Each processor of a multiprocessor system which shares a main storage has a execution circuit for executing a compare and watch instruction provided for watching information for synchronization written into a main storage. When one program being executed by one of the processors issues an instruction, the circuit fetches information for synchronization from a location within the main storage designated by the instruction, compares that fetched information with another information designated by the instruction. If they do not have a specific relation, the fetching and the comparison is repeated. The circuit has a circuit for limiting the repetition with a limited number of times. The circuit further has a circuit for counting a total amount of CPU time spent for execution of plural watching instructions issued by the same program.