Memory leakage in python?

I have python scripts that calls a function foo that performs some operations that uses say 10GB of memory and returns the small object res to the main program. After printing res I delete the res, but top shows the ipython process is sill using about 1GB of memory. How can I ensure the process frees up all unused memory?

def foo():   .... # some complication operation   return resif __name__ == "__main__":    res = foo()    print(res)     del res

I tried using gc.collect() but the it does not change anything.I have read that sometimes the memory becomes fragmented and python can not reclaim all the memory.