An elevator system comprising: a plurality of elevator sub-systems (110; 150), each elevator sub-system comprising at least one elevator car (115, 120; 155, 160) arranged to travel in a loop path comprising a first vertical section and a second vertical section connected to each other with two horizontal sections, wherein the at least one elevator car (115, 120; 155, 160) is configured to travel upwards in the first vertical section of the loop path and downwards in the second vertical section of the loop path, a group controller configured to control the plurality of the elevator sub-systems (110; 150), characterised in that the group controller is configured to: determine an efficiency of the elevator system by: determining a value for each zone, the value representing an efficiency of the elevator system with respect to the zone, comparing each of the determined values to a corresponding reference value, and setting a detection result to represent the efficiency of the elevator system with respect to each zone, and control, in accordance with the efficiency of the elevator system, at least one elevator sub-system (110; 150) to serve elevator calls at different zones from at least one other elevator sub-system (110; 150). a plurality of elevator sub-systems (110; 150), each elevator sub-system comprising at least one elevator car (115, 120; 155, 160) arranged to travel in a loop path comprising a first vertical section and a second vertical section connected to each other with two horizontal sections, wherein the at least one elevator car (115, 120; 155, 160) is configured to travel upwards in the first vertical section of the loop path and downwards in the second vertical section of the loop path, a group controller configured to control the plurality of the elevator sub-systems (110; 150), characterised in that the group controller is configured to: determine an efficiency of the elevator system by: determining a value for each zone, the value representing an efficiency of the elevator system with respect to the zone, comparing each of the determined values to a corresponding reference value, and setting a detection result to represent the efficiency of the elevator system with respect to each zone, and control, in accordance with the efficiency of the elevator system, at least one elevator sub-system (110; 150) to serve elevator calls at different zones from at least one other elevator sub-system (110; 150). determine an efficiency of the elevator system by: determining a value for each zone, the value representing an efficiency of the elevator system with respect to the zone, comparing each of the determined values to a corresponding reference value, and setting a detection result to represent the efficiency of the elevator system with respect to each zone, and determining a value for each zone, the value representing an efficiency of the elevator system with respect to the zone, comparing each of the determined values to a corresponding reference value, and setting a detection result to represent the efficiency of the elevator system with respect to each zone, and control, in accordance with the efficiency of the elevator system, at least one elevator sub-system (110; 150) to serve elevator calls at different zones from at least one other elevator sub-system (110; 150).