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Chapter 19: RealTime Systems

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Title: Chapter 19: RealTime Systems


1
Chapter 19 Real-Time Systems
2
Chapter 19 Real-Time Systems
  • Overview and Introduction
  • System Characteristics
  • Features of Real-Time Systems
  • Implementing Real-Time Operating Systems
  • Real-Time CPU Scheduling

3
Objectives
  • To explain the timing requirements of real-time
    systems
  • To distinguish between hard and soft real-time
    systems
  • To discuss the defining characteristics of
    real-time systems
  • To describe scheduling algorithms for hard
    real-time systems

4
Overview of Real-Time Systems
  • The differences between real time computing
    systems and general purpose computing systems are
    very profound.
  • We will examine many of these differences in the
    upcoming slides.
  • A real time computing system is one that requires
    that correct results be produced within specified
    deadline periods. (Deadline is a key word)
  • Results produced after a specific time period has
    elapsed may well be of absolutely no value and
    may mean loss of life or an aircraft crash yet
    other failures might not be quite as disastrous
    if real time system is not quite as responsive.
    .
  • Real time
  • Run on wide range of computer hardware
  • Used in many different kinds of applications.
  • Some real time systems are embedded in aircraft
    instrumentation
  • your microwave, cell phone, cruise control, and
    hosts of other applications.
  • Are often part of a larger system
  • Oftentimes their presence is not obvious to a
    user.

5
Definitions
  • A real-time system requires results produced
    within a specified deadline period.
  • A hard real time system has stringent
    requirements, guaranteeing that critical
    real-time tasks be completed within their
    deadlines.
  • E.g. Safety-critical systems health systems
    etc.
  • A soft real-time system is less restrictive and
    guarantees simply that a critical real-time task
    will receive priority over other tasks
  • Further, these tasks retain priority over other
    tasks until the tasks complete.
  • An embedded system is a computing device that is
    part of a larger system (I.e. automobile,
    airliner.)
  • A safety-critical system is a real-time system
    with catastrophic results in case of failure.
  • Again, a hard real-time system guarantees that
    real-time tasks be completed within their
    required deadlines, while a soft real-time system
    provides priority of real-time tasks over non
    real-time tasks.

6
System Characteristics
  • Will look at both soft and hard real-time
    operating systems..
  • Real time systems typically exhibit the following
    characteristics
  • Single purpose
  • Small size
  • Inexpensively mass-produced
  • Specific timing requirements
  • And, so many other rather unique features spring
    from these characteristics.
  • These are the big four defining characteristics
  • Lets look at those

7
Characteristics - 1
  • Single Purpose
  • Single purpose is typical controlling anti-lock
    brakes toaster, cell phone.
  • This makes the operating system simple too, as
    many characteristics integral to general
    purpose operating systems are not available or
    needed.
  • Size
  • Often found in severely cramped space but
    sufficient for operations.
  • Examples wrist watches, cell phones, toys
  • Thus, CPU processing power is minimal
  • Amount of primary memory is also minimal.
  • Architectures Compare 32 / 64-bit architectures
    with 8/16 bit processors.
  • Architecture Compare several gigs of memory
    with

8
Characteristics - 2
  • Cost
  • Typically mass produced as in microwave ovens,
    thermostats.
  • Thus, real time microprocessors are often
    inexpensive
  • Organization of real time systems are designed to
    minimize cost
  • To eliminate bus architectures, the physical
    organization for embedded controllers are often
    organized as a system-on-a-chip, which has all
    necessary interconnections.
  • Chip includes memory, cache, a MMU (for possible
    address translation), any peripheral ports
    necessary - all in a single integrated circuit.
  • Such organization is typically much less
    expensive than typical bus-oriented architectures.

9
Bus-Oriented System
10
Characteristics - 3
  • Timing
  • This is the feature that impacts almost
    everything else and makes real time systems what
    they really are!
  • Both hard and soft real time systems have
    timing requirements.
  • So, we will need to develop real time scheduling
    algorithms that provide priority to the highest
    scheduled processes.
  • Schedulers absolutely must ensure that the
    priority of real time tasks does not degrade over
    time.
  • So, we must minimize the response time to
    interrupts recognize the interrupt, save
    context, transfer control to the handler, etc
    as one can easily imagine.

11
Features of Real-Time Kernels
  • Most real-time systems do not provide features
    found in desktop systems.
  • Simply not needed.
  • Do not need (in general) support for
  • A variety of peripheral devices graphical
    displays, CD, DVD drives
  • Protection and security mechanism
  • Support for multiple users
  • Note Windows XP has 40,000,000 lines of code
    a typical real-time operating system usually is
    written in thousands of source code lines!
  • Reasons include
  • Real-time systems are typically single-purpose.
  • Real-time systems often do not require
    interfacing with a user.
  • General-purpose features often require more
    substantial hardware than that found in a RT
    system.

12
Memory Mapping Schemes
  • (1) Real-addressing mode where programs generate
    actual addresses.
  • But there is no memory protection between
    processes.
  • We might also need to specify exactly where in
    memory the program is to be loaded.
  • But the speed is very difficult to beat!
  • No time spent in address translations.
  • Very commonly found in real time systems having
    hard, real-time constraints.
  • Some real time operating systems running on
    microprocessors containing a MMU, actually
    disable the MMU to gain performance benefit in
    referencing physical addresses directly.
  • (2) Relocation register mode.
  • In this scheme, we have a relocation register is
    set to the process load point.
  • Physical addresses are added to contents of the
    relation register formed by adding logical (L) to
    R (relocatable) to form P (Physical) addresses..
  • But here again, there is no protection between
    processes.
  • (3) Implementing full virtual memory.
  • But here we may have page tables and translation
    look-aside buffers.
  • While this strategy does indeed provide memory
    protection, it is costly

13
Address Translation
Diagram of the previous three memory access
techniques.
14
Implementing Real-Time Operating Systems
  • In general, real-time operating systems must
    provide the following features
  • (1) Preemptive, priority-based scheduling
  • (2) Preemptive kernels
  • (3) Minimal latency
  • Lets look at these three important
    characteristics.

15
1. Priority-Based Scheduling
  • A RTOS must respond immediately to a real time
    process as soon as that process requires the CPU.
  • So, there must be a scheduler to support a
    priority-based algorithm with preemption.
  • In priority-based scheduling, processes are
    assigned a priority based on importance.
  • More important processes are assigned higher
    priorities.
  • So, if the scheduler supports preemption, a
    process running on a CPU can be preempted if a
    higher-priority process arrives.
  • Recall Windows XP has 32 priority levels, with
    the highest levels having priority values 16 to
    31 used for RT processes..

16
2. Preemptive Kernels
  • Problem with non-preemptive kernels is process
    doesnt have to give up the CPU. This can be
    disastrous in a real time system.
  • Preemptive kernels allow the preemption of a task
    running in kernel mode!
  • But designing preemptive kernels is complex
  • Many common modern-day applications
    (spreadsheets, browsers, ..) simply do not
    require preemptive kernels.
  • So why have the complexity of such kernels?
  • Thus most commercial desktop OSs are not
    preemptive, such as XP.
  • So, non-preemptive kernels are not acceptable for
    hard RT systems.
  • We need preemption!

17
2. Preemptive Kernels - 2
  • One approach
  • to insert preemption points within long duration
    system calls, so when a process is preempted,
    and, if a context-switch takes place, when the
    process resumes after the high priority process
    runs, it may do so at the point of preemption!
  • Important to note that preemption points only
    occur at carefully architected points in the
    kernel, where kernel structures are not
    undergoing any kind of modification.
  • Second approach for supporting preemptive kernels
    is
  • to implement synchronization mechanisms which
    protect kernel data structures from modification
    from a high-priority process.

18
3. Minimizing Latency
  • Event latency is the amount of time from when an
    event occurs to when it is serviced.
  • Here, we consider the event-driven nature of a RT
    system, and we recognize that the application is
    usually waiting for an event. .
  • The system must respond and service the event as
    quickly as possible!

19
Minimizing Latency continued
  • But all events are not created equal.
  • Different events have different latency times.
  • Some have a very short latency time others
    longer.
  • Example an embedded system controlling
    something like a toaster or some kind of radar,
    might tolerate a latency of several seconds.
  • In RT systems weve got two types of latencies to
    consider
  • Interrupt latency, and
  • Dispatch latency.

20
Interrupt Latency
  • Interrupt Latency refers to the period of time
    from the arrival of an interrupt at the CPU to
    the start of the servicing routine. (see figure
    below)
  • Getting an interrupt, the CPU must finish the
    current instruction, determine type of interrupt,
    save the state of the current process (context
    switch likely) and then jump to the interrupt
    service routine.
  • We clearly need to minimize interrupt latency and
    service the interrupt immediately.

21
Dispatch Latency
  • Dispatch latency is the amount of time required
    for the scheduler to stop one process and start
    another.
  • RTOSs must minimize this latency, and the most
    effective approach for keeping dispatch latency
    at a minimum is via preemptive kernels.
  • See figure. The conflict phase has two parts
  • Preemption of any process running in the kernel,
    and
  • Release by low-priority process resources needed
    by a high-priority process.
  • (e.g. dispatch latency w/preemption disabled in
    Solaris is 100 msec with preemption enabled,
    preemption is reduced to

22
Real-Time CPU Scheduling
  • We must change gears to address the reality that
    so far we are only ensuring that real time
    systems provide priority processing for critical
    processes.
  • But hard real-time systems need much stronger
    guarantees!
  • Tasks MUST be serviced by deadline w/knowledge
    that missing deadline no service at all!
  • So lets consider scheduling considerations for
    hard real-time systems.

23
Real-Time CPU Scheduling - more
  • To understand whats going on, lets consider the
    following assumptions and definitions.
  • First, we will assume that RT processes are
    periodic.
  • This means that they need the CPU at constant
    intervals.
  • Each of these periods processes a fixed
    processing time, t, once the CPU acts on it,
  • Each has a deadline, d, when it must be serviced
    by the CPU, and
  • Each has a period, p
  • The relationship of the processing time t, the
    deadline, and the period can be expressed as
  • 0
  • The rate of a periodic task is 1/p.
  • See next figure to see these relationships.
  • .

24
Real-Time CPU Scheduling
  • Periodic processes require the CPU at specified
    intervals (periods)
  • p is the duration of the period
  • d is the deadline by when the process must be
    serviced
  • t is the processing time
  • Clearly, we need the processing time t to be less
    than the deadline (be completed before deadline
    expires).

Lets look at scheduling algorithms that address
deadline requirements of hard, real-time systems.
25
Preface to Scheduling Algorithms
  • We will look at the
  • 1. Rate Monotonic Scheduling Algorithm, and
  • 2. Earliest Deadline First Scheduling.
  • The rate-monotonic scheduling algorithm schedules
    these periodic tasks using a static priority
    policy with preemption.
  • If we are running a lower priority process when a
    higher priority process needs to be run, the
    scheduler will preempt the lower priority
    procedure.
  • When a periodic task enters the system, it is
    assigned a priority number based on its period.
  • Shorter the period, higher the priority, and vice
    versa.
  • Gives higher priority to the process that needs
    the CPU more often.
  • Procedure also assumes processing time of a
    periodic process is the same for each CPU burst!
  • So every time a process acquires the CPU,
    duration of its CPU burst is the same.
  • As it turns out using the simulation in the book,
    it seems that we may be able to assert that we
    can schedule real time tasks in such a way that
    using this algorithm may be used to meet real
    time task deadlines and still have the CPU with
    available cycles
  • We shall see.
  • Now lets consider two cases We will assign P2
    a higher priority than P1 and then P1 a higher
    priority than P2.

26
Scheduling of tasks when P2 has a higher priority
than P1
Without the rate-monotonic scheduler We will
first assume that P2 has a higher priority than
P1. We see the execution scenario below. P2
starts executing first and completes at time 35.
Then P1 starts. It completes its burst at time
55 but the first deadline for P1 was at 50, so
the scheduler causes P1 to miss its
deadline! Now, suppose we continue to
use the rate-monotonic scheduling approach where
we assign P1 a higher priority than P2, (since
the period of P1 is shorter than P2).
27
Rate Monotonic Scheduling
  • Continuing (P1 period shorter than P2).
  • P1 starts and completes its CPU burst at time 20.
    Meets its first deadline.
  • P2 starts and running until time 50 but at this
    time, it is preempted by P1, even though it only
    needed 5 msec to finish its first CPU burst.
  • P1 finishes its CPU burst at time 70 (meets its
    deadline) scheduler resumes P2, which completes
    its burst at time 75, also meeting its first
    deadline.
  • System is then idle until time 100 when P1 is
    scheduled again.
  • This rate monotonic scheduling is considered
    optimal in the sense that if a set of processes
    cannot be scheduled by this algorithm, it cannot
    be scheduled by any other algorithm that assigns
    static priorities.
  • Your book then goes into another example and
    arrives at the important conclusion the
    rate-monotonic scheduling cannot guarantee
    processes can be scheduled so that they will
    always meet their deadlines.
  • So, enter the Earliest Deadline First
    Scheduler. (EDFS)

28
Earliest Deadline First Scheduling
  • This scheduling algorithm dynamically assigns
    priorities according deadline.
  • Earlier the deadline, the higher the priority
    later deadline?, lower priority.
  • In this algorithm, when a process becomes
    runnable, it must announce its deadline
    requirements to the system.
  • Priorities may have to be dynamically adjusted to
    reflect deadlines of newly runnable processes.
  • This differs from rate-monotonic scheduling,
    where priorities are fixed.

29
  • The EDF scheduling algorithm does not require
    that processes by periodic nor must a process
    require a constant amount of CPU time per burst.
  • The only requirement is that a process announce
    its deadline to the scheduler when it becomes
    runnable.
  • The attraction of the EDF scheduling is that it
    is theoretically optimal it can schedule
    processes so that each process can meet its
    deadline requirements and CPU utilization will be
    100 percent.
  • In practice, as it turns out, it is impossible to
    achieve this level of CPU utilization due to the
    cost of context switching between processes and
    interrupt handling.

30
End of Chapter 19
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