Operating System 简明教程

Operating System - Process Scheduling

Definition

进程调度是进程管理器的一种活动，负责从CPU中移除正在运行的进程，并根据特定策略选择另一个进程。

The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy.

进程调度是多道程序操作系统的必然部分。此类操作系统允许一次将多个进程加载到可执行内存中，并且已加载的进程使用时分复用共享CPU。

Process scheduling is an essential part of a Multiprogramming operating systems. Such operating systems allow more than one process to be loaded into the executable memory at a time and the loaded process shares the CPU using time multiplexing.

Categories of Scheduling

有两类调度：

There are two categories of scheduling:

Process Scheduling Queues

操作系统在进程调度队列中维护所有进程控制块（PCB）。操作系统为每个进程状态维护一个独立的队列，并且相同执行状态下的所有进程的PCB被放入同一队列中。当进程的状态更改时，其PCB将从其当前队列中取消链接并移至其新的状态队列。

The OS maintains all Process Control Blocks (PCBs) in Process Scheduling Queues. The OS maintains a separate queue for each of the process states and PCBs of all processes in the same execution state are placed in the same queue. When the state of a process is changed, its PCB is unlinked from its current queue and moved to its new state queue.

操作系统维护着以下重要的进程调度队列−

The Operating System maintains the following important process scheduling queues −

Job queue − This queue keeps all the processes in the system.
Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.
Device queues − The processes which are blocked due to unavailability of an I/O device constitute this queue.

操作系统可以使用不同的策略来管理每个队列（FIFO、循环优先级等）。操作系统调度程序确定如何在就绪队列和运行队列之间移动进程，这在系统上的每个处理器核上只能有一个条目；在上图中，它已与CPU合并。

The OS can use different policies to manage each queue (FIFO, Round Robin, Priority, etc.). The OS scheduler determines how to move processes between the ready and run queues which can only have one entry per processor core on the system; in the above diagram, it has been merged with the CPU.

Two-State Process Model

两态进程模型是指下面描述的运行态和非运行态：

Two-state process model refers to running and non-running states which are described below −

S.N.

State & Description

Running When a new process is created, it enters into the system as in the running state.

Not Running Processes that are not running are kept in queue, waiting for their turn to execute. Each entry in the queue is a pointer to a particular process. Queue is implemented by using linked list. Use of dispatcher is as follows. When a process is interrupted, that process is transferred in the waiting queue. If the process has completed or aborted, the process is discarded. In either case, the dispatcher then selects a process from the queue to execute.

Schedulers

调度程序是特殊系统软件，以各种方式处理进程调度。它们的主要任务是选择要提交给系统的作业并决定运行哪个进程。调度程序有三种类型：

Schedulers are special system software which handle process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. Schedulers are of three types −

Long-Term Scheduler
Short-Term Scheduler
Medium-Term Scheduler

Long Term Scheduler

它也被称为 job scheduler 。长期调度程序确定哪些程序被允许进入系统进行处理。它从队列中选择进程并将它们加载到内存中以执行。进程加载到内存中以进行 CPU 调度。

It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling.

作业调度程序的主要目标是提供平衡的作业组合，如 I/O 绑定和处理器绑定。它还控制多道程序设计的程度。如果多道程序设计的程度稳定，那么进程创建的平均速率必须等于离开系统的进程的平均离开率。

The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.

在某些系统上，长期的调度程序可能不可用或是最小的。分时操作系统没有长期调度程序。当进程将状态从新建更改为就绪时，则使用长期调度程序。

On some systems, the long-term scheduler may not be available or minimal. Time-sharing operating systems have no long term scheduler. When a process changes the state from new to ready, then there is use of long-term scheduler.

Short Term Scheduler

它也被称为 CPU scheduler 。其主要目的是根据所选的一组标准来提高系统性能。它是进程从就绪态变为运行态的变化。CPU 调度程序从准备执行的进程中选择一个进程并将 CPU 分配给其中之一。

It is also called as CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.

短期调度程序（也称为分派程序）决定接下来执行哪个进程。短期调度程序比长期调度程序更快。

Short-term schedulers, also known as dispatchers, make the decision of which process to execute next. Short-term schedulers are faster than long-term schedulers.

Medium Term Scheduler

中期调度是 swapping 的一部分。它从内存中删除进程。它降低了多道程序设计的程度。中期调度程序负责处理换出进程。

Medium-term scheduling is a part of swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium-term scheduler is in-charge of handling the swapped out-processes.

如果一个正在运行的进程发出 I/O 请求，则该进程可能会被挂起。被挂起的进程无法继续执行。在这种情况下，为了从内存中删除进程并为其他进程腾出空间，将被挂起的进程移到辅助存储器中。此过程称为 swapping ，并且该进程据说是被换出或注销的。交换可能需要改进进程组合。

A running process may become suspended if it makes an I/O request. A suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other processes, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix.

Comparison among Scheduler

S.N.

Long-Term Scheduler

Short-Term Scheduler

Medium-Term Scheduler

It is a job scheduler

It is a CPU scheduler

It is a process swapping scheduler.

Speed is lesser than short term scheduler

Speed is fastest among other two

Speed is in between both short and long term scheduler.

It controls the degree of multiprogramming

It provides lesser control over degree of multiprogramming

It reduces the degree of multiprogramming.

It is almost absent or minimal in time sharing system

It is also minimal in time sharing system

It is a part of Time sharing systems.

It selects processes from pool and loads them into memory for execution

It selects those processes which are ready to execute

It can re-introduce the process into memory and execution can be continued.

Context Switching

上下文切换是将 CPU 的状态或上下文存储和恢复到进程控制块中的机制，以便稍后可以从同一位置恢复进程执行。使用此技术，上下文切换器使多个进程能够共享单个 CPU。上下文切换是多任务操作系统功能的本质部分。

A context switching is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. Using this technique, a context switcher enables multiple processes to share a single CPU. Context switching is an essential part of a multitasking operating system features.

当调度程序将 CPU 从执行一个进程切换到执行另一个进程时，从当前正在运行的进程中的状态将被存储到进程控制块中。此后，要执行的下一次进程的状态将从其自身的 PCB 中加载并用于设置 PC、寄存器等。在这一点上，第二个进程可以开始执行。

When the scheduler switches the CPU from executing one process to execute another, the state from the current running process is stored into the process control block. After this, the state for the process to run next is loaded from its own PCB and used to set the PC, registers, etc. At that point, the second process can start executing.

上下文切换在计算上是密集的，因为必须保存和还原寄存器和内存状态。为了避免上下文切换时间，一些硬件系统采用两组或更多组处理器寄存器。当进程切换时，以下信息将被存储以供以后使用。

Context switches are computationally intensive since register and memory state must be saved and restored. To avoid the amount of context switching time, some hardware systems employ two or more sets of processor registers. When the process is switched, the following information is stored for later use.

Program Counter
Scheduling information
Base and limit register value
Currently used register
Changed State
I/O State information
Accounting information