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1 Institut Mines-Telecom Ludovic Apvrille Eurecom, office Outline 2/66 Fall 2014 Institut Mines-Telecom

2 Definitions What is an Embedded System? Computer system designed for specific purpose Tightly coupled hardware and software integration An embedding system may include several embedded systems PC vs. embedded systems PC: general-purpose microprocessor and OS High power consumption, heat production, large size Embedded system Dedicated system Dedicated hardware (e.g., DSPs) and software Constraints of consumption, heat, size, performance, price,... 3/66 Fall 2014 Institut Mines-Telecom A Few Characteristics of Embedded Systems Environment The system evolves in an environment that must be taken into account: Temperature, vibration, impacts, variable power supply, interferences, corrosion, fire, water, radiations, etc. Criticality Failures may have major consequences The system should always perform correctly! Hardware level: for example, a specification could be that the system should continue to work even if an electronic component fails (redundancy) Safety at software level: e.g., handling of possible bugs and crash Various execution modes 4/66 Fall 2014 Institut Mines-Telecom

3 Highly Available Systems Number of 9s Downtime per year Typical application 3 Nines (99.9%) 9h Desktop 4 Nines (99.99%) 1h Enterprise server 5 Nines (99.999%) 5mn Carrier-class server 6 Nines ( %) 30s Carrier-switch equipment Source: Providing Open Architecture High Availability solutions, Revision 1.0, Published by HA Forum, Feb /66 Fall 2014 Institut Mines-Telecom A Few Characteristics of Embedded Systems (Cont.) Reduced space, weight and consumption Example: smartphones Memory size is small with regards to PCs Reduced computation power Various techniques Mix of analog, digital, software functions Hardware / software co-design System on a Chip (SoC) 6/66 Fall 2014 Institut Mines-Telecom

4 High-level Architecture Embedded system = controlling system + controlled system + environment 7/66 Fall 2014 Institut Mines-Telecom Response to External Events 8/66 Fall 2014 Institut Mines-Telecom

5 Example of Embedded Systems 9/66 Fall 2014 Institut Mines-Telecom Typical Cross-Platform Environment for the Development of Embedded Systems 10/66 Fall 2014 Institut Mines-Telecom

6 View of the Target Embedded System 11/66 Fall 2014 Institut Mines-Telecom Debugging ROM Monitor Uses serial line On-Chip Debugging (OCD) = built-in microprocessor debugging capabilities BDM - Background Debug Mode JTAG - Joint Test Action Group No need for underlying software support on chip OCD debugging information: Content of processor registers System memory dump Instruction under execution... 12/66 Fall 2014 Institut Mines-Telecom

7 What is a Real-Time System? Real-Time Systems have been defined as: Those systems in which the correctness of the system depends not only on the logical results of the computation, but also on the time at which the results are produced (J. Stankovic, Misconceptions About Real-Time Computing, IEEE Computer, 21(10), October 1988) 13/66 Fall 2014 Institut Mines-Telecom What is a Real-Time System? (Cont.) More precisely, a system is said to be a Real-Time System if: It reacts to external stimuli System interacting with its environment It reacts to these stimuli within a limit of time, regardless of the load of the system WCET = Worst Case Execution Time Timing correctness Vs. usual systems Only logical correctness Manage the average case Best effort 14/66 Fall 2014 Institut Mines-Telecom

8 Response to External Events 15/66 Fall 2014 Institut Mines-Telecom Definition Jobs and Tasks A task is a set of related jobs joined to provide functions A job is a unit of work executed by the system Parameters Temporal behavior E.g., a job lasting less than 20ms, a periodic task with period = 30ms Functional: intrinsic properties of the job (e.g., taps of filter) Resource requirements (e.g., 10kb of memory) Interconnection: its communication with other jobs E.g., a decoding job must send its result to a displaying job Relies on shared memory, message queues, etc. 16/66 Fall 2014 Institut Mines-Telecom

9 Time Constraints Time constraint = Constraint imposed on the timing behavior of a job Release time Instant of time when a job becomes available for execution Deadline Instant of time at which a job is required to be completed Absolute deadline = release time + relative deadline Response Time Difference between the completion time and the release time of a job 17/66 Fall 2014 Institut Mines-Telecom System Latency System latency = End-to-end delay between a change of state in the environment, and the corresponding output reaction produced by the system Latency involves: delay for scanning the environment, delay due to the OS, delay for executing calculations, delay for producing output results (communication delay) Jitter = incertitude on delays Load of the system, etc. Should be low with regards to the latency 18/66 Fall 2014 Institut Mines-Telecom

10 Determinism Issue Computing time Each activity can use various execution branches Communication time Status of messages queues, delay of signals, etc. Interrupts Software or hardware exceptions 19/66 Fall 2014 Institut Mines-Telecom Hard and Soft Real-Time Systems Comparison between hard and soft systems 1. The degree of tolerance of missed deadlines 2. The usefulness of computed results after missed deadlines 3. The severity of penalty incurred for failing to meet deadlines Hard system Soft system 1 Zero tolerance Non-zero 2 Useless Not zero right after passing deadline 3 Catastrophic Non-catastrophic Complex real-time and embedded systems A complex real-time and embedded system is commonly a mix of hard and soft subsystems 20/66 Fall 2014 Institut Mines-Telecom

11 Example 1: A GSM Phone Specification Software is in charge Of some operations performed at physical level (receiving, sending, measuring the level of the electromagnetic field, etc.) Of logical operations such as scanning for incoming calls, maintaining connections when changing of beams, etc. Sending and receiving voice data is a critical task 577µs of voice data are transmitted every 4.6ms 577µs of voice data are received every 4.6ms Management of the mobility issue If the distance to the relay is increasing, data should be emitted earlier to remain synchronized with the relay (Doppler effect) 21/66 Fall 2014 Institut Mines-Telecom Example 1: A GSM Phone (Cont.) Constraints Temporal constraints on sending and receiving Global constraint: Audio quality 22/66 Fall 2014 Institut Mines-Telecom

12 Example 2: A Multimedia System Discrete media vs. continuous media Discrete media (= static): Media characterized by their spatial dimension: text, images Continuous media (= dynamic ): Media with a temporal dimension: sound, video and animations Quality of Service (QoS) Fundamental issue! Use of synchronization techniques 23/66 Fall 2014 Institut Mines-Telecom Example 2: A Multimedia System (Cont.) 24/66 Fall 2014 Institut Mines-Telecom

13 True or False? A real-time system runs faster than usual systems? An elevator controlling system is a real-time system? A real-time application can be executed on every operating systems? 25/66 Fall 2014 Institut Mines-Telecom Outline 26/66 Fall 2014 Institut Mines-Telecom

14 Limitations of General-Purpose OS for Real-Time Systems Scheduling policies share CPUs in fair way They don t take into account temporal constraints Communication mechanisms introduce non-deterministic temporal behavior Time is wasted at input / output management level Memory management (MMU, cache, dynamic allocation) introduces non-deterministic execution time Software timers used for managing time are not fine-grained enough Use of 27/66 Fall 2014 Institut Mines-Telecom Key Characteristics Reliability Need to operate for a long period of time System reliability depends on all system elements Hardware, BSP, RTOS and application(s) Predictability To a certain degree, and at least better than in GPOS Completion of system calls occurs within known frames Benchmark on the variance of the response times for each system calls WCET 28/66 Fall 2014 Institut Mines-Telecom

15 Key Characteristics (Cont.) Performance CPU performance: MIPS Throughput performance: bps Performance should be measured as a whole Compactness RTOS memory footprint should be small Scalability, i.e handling application-specific requirements Modular components File systems, protocol stacks, etc. 29/66 Fall 2014 Institut Mines-Telecom Target System Software Initialization Sequence 30/66 Fall 2014 Institut Mines-Telecom

16 Examples of RTOS VxWorks LynuxWorks QNX Windows Mobile, ios, Symbian, BlackBerry Operating System 1 Linux for RT and embedded systems µclinux RTLinux RTAI Android 1 It is based on QNX 31/66 Fall 2014 Institut Mines-Telecom Developing an Application for VxWorks with Tornado 32/66 Fall 2014 Institut Mines-Telecom

17 Debuging Using WindView 33/66 Fall 2014 Institut Mines-Telecom Using Free and Open-Source OS for Embedded Systems No royalties when distributing the embedded system Source code is available Maintenance can be performed for as long as desired Systems can be built from this source code 34/66 Fall 2014 Institut Mines-Telecom

18 How to Make UNIX more Real-Time UNIX Real-Time UNIX Time-sharing based scheduling Real-time scheduling classes Services not reentrant Fully reentrant libraries IPC non deterministic Reworking IPC with deterministic communication time Large memory footprint Modularity (micro-kernels) Timing issues, timer accuracy Preemptive kernel /66 Fall 2014 Institut Mines-Telecom Scheduling Limitations of schedulers in general-purpose OS Schedulers are defined with interactivity in mind Dynamic priorities Calculation tasks are penalized Temporal constraints are not taken into account Schedulability analysis cannot be performed Schedulers for Real-Time Systems RMA, EDF, FCFS, Round-Robin, Fixed priority 36/66 Fall 2014 Institut Mines-Telecom

19 Preemption Latency in a Priority-Based System 37/66 Fall 2014 Institut Mines-Telecom Preemption Points Adding preemption points in the kernel ( Fully preemptive kernel) Define a slice of time during which no preemption is allowed (e.g., 2,000 instructions) Rewrite all kernel primitives: The duration during which no preemption is allowed must always be shorter than the defined slice of instructions After each slice, call the scheduler (just in case) Limitation: Defining the slice is not obvious Too long average preemption latency is higher Too short important overhead 38/66 Fall 2014 Institut Mines-Telecom

20 Memory Management Dynamic allocations Should be avoided at run-time Algorithms are based on fixed-size blocks Pools of objects Avoid frequent allocation and fragmentation No memory compaction Real-time compaction algorithms might be used Most of the time, virtual addressing (i.e., MMU) is not used And sometimes even not implemented in RTOS! Page fault introduces non-determinism 39/66 Fall 2014 Institut Mines-Telecom IPC - Inter Process Communications Limitations for RT systems No deterministic transmission delay for: Signals, pipes, message passing, semaphores, shared memory A Few Solutions Implementation with deterministic processing time Example: RT-FIFO RT-FIFO = FIFO with predefined maximum size and with pre-allocated messages 40/66 Fall 2014 Institut Mines-Telecom

21 Input / Output Management Limitations for RT systems Difficult or impossible to bound the time to perform an I/O operation Solutions Drivers working closely with the kernel Deterministic delay in all inter-process communications Communication structures with basic and deterministic algorithms RT-FIFO, RT-signals, etc. 41/66 Fall 2014 Institut Mines-Telecom Issue Reentrant Runtime Libraries Libraries offer services such as: Input / Output operations, dynamic memory allocations, etc. Services may have an internal state: buffer (I/O), pointer to the next chunk of free memory, etc. If preemption occurs when a service is being called, internal state of the service may be corrupted Solutions Protection at application level: mutex, semaphore, etc. Safe, but costly Use a reentrant version of the library (if available) Safe, less costly (but more costly than non-reentrant librairies) 42/66 Fall 2014 Institut Mines-Telecom

22 Exceptions and Interrupts Exceptions Events that disrupt the normal execution flow of the processor and force the processor to execute special instructions in a privileged state Synchronous exceptions Divide by zero Asynchronous exceptions Interrupts Pushing the reset button of the embedded system = Asynchronous exceptions triggered by events external hardware devices generate 43/66 Fall 2014 Institut Mines-Telecom Programmable Interrupt Controller Hardware-implemented Prioritizing multiple interrupt sources so that at any time the highest priority interrupt is presented to the core CPU for processing PICs have a set of Interrupt Request Lines Interrupt = physical signal on a given line Handlers specified at PIC level ESR = Exception Service Routine ISR = Interrupt Service Routine 44/66 Fall 2014 Institut Mines-Telecom

23 Source Airbag sensor Brake sensor Fuel Level Sensor Programmable Interrupt Controller: Example Priority Vector Address IRQ Max freq. Description Highest 14h 8 N/A Detects the need to deploy airbags High 18h 7 N/A Informs about the current braking power Med 1Bh 6 20Hz Detects the level of fuel RTC Low 1Dh 5 100Hz Clock runs at 10ms tick 46/66 Fall 2014 Institut Mines-Telecom

24 Masking Interrupts Why masking? Reduce the total number of interrupts raised by devices Interrupts are lost when masked Function being processed is non-reentrant Atomic operations How is masking done? Disable the device so that it cannot assert additional interrupts Mask the interrupts of equal or lower priority levels Disable the line between the PIC and the core processor Interrupts of any priority level do not reach the processor 47/66 Fall 2014 Institut Mines-Telecom Classification of Exceptions Asynchronous Non-maskable Cannot be blocked or enabled by software Maskable Can be blocked or enabled by software Synchronous Precise Program counter points to the exact instruction which caused the exception (Offending instruction) Processor knows where to resume the execution Non-precise Because of prefetching and pipelining techniques, no knowledge about the exact instruction that caused the exception 48/66 Fall 2014 Institut Mines-Telecom

25 General Exceptions Priorities 49/66 Fall 2014 Institut Mines-Telecom Nested Interrupts 50/66 Fall 2014 Institut Mines-Telecom

26 Exceptions Timing ESR and ISR should be short Designers Should avoid situations where interrupts could be missed Should be aware of the interrupt frequency of each device Maximum allowed duration can be deduced from this information Should take into account the fact that real-time tasks cannot execute when interrupts are being processed 51/66 Fall 2014 Institut Mines-Telecom Exceptions Timing (Cont.) 52/66 Fall 2014 Institut Mines-Telecom

27 Cutting ISRs in Two Parts 53/66 Fall 2014 Institut Mines-Telecom Figure: The caption Conclusion on ISRs Benefits about cutting ISRs in two parts Lower priority interrupts can be handled Reduces the chance of missing interrupts Increase concurrency because devices can start working on next operations earlier General guide for implementing ISRs An ISR should disable interrupts of the same level An ISR should mask all interrupts if it needs to execute a sequence of code as one atomic operation An ISR should avoid calling non-reentrant functions and blocking system calls 54/66 Fall 2014 Institut Mines-Telecom

28 Timer and Timer Services Use of Applications often have to perform jobs after a given delay has elapsed Programmable Interval (PIT) 55/66 Fall 2014 Institut Mines-Telecom Timer Interrupt Service Routine Updating the system clock Absolute time (calendar) Elapsed time (since power up) Calling a registered kernel function to notify the occurrence of a pre-programmed period announce time tick() Calls the Scheduler and the Soft-timer facility Acknowledging the interrupt, reinitializing the necessary timer control register and returning from interrupt 56/66 Fall 2014 Institut Mines-Telecom

29 Steps in Servicing Timer Interrupt 57/66 Fall 2014 Institut Mines-Telecom Goal Implementing the Soft-Timer Handling Facility Allowing applications to start a timer A callback function is called when the timer expires Allowing applications to stop or cancel a previously installed timer Internally maintaining the application timers Implementation: timer ISR The timer ISR should be as fast as possible to avoid missing next timer interrupts 58/66 Fall 2014 Institut Mines-Telecom

30 Timer Inaccuracy Due to Processing Delays ISR triggers asynchronous events to signal a task of the expiration of its timer E.g., sending of a message, release of a semaphore Task scheduling delay, context switch Higher priority task may be running 59/66 Fall 2014 Institut Mines-Telecom Limitations A few solutions RTAI Outline Limitations A few solutions RTAI 60/66 Fall 2014 Institut Mines-Telecom

31 Limitations A few solutions RTAI Limitations of Linux for Real-Time Systems Time-sharing system Interactivity-based scheduling: strict priority-based approach is needed in real-time systems! Long code sections where all external events are masked Preemption is not possible Kernel cannot be preempted When executing a system call (so, interrupts may be missed!) When executing an ISR Device management Partial support for POSIX.4 61/66 Fall 2014 Institut Mines-Telecom Limitations A few solutions RTAI Towards a Linux for Real-Time Systems Introducing a real-time scheduler and preemption points Patching the kernel Adding new kernel modules 62/66 Fall 2014 Institut Mines-Telecom

32 Limitations A few solutions RTAI Introducing a Real-Time Scheduler (Patch) Patchs are said to be preemptive Kernel s latency is reduced Limited to soft real-time systems? RT-Preempt Based on call to the scheduler whenever possible Kernel s data are protected with mutex Low Latency Adds preemption points at carefully chosen places in the code Approach is less systematic Performance study demonstrates its efficiency with regards to the Preempt kernel patch 63/66 Fall 2014 Institut Mines-Telecom Limitations A few solutions RTAI Modifying the Kernel (Modules) Basic idea: the Linux kernel will never be real-time Another scheduler is added to the kernel Priority-based scheduling policy Non real-time tasks are scheduled by the default Linux scheduler Real-time tasks are programmed as linux kernel modules Hard real-time system Two main implementations RTAI ( RTLinux (FSMLabs) 64/66 Fall 2014 Institut Mines-Telecom

33 Limitations A few solutions RTAI Architecture of the Linux Kernel 65/66 Fall 2014 Institut Mines-Telecom Limitations A few solutions RTAI Architecture of RTAI 66/66 Fall 2014 Institut Mines-Telecom

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