1. A computer system comprising: a main storage unit configured to hold data; a first processing executing unit provided in a first processor capable of virtual multiprocessing, and configured to sequentially execute processing to be executed on virtual processors; a first shared cache memory which is provided in the first processor and shared among the virtual processors, and which holds data to be used by the virtual processors; a partitioned cache memory which is provided on a storage level, outside the first processor, farther from the first processing execution unit than the first shared cache memory, and which includes storage areas that are partitioned based on the number of the virtual processors, the storage areas each (i) corresponding to one of the virtual processors and (ii) holding the data to be used by the corresponding one of the virtual processors; a first context memory which holds, for each of the virtual processors, a context item corresponding to the virtual processor; a virtual processor control unit configured to, according to a request for switching of the virtual processors that execute processing in the first processing executing unit, (i) save on the first context memory a context item of one of the virtual processors executing processing in the first processing executing unit and (ii) set, to the first processing executing unit, a context item held in the first context memory and designated to another one of the virtual processors to execute processing next in the first processing executing unit; a first shared cache control unit configured to, in the case where a cache miss occurs in the first shared cache memory, perform control for (i) reading missing data from the partitioned cache memory and (ii) writing the read missing data to the first shared cache memory; and a partitioned cache control unit configured to, in the case where a cache miss occurs in the partitioned cache memory, perform control for (i) reading missing data from the main storage unit and (ii) writing the read missing data to one of the storage areas which corresponds to one of the virtual processors executing processing in the first processing executing unit.