Also, your pre-allocation of matr_C isn’t effective or needed, since that allocation is not used. Unlike in Fortran or Matlab, when you write matr_C = ..., this does not copy the right-hand side into matr_C. Instead, the label matr_C in effect is “re-pointed” to whatever is on the right-hand side, in this case a KroneckerProduct type as defined in the Kronecker package. So the memory you allocated for matr_C is discarded and eventually cleaned up by Julia’s garbage collector.
This is an important point to understand about Julia, and it is often discussed in this forum, e.g. here.