Computer system including storage unit and processing unit including performance monitoring unit (PMU) for performance monitoring on tasks is provided. Processing unit is configured to provide a callback function at location at which a task switch is being performed, reset a counter for external access counting of PMU when first task switch which is to switch execution of tasks to first task occurs, read counted value from counter and record read value into a log of storage unit as record of first task when second task switch which is to switch execution of tasks from first task to second task occurs, and reset counter for second task after log is generated. Records of first task in log within a predetermined time period are summed to analyze performance of first task within predetermined time period so as to determine performance of external access of first task for specific event.