Contents
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Real Time System Fundamentals
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Real Time Operating System (RTOS)
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Linux as RTOS
What are Real-time Systems ?
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Real-time systems
Those systems in which the correctness of
the system depends not only on the logical
result of computation, but also on the time at
Types of Real Time Systems
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Based on Hard deadline: Penalty due to
missing deadline is a higher order of
magnitude than the Reward in meeting the
deadline.
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Based on Soft deadline: Penalty often equal /
A Sample Real Time System (2)
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Mission: Reaching the destination safely.
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Controlled System: Car.
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Operating environment: Road conditions.
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Controlling System
- Human driver: Sensors - Eyes and Ears of the
driver.
- Computer: Sensors - Cameras, Infrared receiver,
A Sample Real Time System (3)
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Controls: Accelerator, Steering wheel, Break-pedal.
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Actuators: Wheels, Engines, and Brakes.
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Critical tasks: Steering and breaking
A Sample Real Time System (4)
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Performance is not an absolute one. It measures
the goodness of the outcome relative to the best
outcome possible under a given circumstance.
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Cost of fulfilling the mission → Efficient solution.
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Reliability of the driver → Fault-tolerance is a
Contents
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Real Time System Fundamentals
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Real Time Operating System (RTOS)
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Linux as RTOS
RTOS Kernel
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RTOS Kernel provides an Abstraction layer that
hides from application software the hardware
details of the processor / set of processors upon
which the application software shall run.
Application Software
RTOS
Task Management
• Set of services used to allow application software developers to design their software as a number of separate chunks of software each handling a distinct topic, a distinct goal, and sometimes its own real-time deadline.
• Main service offered is Task Scheduling
– controls the execution of application software tasks
Task Scheduling
• Non Real -time systems usually use Non-preemptive Scheduling
– Once a task starts executing, it completes its full execution
• Most RTOS perform priority-based preemptive task scheduling.
• Basic rules for priority based preemptive task scheduling
– The Highest Priority Task that is Ready to Run, will be the Task that Must be Running.
Priority based Preemptive Task Scheduling
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Every Task in a software application is assigned a
priority.
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Higher Priority = Higher Need for Quick Response.
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Follows nested preemption
Nested Preemption
Task Switch (1)
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Each time the priority-based preemptive scheduler
is alerted by an External world trigger / Software
trigger it shall go through the following steps that
constitute a Task Switch:
– Determine whether the currently running task should continue to run.
– Determine which task should run next.
– Save the environment of the task that was stopped (so it can continue later).
– Set up the running environment of the task that will run next.
Task Switch (2)
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A Non Real time operating system might do task
switching only at timer tick times.
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Even with preemptive schedulers a large array of
tasks is searched before a task switch.
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A Real time OS shall use Incrementally arranged
tables to save on time.
Task Switch (3)
Intertask Communication &
Synchronization
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These services makes it possible to pass
information from one task to another
without information ever being damaged.
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Makes it possible for tasks to coordinate &
productively cooperate with each other.
nicatInter-Task commuion &
Synchronization
• The most important communication b/w tasks in an OS is the passing of data from one task to another.
• If messages are sent more quickly than they can be handled, the OS provides message queues for holding the messages until they can be processed.
Message Producer Task Message Receiver Task
Message passing in OS
• Most General Purpose OS actually copy messages twice as they transfer them from task to task via a message queue.
Message Sender
Task RAM
Messag e
RAM Message Receiver Task Messag
Message passing in RTOS
• In RTOS, the OS copies a pointer to the message,
delivers the pointer to the message-receiver task, and then deletes the copy of the pointer with
message-sender task. Message Sender Task RAM Message Receiver Task msg_pt RTOS r msg_pt r Messag e Message
Dynamic Memory Allocation
in General Purpose OS
• Non-real-time operating systems offer memory allocation services from what is termed a Heap.
• Heaps suffer from a phenomenon called External Memory
Fragmentation.
• Fragmentation problem is solved by Garbage collection /
Defragmentation.
• Garbage collection algorithms are often wildly non-deterministic.
Dynamic Memory Allocation
in RTOS
• RTOS does it by a mechanism known as Pools.
• Pools memory allocation mechanism allows application software to allocate chunks of memory of 4 to 8 different buffer sizes per pool.
• Pools avoid external memory fragmentation, by not
permitting a buffer that is returned to the pool to be broken into smaller buffers in the future.
• When a buffer is returned the pool, it is put onto a free
buffer list of buffers of its own size that are available for
Contents
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Real Time System Fundamentals
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Real Time Operating System (RTOS)
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Linux as RTOS
Dark Side of Linux
in Real-Time
• The Linux kernel (< version 2.6) is neither preemptive nor
reentrant by user processes.
• Processes are heavy-weight: Linux has insufficient resource handling.
• Fair share scheduling algorithm .
• Unbound amount of CPU time used by interrupt handlers under Linux, and these run at a higher priority than any user process.
Typical Solutions for Linux
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3 ways to deal with Linux’ s poor real-time
performance :
– Ignore the problem (only for soft real-time).
– Work around and run real-time applications under an
RTOS with Linux itself as a separate task. e.g. RT-Linux.
– Adapt Linux and run a Linux compatible RTOS kernel
Ignore the problem Approach
• The most popular approach is to ignore the problem.
• Mostly works for soft real-time systems.
• Design recommendations to reduce latency
– If possible, address all real-time response needs directly with interrupt service routines.
– Avoid known excessive interrupt-off periods in Linux. – If a process component is required in the real-time
control path, then consider aggregate system loading & Adapt processes.
Work Around Approach
• Linux kernel runs as a task under a small real-time executive.
• Real-time tasks are run directly under the real-time executive
• Non-real-time tasks are run under Linux.
• Applications that work on such a system can include machine control, process control, and instrumentation applications.
Work Around - Advantages
• The real-time executive can be small and simple making it easier to verify its real-time performance.
• The Linux kernel running non-real-time tasks is "standard" Linux so it is compatible with other Linux distributions and can easily be updated.
Work Around - Disadvantages
• Not applicable if real-time part of the software is large / not easily separated from the code that needs a Linux environment
• Tasks running under the real-time executive do not have access to the Linux facilities, device drivers, etc.
• Tasks running under the Linux kernel and can access the facilities are not real-time.
• Programmers tend to increase the number of facilities in the real-time kernel until the real-time executive
Adapt Linux
• The original Linux kernel is replaced with a compatible kernel with hard real-time performance characteristics.
• The kernel is only a small part of a whole operating system. If just the Linux kernel is replaced and the libraries, utilities, and file structure remain the same.
• The real-time kernel must support all the facilities of a Linux kernel while still remaining fully preemptive and reentrant.
Adapt Linux Approach
Advantages & Disadvantages
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Advantages:
– No limits on the size and complexity of the real-time application code.
– Third party software not originally envisioned for real-time use can be invoked by real-time tasks
– With the same programming interfaces, programmers who understand Linux would already be experts in the real-time environment.
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Disadvantages:
– With modified Linux you are NOT sure of full compatibility with standard Linux application
