本文是计算机专业的留学生作业范例，题目是“Characteristics of Real Time Systems（实时系统的特性）”，在本文中，我们描述了可表示为时间函数的实时系统的显著特征。本文也以实时操作系统的时间驱动模型的形式进行了描述，并提供了一个测量实时系统有效性的工具。对于这个模型，我们已经生成了一个实时系统，在这个系统中我们测量了许多著名的调度算法。为了满足任务调度的实时性约束，采用了不同的调度算法。实时系统大多采用基于优先级的抢占式调度和最坏情况下的执行时间。
Abstract 摘要
In this paper we describe the distinguished characteristics of real time systems which can be expressed as a function of time. This paper is also described in the form of a time driven model for a real time operating system and also provides a tool for measuring the effectiveness of a real time system. For this model, we have generated a real time system in which we measure a number of well known scheduling algorithms. To meet the real time constraints for scheduling the task different algorithms were used. Most of the real time system are designed using prioty based preemptive scheduling and worst case execution time.
Scheduling refers to the set of policies and mechanism to control the order of work to be performed by a computer system. process scheduling in real time system has almost used many more algorithms like FIFO, Round ROBIN, Uniprocessor Multiprocessor etc. The choices for these algorithms are simplicity and speed in the operating system but the cost to the system in the form of reliability and maintainability have not been assessed.
调度是指控制计算机系统工作顺序的一组策略和机制。在实时系统中，进程调度几乎使用了更多的算法，如FIFO、轮询、单处理器多处理器等。这些算法的选择是操作系统的简单性和速度，但对系统的可靠性和可维护性的成本尚未评估。
1.INTRODUCTION引言
In real time operating systems like embedded system the scheduler also perform that the process can meet deadline that is necessary for stabling the system. Schedular are sent to mobile devices and managed by an administrative back end. A common characteristic of many real-time systems is that their requirements specification includes timing information in the form of deadlines. The time taken to complete an event is mapped against the “value”this event has to the system. Here “value” is loosely defined to mean the contribution this event has to the system’s objectives. The mapping of time to value between start-time and deadline is application dependent.
在嵌入式系统等实时操作系统中，调度器也执行进程能够满足稳定系统所需的最后期限的任务。日程表发送到移动设备，并由管理后端进行管理。许多实时系统的一个共同特征是，它们的需求规范包括期限形式的时间信息。完成一个事件所花费的时间将根据该事件的“值”映射到系统。这里的“值”松散地定义为该事件对系统目标的贡献。开始时间和截止时间之间的时间和值的映射依赖于应用程序。
In computer science, scheduling is the procedure by which threads, process or given the access to system resources. The scheduling is mainly concerned with these things throughout:
Total number of processes that complete their execution per unit time.
Response time: when a request was submitted what amount of time takes when first response produced.
Turnaround time: total time between completion and submission of process.
Fairness: equal time to each process.
2.Scheduling调度
In a general purpose computer system a scheduler is considered good if it is fair and gives execution time to all tasks equally. When scheduling a real-time system what is most important is that all tasks meet their deadlines and are executed so that any task depending on them, meet their deadlines as well.
在一般用途的计算机系统中，如果调度程序是公平的，并且给所有任务同等的执行时间，那么它就被认为是好的。当调度一个实时系统时，最重要的是，所有的任务都要在截止日期前完成，并且要执行，这样任何依赖于它们的任务也都要在截止日期前完成。
Types of scheduling
Network scheduling
DISK Scheduling
job scheduling
Manual scheduling
Multilevel queue scheduling
First in first out
Process scheduling
Process scheduling is divided into four main types
1 long term scheduling which determines which programs are admitted to the system for execution and when, and which ones should be exited. Concept of scheduling in real time system
2 medium scheduling: Which determines when processes are to be a suspended and remain.
3 short term scheduling ordispatcher which determine which process have cpu resources and for how long.
Process scheduling in computer operating system is instance of extensively studied problem from operation research which in form of producing a sequence of jobs which must a common resource. all decisions should be made dynamically for example all jobs have to be scheduled and processing time requirements are available at the start of sequence time =0 the new job will not come during processing. if it happens the priviouly computed job is invalidated and scheduling must be started overif is maintained. Now process scheduling in real time system can categorized into two categories:
1 periodic
2 aperiodic
Periodic processes: which arrive at regular intervals are called periodic process and aperiodic are those which arrive at irregular. the main difference between real time systems and other computer system have not understood. the time to complete a process is important in all computer systems but in real time response time play a cruicial part in the correctness of the application software
Real-time systems are divided into two classes: Hard real -time systems and soft real -time systems.
Hard real-time systems are those whose deadlines must absolutely be met and system will be considered to have failed whereas soft real-time systems allow for some more deadlines, at least occasionally, to be missed with only adegradation performance but not a complete failure of the system. In judge a number of existing real time systems we study the DMA cycling ,interrupt processing blocking ang non-blocking. The deadline scheduler gives no reasonable control over the choice of which deadlines are delayed and which lead to unperictable failures result to the impact on reliability and maintainability of the real time system. real process completion is handled by step function in which there is no any value in completing the process after its deadline the characteristic of a real-time system is that correctness is determined not only by what is done, but when itis done, we propose to use a representation of a process completion value to measure the algorithms in real time system.
Computational model consists a set of processes every process has a request time R ,Time interval c and avalue function R.
Its value function become zero or negative. the value function may be negative at R,not rise above zero the request time R may be future time or past time . if request time r is future time then process is not scheduable but attributes in computations asre load in which current scheduling decisions are made classical algorithms are.
Deadline:The earlist critical time in process at each decision point
FIFO:The longest request set is executed in process at decision point
Random:chosen from the request set and executed
stack:the process with the smallest stack time is executed in each decision point
SPT:The shortest completion time is executed in each decision point.
There are many approaches for utilizing a time driven model in real time system. real time operating system support or modify the value for the process or the set of processes during run time. In this way application designr can set and modify scheduling policy for various systems. For describing these processes we assume primtives to creat and kill processes already exist. There are three real time interfaces
Time control primitive: The arguments of these operating systems communicate the information needed to implement the model but the issue is the structure of informative that passed to the operating system. In single primitive each parameter would be flexible but in user might set inconsistent parametres.
Scheduling policies: In real time operating system it is compulsory to provide a mechanism to express the scheduling model to implentour model. the system should also able to modify these policies to take advantage or flexibility of the system.
Periodic policies: There is one way to describe a periodic policies to using optional arguments in a creative process. The creative Process make new instance of process at a specific node 5 ways for scheduling the task in real time systems.
3.Real time system and scheduling techniques实时系统与调度技术
Real time scheduling techniques are divided into two main categories. One is called static scheduling techniqueAnd other is called dynamic scheduling technique. Dynamic may be static perioty or dynamic perioty. Static prioty is divided into two types
实时调度技术主要分为两大类。一种是静态调度技术，另一种是动态调度技术。动态可以是静态的或动态的。静态优先级分为两种类型
1 rate monolithic : rate-monotonic scheduling is a scheduling algorithm used in real- time operating systems with a static-priority scheduling class. [2]
The static priorities are assigned on the basis of the cycle duration of the job: the shorter the cycle duration is, the higher is the job’s priority.
These operating systems are generally preemptive and have deterministic guarantees with regard to response times. Rate monotonic analysis is used in conjunction with those systems to provide scheduling guarantees for a particular application.
2 deadline monolithic : Dynamic prioty is also divided into two types
1)earlist time first
2)least stack time first
4.System and the task model系统和任务模型
Each type and unit of work that is scheduled and execute the system as a job. ALL the tasks are taken to be periodic. the system knows all the things about arrival time ,periodexecution time. the task are ready to execute if it arrives the system. IN soft real time system each task has a real positive value. The main goal of the task is to obtain a value as much as possible.
作为作业调度和执行系统的每种工作类型和工作单元。所有的任务都是周期性的。系统知道所有关于到达时间，周期执行时间的事情。如果任务到达系统，则该任务已准备好执行。在软实时系统中，每个任务都具有实际的正值。任务的主要目标是获得尽可能多的值
There are the two conditions if the task succeed the system acquire that value if the task is not succeed the ystem gain less value in a special case like soft real systems the task has nothing a vale
5.Basic requirements of schedulars in real time operating system实时操作系统中调度程序的基本要求
There are five basic requirements of scheduler in real time operating system .Multitasking and preemptable In real time operating applications real time operating system should be multitask and preemtable. the scheduler are able to preempt any kind of task in the system and give the resources to task that the system needs it .Dynamic deadline identification With the earlist deadline RTOS should be able to identify the task. deadline information may be converted to prioty levels for resource allocation .Predictable synchronization To communicate multiple threads among themselves in a timely fashionsynchronization mechanism also reqired and also the abiliy to lock or unlock is the resource to achieve dta integrity. Sufficient perioty levels The real time operating systems must also have a sufficient number of priority levels for effective implementation. Namelypurety,inheritance,ceiling protocol need sufficient prioty levels .Predefined latencies the timing of system call define the following specifications.
在实时操作系统中，调度程序有五个基本要求。在实时操作应用中，实时操作系统应该是多任务和可抢占的。调度器能够抢占系统中任何类型的任务，并将资源分配给系统需要的任务RTOS应该能够通过最早的截止日期列表来识别任务。截止日期信息可以转换为资源分配的优先级为了及时地在多线程之间进行通信，同步机制也需要，并且能够锁定或解锁是实现dta完整性的资源。足够的周期级别实时操作系统还必须有足够数量的优先级级别来有效实现。即纯粹，继承，天花板协议需要足够的优先级级别、预定义的延迟系统调用的时间定义了以下规范。
Task awitching latency :time to save the context of a current execution time and switch to another
Interrupt latency:the time elaped between first instruction of the handler and execution time of the last instruction of the interrupt task
Dynamic scheduling algorithm: Dynamic algorithm at runtime assign perioties based on the execution parametres of tasks the most important dynamic scheduling with puriotysscheduling algorithms are
1 EArlist deadline first algorithm The perioty of each task based on the value of itsdeadlinethe algorithm is simple and preemptive.
2 ACO Based scheduling algorithms. The ACO algorithms are computational models for the collective foraging behavior of ants . Ant is an agent that generate a path. ANT do not need synchronization. ant moves to the good looking neighbor for the crrent node probabaisatically
6.Time Triggered时间触发
When scheduling a distributed system using offline scheduling the whole system including the communication is scheduled before the start of the system giving a very rbust system but the cost of adding a new node is high, complete rescheduling of the whole system. To schedule a time-triggered distributed real-time system we use the same techniques as we did with the single processor system, but understandably with more complex graphs. For example we might have a precedence graph, where one task is preceded by several tasks on different nodes. Making a schedule that have a task precede by tasks on several nodes requires more of the system then of the schedule, even the best schedule will fail if the nodes time references is not synchronized and tasks are not executed in time. This requires all nodes to synchronize time with each other. This can be done at an application level like in normal distributed systems, with a common time reference with a communication protocol that handles time synchronization TTCAN, TTP and FlexRay to namea few.
当使用脱机调度分布式系统时，整个系统包括通信在系统启动之前就被调度，这给了一个非常健壮的系统，但增加一个新节点的成本很高，整个系统完全重新调度。为了调度一个时间触发的分布式实时系统，我们使用了与单处理器系统相同的技术，但可以理解的是，我们使用了更复杂的图形。例如，我们可能有一个优先图，其中一个任务前面有多个不同节点上的任务。如果在多个节点上先执行一个任务，然后再执行另一个任务，则需要更多的系统时间，如果节点的时间引用没有同步，任务没有及时执行，即使是最好的调度也会失败。这要求所有节点之间同步时间。这可以像正常的分布式系统一样在应用程序级别上完成，使用一个通用的时间引用和一个通信协议来处理时间同步，TTCAN, TTP和FlexRay等等。
We have looked at the most commonly used scheduling techniques and communication protocols used in distributed realtimesystems. When we started this paper we where looking at doing a survey of everything related to distributed real-time systems,but found that to be a to vast and growing field so we narrowed our field to communication protocols and scheduling algorithms used in distributed real-time systems. During our research for this paper we have been looking atschedulers that are created to work better with the FlexRaycommunications bus or in a similar fashion. We have taken a real interest in distributed real-time systems and are looking forward to seeing the advances in scheduling and communication for distributed real-time systems and maybe one day join the research field our selves. With this paper we hoped to introduce the reader to the problem of scheduling real-time tasks in distributed systems. We presented the different interpretations of the problem and the various options available to the solution designers. Our analysis of some of the existing scheduling algorithms tried to focus on the affect of the specific problem on the choices made in the solution. We hope that what we presented provides the reader with a broad understanding of the problem and a range available solutions. This paper was also aimed at providing the reader with a solid foundation for further research on the subject. Finally, we suggested possible future research directions.
我们已经研究了分布式实时系统中最常用的调度技术和通信协议。当我们开始这篇文章的时候，我们想要做一个关于分布式实时系统的调查，但是发现这是一个巨大的和不断增长的领域，所以我们把我们的领域缩小到分布式实时系统中使用的通信协议和调度算法。在本文的研究过程中，我们一直在研究如何创建调度程序，以便更好地与FlexRaycommunications总线或以类似的方式工作。我们对分布式实时系统非常感兴趣，并期待着看到分布式实时系统在调度和通信方面的进步，也许有一天我们自己也会加入这个研究领域。在本文中，我们希望向读者介绍分布式系统中实时任务调度问题。我们给出了对问题的不同解释，以及可供解决方案设计者使用的各种选项。我们对现有的一些调度算法的分析试图集中在特定问题对解决方案中所做的选择的影响。我们希望我们所呈现的内容能够让读者对问题有一个广泛的理解，并提供一系列可用的解决方案。本文也旨在为读者进一步研究这一课题提供一个坚实的基础。最后，提出了未来可能的研究方向。
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