Outline. EE899.6 Embedded System and SOC. Embedded Operating Systems. Operating Systems. Regular Operating Systems. Embedded Operating Systems

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1 Outline EE899.6 Embedded System and SOC RTOS and Scheduling In this slide set, we will learn: The role of embedded operating systems Commercial and free real-time operating systems Department of Electrical Engineering University of Saskatchewan 2 Operating Systems CPU Device Drivers (Real-Time) Operating System Control Software Operating system Controls access to the processors Controls access to the device drivers Embedded Operating Systems Does an embedded system need an OS? Not necessarily We can write our code as one big program, in particular for memory-constrained systems Need to develop device drivers But a (real-time) OS will make your task much easier Otherwise, the code would be too complex Time-to-market, security, reliability 3 4 Embedded Operating Systems Can we use a regular operating system for embedded systems design? Regular OS: on average, make things fast. They are not designed with real-time in mind RTOS: Do everything in a bounded time. (Note the difference between fast and realtime) A regular embedded OS might be fine for many applications. Regular Operating Systems Process/Task Management Memory Management Interrupt Handling Exception Handling Process/Task Synchronization CPU Scheduling Disk Management 5 6 1

2 Real-Time Operating Systems Process/Task Management Tasks might have deadlines (hard or soft) Memory Management Not likely to use virtual memory (why?) Interrupt Handling Very important Exception Handling Process/Task Synchronization Avoid priority inversion (the highest-priority ready task fails to run when it should) CPU Scheduling Time Management Commercial RTOS Wind River: VxWorks Microsoft: Windows Embedded Windows CE, Windows XP Embedded Palm Source: Palm OS RadiSys: Microwave OS-9 LinuxWorks: LynxOS QNX: Neutrino 7 8 Free RTOS Embedded Linux ecos μc/os-ii Selecting a RTOS Network capability Security GUI Hardware platform (x86, ARM, 68K, etc.) Portability IDE (compiler, assembler, debugger, etc.) Size (ROMable, etc.) 9 10 Outline Multitasking In this slide set, we will learn: The goal of processor scheduling Preemptive and non-preemptive scheduling Scheduling criteria The ability of the operating system to handle multiple activities within set deadlines Many tasks of execution appear to be running concurrently; however, the kernel is actually interleaving executions sequentially, based on a preset scheduling algorithm The scheduler must ensure that the appropriate task runs at the right time

3 Scheduler Scheduling Disciplines The scheduler is the heart of a process management unit that determines which task to execute next. Non-preemptive: Once the system has assigned a processor to a task, the system cannot remove the processor from that task. Preemptive: The system can remove the processor from the task Preemptive Scheduling The processor may execute a portion of a task s code and then perform a context switch Useful in systems in which high-priority tasks requires rapid response For example, real-time systems Helps guarantee acceptable user response times in interactive timesharing systems Preemptive Scheduling (cont.) Context switches incur overhead The system must maintain many tasks in main memory so that the next task is ready when a processor becomes available Non-Preemptive Scheduling Each task runs to completion or until it voluntarily relinquishes its processor Errant programs may never relinquish control of the processor Use periodically generated interrupts (interrupt clock or interval timer) to prevent a task from monopolizing the processor Short tasks can experience lengthy service delays Low-priority tasks can make high-priority tasks wait Priority Static priority Priorities remain fixed Relatively easy to implement Not responsive to changes in environment to increase throughput or reduce latency Dynamic priority Responsive to change. For example, increase the priority of a key resource needed by a higher-priority of a process priority inversion. More complex to implement

4 Scheduling Criteria Processor bound or I/O bound processor bound: tasks tend to use all the processor time I/O bound: tasks tends to use processor only briefly and spend most of their time waiting for external resources to service their requests Batch or interactive The scheduler should provide good response time to an interactive process Batch processes generally can suffer reasonable delay Scheduling Criteria (cont.) Priority The scheduler should favor tasks with higher priorities Under some disciplines, frequently preempted processes receive less treatment. Why? Scheduling Criteria (cont.) Preemptive schedulers often maintain information about how much real time each task has received Should a process that has received little execution time should be favored? Or a process that has received much execution time could be near completion and should be favored? Outline In this slide set, we will learn: Common scheduling algorithms FIFO, RR, SRR, SPF, HRRN, SRT Fair share scheduling First-In-First-Out (FIFO) Tasks are dispatched according their arrival time at the ready queue Also called first-come-first-serve Non-preemptive Long tasks make short tasks wait, unimportant tasks make important tasks wait, etc. Rarely used as a master scheme in today s systems, but often found within other schemes Tasks with the same priority are dispatched in FIFO order 23 Round-Robin Tasks are dispatched FIFO but are given a limited amount of time called time slice or quantum If a task does not complete before its quantum expires, the system preempts it and gives the processor to the next waiting task The preempted task is placed at the back of the ready queue Effective for interactive environments Like FIFO, rarely used as the master scheme 24 4

5 Selfish Round-Robin (SRR) Gradually increase task priorities over time Each task enters a holding queue as it enters the system Tasks in the holding queue ages and moved to a active queue when their priorities reaches the level of tasks in the active queue Tasks in the active queue are scheduled round-robin Older tasks are favored over those that have just entered the system Selfish Round-Robin (SRR) Tasks in the holding queue increase their priorities at a rate a, while in the active queue, at a rate b. When a >> b, SRR degenerates to RR When a = b, SRR degenerate to FIFO Tuning the parameters impacts how a process s age affects average latency and throughput Quantum Size Quantum size is critical to preemptive scheduling As the quantum gets larges, RR degenerates to FIFO As the quantum gets small, context-switching overhead dominates Large enough for vast majority of I/Obound tasks to complete their requests Maximizes I/O utilization and provides rapid response times for interactive tasks 27 Quantum Size (cont.) Optimal quantum varies from system to system and under different loads In Linux, default quantum is 100ms, but can vary from 10 to 200ms depending on process priority and behavior In Windows XP, default quantum is 20ms on most systems depending on architecture and behavior 28 Shortest-Process-First (SPF) The task with the smallest estimated runtime-to-completion is selected first Non-preemptive Reduced average waiting time over FIFO The waiting times have a larger variance; i.e., more unpredictable than FIFO Requires precise knowledge of how long a task will run - Usually not available Rely on user estimates 29 Highest-Response-Ratio-Next (HRRN) A modified version of SPF Correct the excessive bias against longer tasks and the excessive favoritism toward short tasks Each task s priority is a function not only of its service time but also of its waiting time Each running task runs to completion time waiting + service time priority = service time 30 5

6 Shortest-Remaining-Time (SRT) The preemptive counterpart of SPF A newly arriving task with a shorter estimated run-time preempts a running task with a longer run-time-to-completion Attempting to increase throughput by servicing small arriving tasks Effective for job-processing systems that receive a stream of incoming jobs No longer useful in most of today s systems Fare Share Scheduling 100% System Resources Fair Share Scheduler 60% 30% 10% Process Scheduler Process Scheduler Process Scheduler Tasks Tasks Tasks Outline In this slide set, we will learn: Deadline scheduling Types of real-time scheduling Overview of two basic real-time scheduling algorithms It is no use running, it is better to leave on time. - La Fontain, Le lièvre et la routue Deadline Scheduling Tasks are to be completed by a specific time Important to real-time scheduling Deadline scheduling is complex The user must supply the precise resource requirements in advance Such information rarely available Must execute the deadline tasks without severely degrading service to other tasks Intensive resource management may generate substantial overhead Must carefully plan system resources through to the deadlines New processes may make unpredictable demands When many deadline tasks are active at once, scheduling could become extremely complex Types of Real-Time Scheduling Algorithms Static real-time scheduling algorithms Do not adjust task priority over time Priorities are calculated only once Tend to be simple and incur little overhead Good for hard real-time systems (?) Dynamic real-time scheduling algorithms Schedule tasks by adjusting their priorities May incur significant overhead Types of Real-Time Scheduling Algorithms Off-line scheduling algorithms A scheduling algorithm is executed on the entire task set before actual task activation On-line scheduling Scheduling decisions are taken at run-time every time a new task enters the system or when a running task terminates

7 Off-Line Scheduling Algorithms The schedule generated is stored in a table and later used by a dispatcher Low run-time overhead Independent of the complexity of the scheduling algorithm Inflexible to changes Static Real-Time Scheduling Rate-monotonic (RM) algorithm Increase task s priority linearly (i.e., monotonically) with the frequency (i.e., the rate) with the frequency it must execute Favors periodic tasks that execute frequently Preemptive Priority-based Dynamic Real-Time Scheduling Earliest-deadline-first (EDF) algorithm Dispatch the task with the earliest deadline Preemptive Maximize throughput by satisfying the deadlines of the largest number of tasks per unit time Minimize average waiting time Outline In this slide set, we will learn: Rate-monotonic (RM) algorithm Optimality of RM algorithm Earliest Deadline First (EDF) algorithm Least Laxity First (LLF) algorithm Release Time Release Time: r 1 = 4 Given two periodic tasks : τ 1 (r 1, 1, 4, 4) : τ 2 (0,10,14,14) Consider different values for r 1, r 1 = 4 r 1 = 2 r 1 = Response time =

8 Release Time: r 1 = 2 Release Time: r 1 = Response time = Response time = Optimal Scheduling Algorithm Feasible schedule A scheduling algorithm results in a schedule for a task set. This schedule is feasible if all the tasks meet their timing constraints. Schedulable task set A task is schedulable when a scheduling algorithm is able to provide a feasible schedule. Optimal scheduling algorithm An algorithm is optimal if it is able to produce a feasible schedule for any schedulable task set. Schedulability Test of RM Algorithm (Off-Line) A set of periodic task with deadlines equal to periods is schedulable with the RM algorithm if the processor utilization factor satisfy n i= 1 C T i i n ( n 1/ n 1) Earliest Deadline First (EDF) Assign priority to task according to their absolute deadlines The task with the earliest deadline will be executed at the highest priority Does not make any assumption about the periodicity Can be used for both periodic as well as aperiodic tasks. Optimal If there exists a feasible schedule for a task set, EDF algorithm is able to find it Earliest Deadline First (EDF) A set of periodic task with deadlines equal to periods is schedulable with the EDF algorithm if and only if the processor utilization factor is less than or equal to 1: n i= 1 C T i i

9 Earliest Deadline First (EDF) A hybrid task set is schedulable with the EDF algorithm if: EDF Example τ 1 (0,3,7,20) τ 2 (0,2.4,5) τ 3 (0,1,8,10) n i= 1 Ci D i 1 Task Nominal Laxity: L Also called slack time L = D C If s: start time of task execution e: finish time of task execution L is the maximum lag for s when the task has sole use of the processor Residual Laxity: L(t) D(t): Residual relative deadline at time t D( t) = dk t 0 D( t) < D C(t): Pending execution time at time t D( t) = dk t 0 D( t) < D Residual Laxity Maximum lag for resuming execution of a task when it has sole use of the processor L( t) = D( t) C( t) = d t C( t) = r + D t C( t) k k Least Laxity First (LLF) Assigns priority to tasks according to their relative laxity The task with the smallest laxity will be executed at the highest priority Optimal The schedulability of a task set can be guaranteed using the EDF schedulability test Least Laxity First (LLF) Relative laxity of ready tasks decreases with time When the laxity of tasks is computed only at arrival times, LLF is equivalent to EDF. More context-switching overhead if the laxity is computed at every time t

10 LLF Example τ 1 (0,3,7,20) τ 2 (0,2.4,5) τ 3 (0,1,8,10) When t=0, L(τ 1 )=7-3=4, L(τ 2 )=4-2=2, L(τ 3 ) = 8-1 = 7 LLF Example When t=5, L(τ 2 )=2, L(τ 3 ) = Task 3 Task Outline In this slide set, we will learn: Hybrid task sets Scheduling of soft aperiodic tasks Background scheduling Task servers Slack stealing and joint scheduling Scheduling of hard aperiodic tasks Acceptance test algorithms Hybrid Task Sets Contain both periodic and aperiodic tasks Periodic tasks can be scheduled with one of the four basic algorithms; i.e., RM, DM, EDF, and LLF How to schedule aperiodic tasks? Aperiodic Tasks Soft aperiodic tasks Aperiodic tasks with soft time constraints Scheduling algorithms aim to provide good average response time for soft aperiodic tasks Hard aperiodic tasks Aperiodic tasks with hard time constraints Scheduling algorithms aims to maximize the guarantee ratio of the aperiodic tasks Scheduling of Soft Aperiodic Tasks Background scheduling Task server Slack stealing and joint scheduling

11 Background Scheduling Aperiodic tasks are scheduled in the background when there are no periodic tasks ready to execute Aperiodic tasks are queued according to a first-come-first-server strategy BG Scheduling Example Periodic tasks: τ 1 (0,2,5,5) τ 2 (0,2,10,10) Aperiodic tasks: τ 3 (4,2) τ 4 (10,1) τ 5 (11,2) Idle Time 62 BG Scheduling Example Idle times: [4,5], [7,10], [14,15], [17,20] Aperiodic tasks: τ 3 (4,2) τ 4 (10,1) τ 5 (11,2) Idle Time Task 3 Task 3 Task 4 Task 5 Aperiodic Tasks Background Scheduling The simplest method for scheduling aperiodic tasks For high loads caused by periodic tasks, response time of aperiodic requests can be high Task 3 Task 4 Task Task Servers A task server is a periodic task to serve aperiodic requests Two primary parameters Period Computation time; also called server capacity Use the same algorithm for the periodic tasks Ordering of aperiodic tasks does not depend on the scheduling algorithm for periodic tasks Several types of task servers: Polling server Sporadic server Polling Server Active at regular intervals equal to its period Serves pending aperiodic request within the limit of its capacity Suspends itself until its next period if no aperiodic requests are pending The time originally reserved for aperiodic tasks is used by periodic tasks

12 Polling Server Example Polling Server Example Periodic tasks: τ 1 (0,3,20,20) τ 2 (0,2,10,10) Aperiodic tasks: τ 3 (4,2) τ 4 (10,1) τ 5 (11,2) (0,3,20,20) Add a task server: τ s (0,2,5,5) Schedulability test: = 0.75 < 3( / 3 1) = (0,2,10,10) Task 3 Task 4 Task 5 Task Server Task 5 (0,2,5,5) Task 3 Task 4 Task 5 Server Capacity Polling Server Main drawback If aperiodic tasks enter the system just after the polling server suspends itself, the aperiodic tasks must wait until the next period of the task server. Sporadic Server The sporadic server preserve its capacity if no aperiodic requests are pending Improved response time for aperiodic tasks The replenishment time is set to t R plus the server period The replenishment amount is set to the capacity consumed by the sporadic tasks Sporadic Server Example Sporadic Server Example Periodic tasks: τ 1 (0,3,20,20) τ 2 (0,2,10,10) Aperiodic tasks: τ 3 (4,2) τ 4 (10,1) τ 5 (11,2) Task server: τ s (0,2,5,5) (0,3,20,20) (0,2,10,10) Task 3 Task 4 Task 5 Task Server Task 5 (0,2,5,5) Task 3 Task 4 Task 5 Server Capacity

13 Hard Aperiodic Task Scheduling Each time a new aperiodic task enters the system, an acceptance test is carried out to verify whether the new task can be scheduled within its deadline and without jeopardizing the deadlines of periodic tasks and previously accepted aperiodic tasks Background Scheduling with Acceptance Test Similar to the background scheduling, Aperiodic tasks are scheduled in the background Periodic tasks are scheduled with the EDF algorithm An on-line acceptance test is executed each time a new aperiodic request enters the system Acceptance Test Algorithm for Background Scheduling Computes the amount of processor idle time between the arrival time of the aperiodic task and its deadline. The amount of idle time must be at least equal to the computation time of the newly arrived aperiodic task If there is enough idle time, then verify that the execution of the new task does not jeopardize the guarantee of previously accepted tasks. If there is no enough idle time or if the new task will jeopardize the previously accepted tasks, the new task is rejected. Otherwise, the new task is accepted. Background Scheduling with Acceptance Test (0,2,4,5) Task 3 (0,1,8,10) Idle Time Task 4 Task 6 Task 5 Ape riodic Tasks Task 4 (4,2,10) Task 5 (10,1,18) Task 6 (11,2,16) Outline In this slide set, we will learn: Starting µc/os-ii Creating tasks Task states Overview Preemptive, priority-driven real-time scheduling Multitasking kernel that can manage up to 64 tasks. Written in C with microprocessor-specific code written in assembly language. Assembly language was kept to a minimum so that µc/os-ii can easily be ported to other target microprocessors Support many platforms: x86, 68x, MIPS, Memory footprint is about 20K for a fully functional kernel Source is open

14 Tasks Manage up to 64 tasks Each task must be assigned a unique priority level from 0 to OS_LOWEST_PRIO-2, inclusively. The lower the priority number, the higher the priority of the task OS_LOWEST_PRIO is reserved for system use 0, 1, 2, 3, OS_LOWEST_PRIO-3, LOWEST_RPIO-2, OS_PRIO- 1 are not recommended OS_MAX_TASKS and OS_LOWEST_PRIO are unrelated A task priority number also serves as the task identifier Always executes the highest priority task ready to run 79 Tasks Multitasking is started by calling OSStart OSStart must be called only once during startup and starts the highest priority task A task can be created prior to the start of multitasking or by another task You must create at least one task before you start multitasking The priority of a task can be changed by OSTaskChangePrio() A task can delete itself (return to dormant state) when done 80 Starting µc/os-ii void main (void) { /* Initialize µc/os-ii */ OSInit();.. /* Create at least 1 task using either OSTaskCreate() or OSTaskCreateExt() */. /* Start multitasking! OSStart will not return */ OSStart(); } Creating a Task A task is created by OSTaskCreate() or OSTaskCreateExt() OSTaskCreate() is backward compatible with µc/os-ii OSTaskCreateExt() is an extended version of OSTaskCreate() OSTaskCreate() requires four arguments task: pointer to the task code pdata: pointer to an argument that is passed to your task ptos: pointer to the top of the stack that is assigned to the task prio: task priority Task Stack Task States Dependent on processors, set OS_STK_GROWTH in OS_CPU.H to determine the direction that a stack grows Pass the lowest memory location of the stack to the task if OS_STK_GROWTH is set to 0 Pass the highest memory location of the stack to the task if OS_STK_GROWTH is set to 1 Example #define TASK_STK_SIZE 512 OS_STK TaskStk[TASK_STK_SIZE]; OSTaskCreate(Task, (void *)0, &TaskStk[TASK_STK_SIZE-1], 0); OSTaskDel OSMboxPost OSQPost OSSemPOst OSTaskCreate Task Task Dormant Ready OSTaskDel Task Waiting OSStart OSIntExit Task Preempted OSMboxPend OSQpend OSSemPend OStimeDly OSTimeDlyHSM Interrupt Task Running OSIntExit() ISR Running OSTaskDel

15 Task States Dormant: A task resides in program space (ROM or RAM) but has not been made available to µc/os-ii Ready: A task is ready to run when it is created. Tasks can be created before multitasking starts or dynamically by a running task Running: If multitasking has started and a task created by another task has a higher priority than its creator, the created task is given control of the CPU immediately There must be only one running task ISR Running: A task is preempted by an interrupt Waiting: A task is waiting for certain events to occur Ready List Ready list consists of two variables, OSRdyGrp and OSRdyTbl[] Task priorities are grouped (eight tasks per group) in OSRdyGrp Each bit in OSRdyGrp indicates when a task in a group is ready to run When a task is ready to run, the corresponding bit in OSRdyTbl[] is also set to Task Scheduling Always executes the highest task ready to run OSSchedLock() function is used to prevent task rescheduling until OSSchedUnlock() is called OSSchedLock() and OSSchedUnlock() must be used in pairs The task that calls OSSchedLock() keeps control of the CPU even though other higher priority tasks are ready to run Interrupts are still recognized and serviced (assuming interrupts are enabled). OSSchedLock() and OSSchedUnlock() must be used with caution 87 Idle Task µc/os-ii always creates a idle task that is executed when none of the other tasks are ready to run The idle task is always set to the lowest priority, OS_LOWEST_PRIO Maximum application tasks are Statistics Task µc/os-ii contains a task that provides run-time statistics The task is called OS_TaskStat() The statistic task is created by µc/os-ii if OS_TASK_STAT_EN is set to 1 in OS_CFG.H When enabled, OS_TaskStat() executes every second and computes the percentage of CPU usage The value is placed in a signed 8-bit integer variable, OSCPUUSage The resolution is 1 percent If your application uses the statistic task, you must call OSStatInit() from the first and only task created in your application during initialization 89 Critical Sections OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL() are always used together Example { OS_ENTER_CRITICAL();.. /* µc/os-ii critical code section */. OS_EXIT_CRITICAL(); } 90 15

16 Critical Sections Your application might crash (hang) if you disable interrupts before calling a service such as OSTimeDly() As a general rule, always call µc/os-ii services with interrupts enabled Obtaining µc/os-ii Version You can obtain the current version of µc/os-ii from your application by calling OSVersion() OSVersion returns the version number multiplied by 100 µc/os-ii version 2.52 is returned as