A computer controls I/O allocation for partitions independently of CPU allocation and, in each I/O adapter and partition, the computer has a scheduling means controlling allocation for partitions of the I/O adapter by time sharing, a means to allocate the I/O adapter to partitions by space sharing and a means to dynamically change said allocation made by a partition-control program. Further, the computer has a means to monitor input/output performance of each partition, and a means to maintain SLA of a user program according to performance of each partition.