code: def get_memory_usage(self, variables = None):
          
          result = dict(
              total_nb_bytes = 0,
              by_variable = {}
              )
          for entity in self.populations.values():
              entity_memory_usage = entity.get_memory_usage(variables = variables)
              result['total_nb_bytes'] += entity_memory_usage['total_nb_bytes']
              result['by_variable'].update(entity_memory_usage['by_variable'])
          return result
docs: Get data about the virtual memory usage of the simulation