1. A method of controlling a system having a processor and an I/O controller for transmitting or receiving a request, comprising: storing execution information indicating either of the processor and the I/O controller is to supervise initiations of executions of successive requests in accordance with combinations of the types of the successive requests; receiving a first and a second requests successively by the I/O controller; determining whether initiation of each execution of the first and the second requests is to be supervised by either of the processor and the I/O controller in reference to the execution information by the I/O controller; transmitting the first request to the processor from the I/O controller, and upon completion of execution of the first request at the processor, transmitting the second request to the processor from the I/O controller when the initiations of executions of the first and second request is supervised by the I/O controller; and transmitting the first and second requests to the processor regardless of completion of execution of the first request by the processor when the initiations of executions of the first and second requests is supervised by the processor.