolve1 given below I solve Ax=B, where 
> the actual matrix operation takes place in mysolve2. Doing this 5000 
> times takes 3.51 secs. However, if I move the actual matrix inversion 
> part into mysolve1 (by uncommenting the two commented lines and skip the 
> call to mysolve2) then the computations take only 0.03 secs. Can anyone 
> tell me why there is such a "time-overhead" in introducing the
extra
> function call to mysolve2?
>
> I am on windows XP using R 2.6.1
>
> Best regards
> S?ren
>
>
> SEXP mysolve1(SEXP Ain, SEXP Bin, SEXP tolin)
> {
>  int *Adims, *Bdims;
>  double tol = asReal(tolin);
>  SEXP A, B;
>  PROTECT(A = duplicate(Ain));
>  PROTECT(B = duplicate(Bin));
>  Adims = INTEGER(coerceVector(getAttrib(A, R_DimSymbol), INTSXP));
>  Bdims = INTEGER(coerceVector(getAttrib(B, R_DimSymbol), INTSXP));
>  int nrA, ncB;
>  double *Ap, *Bp;
>  nrA     = Adims[0];
>  ncB     = Bdims[1];
>  Ap      = REAL(A);
>  Bp      = REAL(B);
> /*   int info, *ipiv  = (int *) R_alloc(nrA, sizeof(int)); */
> /*   F77_CALL(dgesv)(&nrA, &ncB, Ap, &nrA, ipiv, Bp, &nrA,
&info); */
>  mysolve2(Ap, &nrA, Bp, &ncB, &tol);
>  UNPROTECT(2);
>  return B;
> }
>
> void mysolve2(double *A, int *nrA, double *B, int *ncB, double *tolin)
> {
>  int info;
>  int *ipiv  = (int *) R_alloc((int) nrA, sizeof(int));
>  F77_CALL(dgesv)(nrA, ncB, A, nrA, ipiv, B, nrA, &info);
> }
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595