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Scheduling

Key to multi-programming 多道程序设计的关键是调度 Four types of scheduling 调度有 4 种类型

Long term 长项调度 Medium term 中项调度 Short term 短项调度 I/O I/O 调度

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Scheduling

Process: 进程定义 A program in execution 一个运行的程序 The “animated spirit” of a program 程序的“活灵魂” That entity to which a processor is

assigned 处理器分配的实体

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Long Term Scheduling

Determines which programs are submitted for processing 确定哪一些程序被提交到系统处理 i.e. controls the degree of multi-

programming 它控制多道程序设计的度 ( 内存中的进程数）

Once submitted, a job becomes a process for the short term scheduler

一旦提交，作业就成为进程加入到短项调度程序的队列中等待短项调度(or it becomes a swapped out job for the medium term scheduler)

加入到中项调度程序的队列中

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Long Term Scheduling

Batch system 在批处理系统中 newly submitted jobs are held in a batch

queue 新提交的作业保存在批处理队列中 Long term scheduler creates processes from

the queue 长项调度程序从队列中选择任务为其创建进程 Criteria include priority, expected execution

time, I/O requirements 选择采用的标准有：优先级、期望的执行时间和 I/O 需求

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Long Term Scheduling

Time sharing system 分时系统中 When a user attempts to connect to the

system, a process request is generated 当用户希望连接系统时，产生一个进程请求 Operate system will accept all authorized

comers 分时用户不是简单地排队等待，而是操作系统接收所有允

许的请求 “the system is full and user should try again

later” when system is saturated 当系统饱和时连接请求会遇到“系统已满和以后再试”的信

息

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Medium Term Scheduling

Part of the swapping function (later…) 中项调度是 7 ． 3 节内存管理将介绍的换入裁决的一部分 Usually based on the need to manage multi-

programming 一般，换入裁决是基于多道程序“度”的管理需要 With or without virtual memory, memory

management is an issue 无论有没有虚拟存储系统，存储管理都是一个重要问题，

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Short Term Scheduler

Dispatcher （也称派遣程序） Execute frequently 执行频繁 Fine grained decisions of which job to execute next 并细粒度地判决下次执行哪个作业

i.e. which job actually gets to use the processor in the next time slot

在下一个时间片哪个作业将获得处理器的使用权

High-level scheduler 高级调度程序 Execute relatively infrequently 执行相对较少 Coarse grained decision of whether or not to take on

a new process 粗粒度地决定是否接受一个新进程

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Process States

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Process Control Block 进程控制块（ PCB ）

Identifier 标识符State 状态 Priority 优先级Program counter 程序计数器Memory pointers 存储器指针Context data 现场数据I/O status I/O 状态信息Accounting information 统计信息

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PCB Diagram

When the scheduler accepts a new job, it creates a blank process control block and places the associated process in the new state.After the system has properly filled in the process control block, the process is transferred to the ready state.

当调度程序接收到一个新的作业时，它产生一个空的进程控制块，并使相关进程处于新的状态。当系统填完 PCB 后，进程进入就绪状态。

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Process Scheduling

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Memory Management

Uni-program 在单道程序设计系统中 Memory split into two 主存划分成两大部分 One for Operating System (monitor) 一部分给操作系统 ( 常驻监控程序 ) One for currently executing program 另一部分结当前正在执行的程序

Multi-program 在多道程序设汁系统中 “User” part is sub-divided and shared among

active processes 存储器的“用户”部分必须进一步划分以适应多个进程

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Swapping Problem: I/O is so slow compared with CPU that

even in multi-programming system, CPU can be idle most of the time

由于处理器速度比 I／ o 快得多，因此，即使在多道程序设计中处理器仍可能有许多时间处于空闲状态。

Solution Increase the degree of multiproramming 增加

多道程序度 Approaches 解决办法

Increase main memory 扩充主存Expensive 主存昂贵Leads to larger programs 程序对主存容量需求增

长更快 Swapping 交换技术

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What is Swapping? Long term queue of processes stored on disk, Processes

are brought in as space becomes available, As a process completes it is moved out of main memory

有一个进程请求的长项队列通常存储在磁盘上，当主存有空间时，进程被调入，每次调入一个。当进程完成时，移出主存。

If none of the processes in memory are ready (i.e. all I/O blocked)

当存储器中无进程处于就绪状态时 ( 如： I/O 阻塞 ) Swap out a blocked process to intermediate queue 处理器把这些进程中的一个调回磁盘，排人中间队列 Swap in a ready process or a new process 然后从中间队列中调入另一个进程执行 But swapping is an I/O process... 然而，交换是—种 I/O 操作

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Disk Storage

Long-Term Queue

Main MemoryOperating

System

Completed Job

Disk Storage

Long-Term Queue

Main Memory

Operating System

Completed Job

Intermediate Queue

Swapping

Simple Job Scheduling Swapping

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Partitioning

Splitting memory into sections to allocate to processes (including Operating System)

将内存分成几部分，分配给多个进程，包括操作系统

Fixed-sized partitions 定长分区 May not be equal size 分区是固定长度，但是大小可以不等 Process is fitted into smallest hole that will take it

(best fit) 当一个进程调入主存时，分配给它一个能容纳它的最小的分区 Some wasted memory 但是主存仍然有浪费 Leads to variable-sized partitions 产生了变长分区

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FixedPartitioning

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Variable Sized Partitions (1)

Allocate exactly the required memory to a process 当—个进程调入主存时，分配的分区大小与进程所需大小一样 This leads to a hole at the end of memory, too

small to use Only one small hole - less waste 开始所有程序按顺序连续分配内存，只在存储器末端留下一空块

，浪费少 When all processes are blocked, swap out a

process and bring in another 当所有进程都阻塞后，换出一个进程，然后换进一个新的 New process may be smaller than swapped out

process 新换进来进程的大小可能小于换出后留下的空间 Another hole 这样就又形成另一个空块

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Effect of Dynamic Partitioning

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Variable Sized Partitions (2)

Eventually have lots of holes (fragmentation)

最后内存中会留下很多空块（碎片）Solutions:

Compaction - From time to time go through memory and move all hole into one free block (c.f. disk de-fragmentation)

为了解决这个问题．采用一种紧缩技术。 操作系统一次次移动存储器中的进程，把所有空闲的空

间放在起，组成新的块。如计算机磁盘碎片整理

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Relocation 重定位 No guarantee that process will load into the

same place in memory 当进程它换入时一般都分配到不同的分区

Instructions contain addresses 指令中包含地址 Locations of data 数据地址 Addresses for instructions (branching) 指令地址 ( 分支指令 )

Logical address - relative to beginning of program

逻辑地址表示相对于程序起始的单元 Physical address - actual location in

memory (this time) 物理地址是主存中的一个实际单元地址 Automatic conversion using base address 当处理器执行进程时，通过把当前进程的起始单元地址 (

称为基地址 ) 加到每个逻辑地址上，自动地把逻辑地址转换成物理地址。

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Paging 分页 Split memory into equal sized, small chunks -page

frames 把存储器分成相等的固定长且比较小的存储块（页帧） Split programs (processes) into equal sized small

chunks – pages 把每个进程也划分成小的固定长的程序块 （页）

Allocate the required number page frames to a process

为每个程序分配必须数量的页帧 Operating System maintains list of free frames 操作系统负责维护空闲帧列表 A process does not require contiguous page frames 一个进程并不需要连续的页帧 Use page table to keep track 它通过页表进行维护

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Logical and Physical Addresses

frame

0123

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Paging Example

The size of a physical main memory is 128 bytes, A process has 8 pages, and each page has 4 bytes

a) What is the length of a logical address?

b) What is the length of a physical address?

c) Given the following page table, what is the physical address of logical address 13?

01234567

2 4 830 5151627

Page Table

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Paging Example

c) logical address 13 = 011 01 physical address = 11110 01 = 121

Ans: a) page# offset

page#: 3 bits, offset: 2 bits 5 bits

b) frame# offset

frame#: 5 bits, offset: 2 bits 7 bits

01234567

2 4 830 5151627

Page Table

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Virtual Memory 虚拟存储器 Demand paging （请求分页）

Do not require all pages of a process in memory, bring in pages as required

一个进程的所有页不需要都装入内存，每个页只在需要时调人即可 Page fault 缺页处理

Required page is not in memory当所需要的页不在内存时

Operating System must swap in required pageOS 需要将其调入

May need to swap out a page to make space这时可能因为没有空的页帧而需要调出一页

Select page to throw out based on recent history要调出的页可以根据最近使用的原则进行选择

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Thrashing （系统抖动） Too many processes in too little memory,

Operating System spends all its time swapping, Little or no real work is done, Disk light is on all the time

当有大量进程在内存时， OS 将忙于进行页交换，真正用于工作的时间就会减少，硬盘也会不停的工作

Solutions Good page replacement algorithms 好的页替代算法

Reduce number of processes running 减少运行的进程数 More memory 增大内存

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Bonus of Virtual Memory 虚存的优点 We do not need all of a process in memory for it

to run. We can swap in pages as required 一个进程不需要全部装入主存就可以运行。可以在需要时换入

So - we can now run processes that are bigger than total memory available!

可以运行大于整个主存的进程

Main memory is called real (or physical) memory 主存称为实 ( 物理 ) 存储器 User/programmer sees much bigger memory -

virtual memory 程序员 /用户看到一个分配在磁盘上的太得多的存储器称为虚拟

存储器

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Page Table Structure 页表结构

Problem Each process needs one page table 一个进程需要一张页表 Page tables need to be stored in main

memory 页表必须驻留主存 E.g., 231 virtual memory per process

page size = 29 bytes 231/ 29 = 222 page table entries too big

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Page Table Structure

Solutions Page page tables ( 将页表分页） Multi-level scheme ( 多级分页 ) Inverted page table ( 倒置分页 )

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Page Table Structure

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Translation Lookaside Buffer（转换后备缓冲）

Each virtual memory reference causes two physical memory accesses (why?)

每次虚拟存储器的访问要产生两次物理存储器存取 Fetch page table entry 一次是获取相应的页表项 Fetch data 另一次是获取所需的数据

Solution: TLB Special cache for page table entries recently

used用来存储最近使用的页表项的特殊 cache

For a memory access CPU checks the TLB first 首先 CPU 查询 TLB If not found then access page table (in memory) 如果不出现，则从内存页表中取项 Then access what we want 然后查找我们所需要的数据

P257

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TLB and Cache Operation

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Segmentation 分段

Paging is not (usually) visible to the programmer 分页对程序员来说是不可见的 Segmentation is visible to the programmer 分段可见 Segment: variable, dynamic size 段的长度是可变的动态的 Usually different segments allocated to program and

data 通常把程序和数据分配给不同的段 May be a number of program and data segments 各种类型的程序可以有许多程序段和数据段 Some systems combine segmentation with paging 有些系统装有提供分页和分段两种方法

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Pentium II

Hardware for segmentation and paging 包含用于分段和分页的硬件 Unsegmented unpaged 未分段未分页存储器 Unsegmented paged 分页未分段存储器 Segmented unpaged 分段未分页存储器 Segmented paged 分段分页存储器

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Pentium II Address Translation Mechanism

16 32

10 10 12

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Pentium II Segmentation（ 1 ）

Each virtual address is 16-bit segment and 32-bit offset

每个虚拟地址由一个 16 位段引用和一个 32 位偏移量组成

2 bits of segment are protection mechanism

段中两位用于保护机制14 bits specify segment 余下的 14 位表示一个具体的段

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Pentium II Segmentation（ 2 ） Unsegmented virtual memory 232 = 4Gbytes 对无分段的存储器，用户的虚拟地址是 232 = 4G 字节 Segmented 246=64 terabytes 对有分段的存储器，用户虚拟空间为 246=64T 字节

Can be larger – depends on which process is active 根据当前哪个进程是活动的， 虚拟存储器容量实际能大于

64T字节， 并被分成两部分 Half is global 一半是全局的 ，被所有进程共享 Half is local and distinct for each process， 另一半是局部的．每个进程都不同。

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Pentium II Segmentation

P1空间

P2空间

全局共享虚拟空间 物理存储器

Half (8K segments of 4Gbytes) is global

处理器 P1 的私用虚拟空间 物理存储器

处理器 P2 的私用虚拟空间

Half is local and distinct for each process

Synonym problem

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Pentium II Protection

Each segment has two forms of protection:

与每段有关的两种保护方式是 Privilege level 优先级 Access attribute 存取属性

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Pentium II Protection – privilege（ 1 ）Privilege level

Protection bits give 4 levels of privilege from 0 most protected to 3 least ;

保护优先级有 4 种，从最高 (0级 ) 到最低 (3级 )

An executing program may only access data segments for which its privilege level is ≤ the privilege level of the data segment.

当正在执行的程序许可级低于 ( 更高特权 ) 或等于 ( 相同特权 ) 数据段的优先级时，它可存取此数据段。

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Pentium II Protection – privilege（ 2 ） Privilege level

Usually level 3 for applications, level 1 for O/S and level 0 for kernel (level 2 not used)

通常 3 级用于应用程序，优先级 1 用于操作系统， 0 级用于小部分专用于存储管理、保护和存取控制的操作系统内核， 2 级未用

Level 2 may be used for apps that have internal security e.g. database

2 级可以用于要实现自己的安全机构的特殊且必须受保护的应用子系统

Except for regulating access to data segments,the privilege mechanism limits the use of certain instructions.

优先级机制除了控制对数据段的访问权限外．它还用来限制某些指令的使用

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Pentium II Protection – access Access attribute

Specifies a data segment whether read-write or read-only accesses are permitted; and specifies a program segment read-execute or read-only access.

一个数据段的存取属性表明该数据段是否允许读／写存取或只读存取。对于程序段，存取属性表示该程序段读执行或只读存取。

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Pentium II Paging Segmentation may be disabled 分段可以被禁止

In which case linear address space is used 程序中使用线性地址

Two level page table lookup 二级页表检索 First, page directory 第 1 级是页目录 1024 entries max 包含可达 1024 个项 Splits 4G linear memory into 1024 page groups of

4Mbyte 把 4G字节的线性存储空间分隔成 1024 个长度为 4M字节的页组

Each page table has 1024 entries corresponding to 4Kbyte pages

每个页表包含达 1024 个项，每项对应单个的 4K字节页 Page directory for current process always in

memory 当前任务的页目录总是在主存中

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Pentium II Address Translation Mechanism

16bits 32bits

10bits 10bits 12bits

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作业

7.17.57.107.12