Chapter 19 Multimedia Systems

19 CHAPTER Real-time Systems Chapter 19 introduces real-time computing systems. The requirements of realtime systems differ from those of many of the systems we have described in the previous chapters, largely because real-time systems must produce results within certain deadlines. In this chapter we provide an overview of real-time computer systems and describe how real-time operating systems must be constructed to meet the stringent timing requirements of these systems. Exercises 19.1 Identify the following environments as either hard or soft real-time. a. Thermostat in a household. b. Control system for a nuclear power plant. c. Fuel economy system in an automobile. d. Landing system in a jet airliner. Answer: Hard real-time scheduling constraints are required for the nuclear power plant and for the jet airliner. In both settings, a delayed reaction could have disastrous consequences and therefore the scheduler would need to satisfy real-time scheduling requirements. On the other hand, the thermostat and the fuel economy system can be operated within the context of a scheduler that provides only soft real-time constraints. Delayed triggering of these devices would only result in suboptimal use of resources or a slight increase in discomfort. 19.2 Discuss ways in which the priority inversion problem could be addressed in a realtime system. Also discuss whether the solutions could be implemented within the context of a proportional share scheduler. Answer: The priority inversion problem could be addressed by temporarily changing the priorities of the processes involved. Processes 127 128 Chapter 19 Real-time Systems that are accessing resources needed by a higher-priority process inherit the higher priority until they are finished with the resources in question. When they are finished, their priority reverts to its original value. This solution can be easily implemented within a proportional share scheduler; the shares of the high priority processes are simply transferred to the lower priority process for the duration when it is accessing the resources. 19.3 The Linux 2.6 kernel can be built with no virtual memory system. Explain how this feature may appeal to designers of real-time systems. Answer: By disabling the virtual memory system, processes are guaranteed to have portions of its address space resident in physical memory. This results in a system that does not suffer from page faults and therefore does not have to deal with unanticipated costs corresponding to paging the address space. The resulting system is appealing to designers of real-time systems who prefer to avoid variability in performance. 19.4 Under what circumstances is the rate monotonic scheduling inferior to earliest deadline first scheduling in meeting the deadlines associated with processes? Answer: Consider two processes P1 and P2 where p1 = 50, t1 = 25 and p2 = 75, t2 = 30. If P1 were assigned a higher priority than P2 , then the following scheduling events happen under rate monotonic scheduling. P1 is scheduled at t = 0, P2 is scheduled at t = 25, P1 is scheduled at t = 50, and P2 is scheduled at t = 75. P2 is not scheduled early enough to meet its deadline. The earliest deadline schedule performs the following scheduling events: P1 is scheduled at t = 0, P2 is scheduled at t = 25, P1 is scheduled at t = 55, and so on. This schedule actually meets the deadlines and therefore earliest deadline first scheduling is more effective than the rate monotonic scheduler. 19.5 Consider two processes P1 and P2 where p1 = 50, t1 = 25 and p2 = 75, t2 = 30. a. Can these two processes be scheduled using rate monotonic scheduling? Illustrate your answer using a Gantt chart. b. Illustrate the scheduling of these two processes using earliest deadline first (EDF) scheduling. Answer: Consider when P1 is assigned a higher priority than P2 with the rate monotonic scheduler. P1 is scheduled at t = 0, P2 is scheduled at t = 25, P1 is scheduled at t = 50, and P2 is scheduled at t = 75. P2 is not scheduled early enough to meet its deadline. When P1 is assigned a lower priority than P2 , then P1 does not meet its deadline since it will not be scheduled in time. 19.6 What are the various components of interrupt and dispatch latency? Answer: The interrupt latency comprises of the time required to save the state of the current process before invoking the specific interrupt service handler. It therefore comprises of the following tasks: determine the interrupt time, perform the context switch, and jump to the appro- Exercises 129 priate ISR. The dispatch latency corresponds to the time required to stop one process and start another process. It might typically comprise of the following tasks: preemption of any process running in the kernel, release of resources required by the process to be scheduled, and the context switch cost corresponding to scheduling the new process. 19.7 Explain why interrupt and dispatch latency times must be bounded in a hard real-time system. Answer: Interrupt latency is the period of time required to perform the following tasks: save the currently executing instruction, determine the type of interupt, save the current process state, and then invoke the appropriate interrupt service routine. Dispatch latency is the cost associated with stopping one process and starting another. Both interrupt and dispatch latency needs to be minimized in order to ensure that real-time tasks receive immediate attention. Furthermore, sometimes interrupts are disabled when kernel data structures are being modified, so the interrupt does not get serviced immediately. For hard real-time systems, the time-period for which interrupts are disabled must be bounded in order to guarantee the desired quality of service.