How do I force my std::map to deallocate memory used?

I'm using a std::map, and I can't seem to free the memory back to the OS. It looks like,

int main(){
  aMap m;

  while(keepGoing){
    while(fillUpMap){
       //populate m
    }
    doWhatIwantWithMap(m);
    m.clear();//doesnt free memory back to OS

    //flush some buffered values into map for next iteration
    flushIntoMap(m);
  }
}

Each (fillUpmap) allocates around 1gig, so I'm very much interested in getting this back to my system before it eats up all my memory.

Ive experienced the same with std::vector, but there I could force it to free by doing a swap with an empty std::vector. This doesn't work with map.

When I use valgrind it says that all memory is freed, so its not a problem with a leak, since everything is cleared up nicely after a run.

edit:

The flush has to appear after the clear.