Книга: Distributed operating systems

4.6.1. What Is a Real-Time System?

4.6.1. What Is a Real-Time System?

For most programs, correctness depends only on the logical sequence of instructions executed, not when they are executed. If a C program correctly computes the double-precision floating-point square root function on a 200-MHz engineering workstation, it will also compute the function correctly on a 4.77-MHz 8088-based personal computer, only slower.

In contrast, real-time programs (and systems) interact with the external world in a way that involves time. When a stimulus appears, the system must respond to it in a certain way and before a certain deadline. If it delivers the correct answer, but after the deadline, the system is regarded as having failed. When the answer is produced is as important as which answer is produced.

Consider a simple example. An audio compact disk player consists of a CPU that takes the bits arriving from the disk and processes them to generate music. Suppose that the CPU is just barely fast enough to do the job. Now imagine that a competitor decides to build a cheaper player using a CPU running at one-third the speed. If it buffers all the incoming bits and plays them back at one-third the expected speed, people will wince at the sound, and if it only plays every third note, the audience will not be wildly ecstatic either. Unlike the earlier square root example, time is inherently part of the specification of correctness here.

Many other applications involving the external world are also inherently real time. Examples include computers embedded in television sets and video recorders, computers controlling aircraft ailerons and other parts (so called fly-by-wire), automobile subsystems controlled by computers (drive-by-wire?), military computers controlling guided antitank missiles (shoot-by-wire?), computerized air traffic control systems, scientific experiments ranging from particle accelerators to psychology lab mice with electrodes in their brains, automated factories, telephone switches, robots, medical intensive care units, CAT scanners, automatic stock trading systems, and numerous others.

Many real-time applications and systems are highly structured, much more so than general-purpose distributed systems. Typically, an external device (possibly a clock) generates a stimulus for the computer, which must then perform certain actions before a deadline. When the required work has been completed, the system becomes idle until the next stimulus arrives.

Frequently, the stimulii are periodic, with a stimulus occurring regularly every AT seconds, such as a computer in a TV set or VCR getting a new frame every 1/60 of a second. Sometimes stimulii are aperiodic, meaning that they are recurrent, but not regular, as in the arrival of an aircraft in a air traffic controller's air space. Finally, some stimulii are sporadic (unexpected), such as a device overheating.

Even in a largely periodic system, a complication is that there may be many types of events, such as video input, audio input, and motor drive management, each with its own period and required actions. Figure 4-25 depicts a situation with three periodic event streams, A, B, and C, plus one sporadic event, X.

Fig. 4-25. Superposition of three event streams plus one sporadic event.

Despite the fact that the CPU may have to deal with multiple event streams, it is not acceptable for it to say: It is true that I missed event B, but it is not my fault — I was still working on A when B happened. While it is not hard to manage two or three input streams with priority interrupts, as applications get larger and more complex (e.g., automated factory assembly lines with thousands of robots), it will become more and more difficult for one machine to meet all the deadlines and other real-time constraints.

Consequently, some designers are experimenting with the idea of putting a dedicated microprocessor in front of each real-time device to accept output from it whenever it has something to say, and give it input at whatever speed it requires. Of course, this does not make the real-time character go away, but instead gives rise to a distributed real-time system, with its own unique characteristics and challenges (e.g., real-time communication).

Distributed real-time systems can often be structured as illustrated in Fig. 4-26. Here we see a collection of computers connected by a network. Some of these are connected to external devices that produce or accept data or expect to be controlled in real time. The computers may be tiny microcontrollers built into the devices, or stand-alone machines. In both cases they usually have sensors for receiving signals from the devices and/or actuators for sending signals to them. The sensors and actuators may be digital or analog.

Fig. 4-26. A distributed real-time computer system.

Real-time systems are generally split into two types depending on how serious their deadlines are and the consequences of missing one. These are:

1. Soft real-time systems.

2. Hard real-time systems.

Soft real-time means that missing an occasional deadline is all right. For example, a telephone switch might be permitted to lose or misroute one call in 105 under overload conditions and still be within specification. In contrast, even a single missed deadline in a hard real-time system is unacceptable, as this might lead to loss of life or an environmental catastrophe. In practice, there are also intermediate systems where missing a deadline means you have to kill off the current activity, but the consequence is not fatal. For example, if a soda bottle on a conveyor belt has passed by the nozzle, there is no point in continuing to squirt soda at it, but the results are not fatal. Also, in some real-time systems, some subsystems are hard real time whereas others are soft real time.

Real-time systems have been around for decades, so there is a considerable amount of folk wisdom accumulated about them, most of it wrong. Stankovic (1988) has pointed out some of these myths, the worst of which are summarized here.

Myth 1: Real-time systems are about writing device drivers in assembly code.

This was perhaps true in the 1970s for real-time systems consisting of a few instruments attached to a minicomputer, but current real-time systems are too complicated to trust to assembly language and writing the device drivers is the least of a real-time system designer's worries.

Myth 2: Real-time computing is fast computing.

Not necessarily. A computer-controlled telescope may have to track stars or galaxies in real time, but the apparent rotation of the heavens is only 15 degrees of arc per hour of time, not especially fast. Here accuracy is what counts.

Myth 3: Fast computers will make real-time system obsolete.

No. They just encourage people to build real-time systems that were previously beyond the state-of-the-art. Cardiologists would love to have an MRI scanner that shows a beating heart inside an exercising patient in real time. When they get that, they will ask for it in three dimensions, in full color, and with the possibility of zooming in and out. Furthermore, making systems faster by using multiple processors introduces new communication, synchronization, and scheduling problems that have to be solved.

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