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). <EOS>