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1、 Wind River Systems, Inc. 1997,Chapter - 5,Real-Time Multitasking,5-2,Real-Time Multitasking,Introduction Task Basics Task Control Error Status System Tasks,5-3,What is Real-Time ?,A system is described as being deterministic if its response time is predictable.,Real-time denotes the ability of the
2、control system to keep up with the system being controlled.,The lag time between the occurrence of an event and the response to that event is called latency.,Deterministic performance is key to real-time performance.,5-4,Requirements of a Real-time System,Ability to control many external components:
3、,Fast response,High speed execution:,Low overhead,Independent components,Asynchronous components,Synchronous components,A late answer is a wrong answer.,Deterministic operation:,5-5,Design Options,shoulder,elbow,wrist,knuckle1,knuckle2,5-6,Unitasking Approach,One task controlling all the components
4、in a loop. arm () for (;) if (shoulder needs moving) moveShoulder(); if (elbow needs moving) moveElbow(); if (writst needs moving) moveWrist(); . ,5-7,Multitasking Approach,Create a separate task to manipulate each joint: Joint () for (;) wait; /* until this joint needs moving */ moveJoint(); ,Each
5、task alternates between “ready” and “waiting”.,VxWorks allows a task to wait for:,A specific time delay.,An event (e.g., an interrupt).,5-8,Task States,PENDED,SUSP & PENDED,SUSPENDED,SUSP & DELAYED,DELAYED,READY / EXEC,5-9,Multitasking Kernel,Manages tasks.,Transparently interleaves task execution,
6、creating the appearance of many programs executing simultaneously and independently.,Uses Task Control Blocks (TCBs) to keep track of tasks.,One per task.,Defined by WIND_TCB data structure defined in taskLib.h.,O.S. control information e.g., task priority, delay timer, I/O assignments for stdin, st
7、dout, stderr,CPU Context Information e.g., PC, SP, CPU registers, FPU registers,5-10,Kernel Function,Kernel,TCB,TCB,TCB,Suspended,Pended,Delayed,Ready,CPU,Executing,5-11,Context Switch,To schedule a new task to run, the kernel must:,Complete context switch must be very fast.,Save context of old exec
8、uting task into associated TCB.,Restore context of next task to execute from associated TCB.,5-12,Priority Scheduling,Different application jobs may have different precedences, which should be observed when allocating the CPU.,Preemptive scheduling is based on task priorities chosen to reflect job p
9、recedence,The highest priority task ready to run (not pended or delayed) is allocated the CPU.,Rescheduling can occur at any time as a result of:,Kernel calls,Interrupts (e.g.,System clock tick),Context switch latency is minimized by invoking scheduler when state transitions occur.,5-13,Priority Bas
10、ed Preemption,High Priority Task A,Medium Priority Task B,Low Priority Task C,Equal priority tasks wont preempt each other (by default).,Event,Event,5-14,Round-Robin Scheduling,Time Slice Period (Constant - Programmable),5-15,Kernel Time Slicing,To allow equal priority tasks to preempt each other, t
11、ime slicing must be turned on: kernelTimeSlice (ticks) If ticks = 0, time slicing turned off.,Priority scheduling always takes precedence.,Round-robin only applies to tasks of the same priority.,Priority based rescheduling can happen any time.,Round-robin rescheduling can only happen every few clock
12、 ticks.,5-16,Performance Enhancements,All tasks reside in a common address space.,All tasks run in supervisor (privileged) mode.,text,data,bss,RAM,fooLib,int fooVal; void fooSet (int x) fooVal = x; ,fooSet (4),tTaskA,fooSet (99),tTaskB,5-17,Multitasking Facilities,Interrupt,Device,Device,Network,Tas
13、k,ISR,Task,Interrupt,Task,Buffer,Task,5-18,How VxWorks Operating System Meets Real-time Requirements,High speed execution,Tasks are cheap (light-weight).,Deterministic operation.,Preemptive priority scheduling assures response for high priority tasks.,Able to control many external components.,Multit
14、asking allows solution to mirror the problem.,Independent functions are assigned to different tasks.,Intertask communication allows tasks to cooperate.,Fast context switch reduces system overhead.,5-19,Real-Time Multitasking,Introduction Task Basics Task Control Error Status System Tasks,5-20,Overvi
15、ew,Low level routines to create and manipulate tasks are found in taskLib.,Do not confuse executable code with the task(s) which execute it.,A VxWorks task consists of:,A stack (for local storage of automatic variables and parameters passed to routines).,A TCB (for OS control),Code is downloaded bef
16、ore tasks are spawned.,Several tasks can execute the same code (e.g., printf().,5-21,Creating a Task,taskSpawn,foo () . ,Stack,TCB PC,5-22,Creating a Task,int taskSpawn (name, priority, options, stackSize, entryPt, arg1, ., arg10) name Task name, if NULL gives a default name. priority Task priority, 0-255. options Task options e.g. VX_UNBREAKABLE. stackSize Size o
