A plurality of subsystems (21, 22, 23) have one or more internal states. A plurality of local optimizers (11, 12, 13) control the corresponding subsystems based on the internal states thereof and data exchange due to a coupling between the subsystems, respectively. A mediator (10) monitors outputs (y 1 , y 2 , y 3 ) and/or some internal states of the plurality of subsystems (21, 22, 23) and controls operations of the plurality of local optimizers (11, 12, 13) based on the internal states and/or outputs thereof. Each of a first subsystem and a second subsystem is one of the plurality of subsystems (21, 22, 23). A first internal state that is the close to the second subsystem in the first subsystem is affected by a second internal state that is close to the first subsystem in the second subsystem, or the first and second subsystems share a common resource.