- •Introduction
- •Applications of Real-Time Systems
- •Voltage
- •Figure 7: Conversion of an Analog Signal to a 16 bit Binary Number
- •Figure 11: Schematic Representation of tmr
- •It is relatively simple to design a hardware equipment to be fault-tolerant. The following are two methods that are popularly used to achieve hardware fault-tolerance:
- •Software Fault-Tolerance Techniques
- •Types of Real-Time Tasks
- •Timing Constraints
- •Events in a Real-Time System
- •Figure 16: Delay Constraint Between Two Events el and e2
- •Examples of Different Types of Timing Constraints
- •Figure 19: Classification of Timing Constraints
- •Real-Time Task Scheduling
- •Figure 1: Relative and Absolute Deadlines of a Task
- •Figure 2: Precedence Relation Among Tasks
- •Types of Real-Time Tasks and Their Characteristics
- •Classification of Real-Time Task Scheduling Algorithms
- •Figure 5: An Example Schedule Table for a Cyclic Scheduler
- •Figure 6: Major and Minor Cycles in a Cyclic Scheduler
- •Comparison of Cyclic with Table-Driven Scheduling
- •Hybrid Schedulers
- •Event-driven Scheduling
- •Is edf Really a Dynamic Priority Scheduling Algorithm?
- •Implementation of edf
- •Figure 10: Priority Assignment to Tasks in rma
- •We now illustrate the applicability of the rma schodulability criteria through a few examples.
- •Deadline Monotonic Algorithm (dma)
- •Handling Aperiodic and Sporadic Tasks
- •Dealing With Task Jitter
- •W Good real-time task scheduling algorithms ensure fairness to real-time tasks while scheduling.
- •State whether the following assertions are True or False. Write one or two sentences to justify your choice in each case.
- •Figure 2: Unbounded Priority Inversion
- •Highest Locker Protocol(hlp)
- •Priority Ceiling Protocol (pcp)
- •Comparison of Resource Sharing Protocols
- •Handling Task Dependencies
- •Fault-Tolerant Scheduling of Tasks
- •Clocks in Distributed Real-Time Systems
- •Clock Synchronization
- •Figure 1: Centralized synchronization system
- •Cn Slave clocks
- •Commercial Real-Time Operating Systems
- •Time Services
- •Clock Interrupt Processing
- •Providing High Clock Resolution
- •Figure 2: Use of a Watchdog Tinier
- •Unix as a Real-Time Operating System
- •In Unix, dynamic priority computations cause I/o intensive tasks to migrate to higher and higher priority levels, whereas cpu-intensive tasks are made to seek lower priority levels.
- •Host-Target Approach
- •Preemption Point Approach
- •Self-Host Systems
- •Windows As a Real-Time Operating System
- •Figure 9: Task Priorities in Windows nt
- •Open Software
- •Genesis of posix
- •Overview of posix
- •Real-Time posix Standard
- •Rt Linux
- •7.8 Windows ce
- •Benchmarking Real-Time Systems
- •Figure 13: Task Switching Time Among Equal Priority Tasks
- •Real-Time Communication
- •Figure 2: a Bus Architecture
- •Figure 4: Logical Ring in a Token Bus
- •Soft Real-Time Communication in a lan
- •Figure 6: Priority Arbitration Example
- •Figure 8: Problem in Virtual Time Protocol
- •Figure 9: Structure of a Token in ieee 802.5
- •Figure 10: Frames in the Window-based Protocol
- •Performance Comparison
- •A Basic Service Model
- •Traffic Characterization
- •Figure 16: Constant Bit-Rato Traffic
- •Routing Algorithms
- •Resource Reservation
- •Resource Reservation Protocol (rsvp)
- •Traffic Shaping and Policing
- •Traffic Distortion
- •Traffic Scheduling Disciplines
- •Figure 20: Packet Service in Jittor-edd
- •Differentiated Services
- •Functional Elements of DiffServ Architecture
- •Real Time Databases
- •Isolation: Transactions are executed concurrently as long as they do not interfere in each other’s computations.
- •Real-Time Databases
- •Real-Time Database Application Design Issues
- •Temporal Consistency
- •Concurrency Control in Real-Time Databases
- •It can bo shown that pcp is doadlock froo and single blocking. Rocolloct that single blocking moans that once a transaction starts executing after being blocked, it may not block again.
- •Speculative Concurrency Control
- •Comparison of Concurrency Control Protocols
- •Commercial Real-Time Databases
- •Figure 16: Uniform Priority Assignment to Tasks of Example 15
- •Version 2 cse, iit Kharagpur
Commercial Real-Time Operating Systems
In tho last throo Chaptors wo discussod tho important real-time task scheduling techniques. We highlighted that timely production of results in accordance to a physical clock is vital to the satisfactory operation of a real-timo system. We had also pointed out that real-timo operating systems are primarily responsible for ensuring that every real-timo task moots its timeliness requirements. A real-timo operating system in turn achieves this by using appropriate task scheduling techniques. Normally, real-timo operating systems provide flexibility to the programmers to select an appropriate scheduling policy among several supported policies. Deployment of an appropriate task scheduling technique out of the supported techniques is therefore an important concern for every real-timo programmer. To be able to determine the suitability of a scheduling algorithm for a given problem, a thorough understanding of the characteristics of various real-timo task scheduling algorithms is important. We therefore had a rather elaborate discussion on real-timo task scheduling techniques and certain related issues such as sharing of critical resources and handling task dependencies.
In this Chapter, we examine the important features that a real-timo operating system is expected to support. Unless these features are adequately supported by an operating system, it becomes to satisfactorily implement certain categories of real-timo applications on this operating system. We discuss to what extent these required features are supported by the various commercially available real-timo operating systems. To gain a bettor insight, wo also investigate the internals of the operating systems to examine the exact ways in which the required features are supported.
To appreciate some of the fundamental issues affecting the design and development of a satisfactory real-timo operating system, we discuss the problems that would crop up if one attempts to use a general purpose operating system such as Unix or Windows for developing real-timo applications. Many real-time operating systems are at present available commercially. We analyze some popular real-timo operating systems, and investigate why these popular systems cannot be used across all applications. We also examine the POSIX standard for real-timo operating systems and its implications.
This chapter is organized as follows. First, we discuss the important features that are usually required to be supported by a real-timo operating system. We start by discussing the time service supports provided by the real- timo operating systems, since accurate and high precision clocks are very important to the successful operation any real-timo application. Subsequently, we discuss the issues that arise if we attempt to use a general purpose operating system such as Unix or Windows in a real-timo applications. Next, we explain how some of the fundamental problems associated with the traditional operating systems (as far as real-timo applications are concerned) are overcome in the the contemporary real-timo operating systems. We then survey some of the important features of the different real-timo operating systems that are being commercially used. Finally, we identify some of the important parameters based on which various real-timo systems can be benchmarked.