操作系统课件第07章 内存管理

内存管理的主要任务⏹空闲物理内存的管理☞用链表或bitmap表示即可⏹物理内存的使用☞进程使用内存的管理只能使用自己的？是系统并发的关键大小不一，预知、未知大小堆、栈、段☞内核自身内存的管理为提高效率，各系统采用多种缓存技术，差异较大各种大小不一的常用数据结构图空闲空闲Chapter 7: Memory Management内存管理⏹Background 背景知识⏹Address Binding 地址绑定⏹Paging 分页☞Demand Paging 请求调页☞Process Creation 进程创建⏹Page Replacement 页替换⏹Allocation of Frames 物理页帧的分配☞Thrashing 抖动⏹Solaris and WindowsNTReview——内存管理的目标操作系统内核挑战：在一维的物理内存空间实现：进程和进程，进程和内核的共享和保护Background 内存管理的背景知识Basic Concepts基础概念⏹Logical Addresses （Space）;逻辑地址（空间）用户程序中的地址⏹Physical Addresses （Space）物理地址（空间）⏹程序须作为进程装入物理内存才能运行进程使用内存的管理⏹进程（程序）在物理内存中放置的位置☞逻辑地址与物理地址间的地址绑定⏹进程（使用内存的）大小☞代码段，基本，固定☞数据段，基本，固定☞栈，可选，变化☞堆，可选，变化地址绑定逻辑地址与物理地址间的映射程序中的逻辑地址Two 16KB Programs 程序分别单独装入右图中的物理地址逻辑地址64KB物理地址Two Programs Loaded Consecutively into Memory 两个程序连续加载到物理内存物理地址编译时实现地址绑定的三个时机加载时运行时编译时地址绑定If memorylocation knowna priori,absolute codecan begenerated;must recompilecode if startinglocationchanges.如果内存位置提前知道,可编译时生成绝对(物理)地址加载时地址绑定16412Mustgeneraterelocatable 可重定位codeif memorylocation isnot known atcompile time.当编译时无法确定内存位置时,须生成可重定位的代码DLL16000运行时地址绑定一种可能的解决方案⏹Binding delayed until runtime if the process can bemoved during its executionfrom one memory segmentto another.进程的内存段允许运行时移动⏹Need hardware support foraddress maps 需要硬件支持, 如：☞Base Register基址寄存器☞Limit Register限长寄存器Dynamic Binding using MMU利用内存管理单元实现动态地址绑定163842816412Memory-Management UnitHardware Address Protection硬件地址保护MMUMemory-Management Unit (MMU)内存管理单元⏹Hardware device硬件设备that maps映射virtual tophysical address.MMU是将逻辑地址映射到物理地址的硬件设备⏹In MMU scheme, the value in the relocation register重定位寄存器is added to every address generated by a user process at the time it is sent to memory.MMU将用户进程的每一地址与基址寄存器相加后使用⏹The user program deals with logical addresses; it neversees the real physical addresses.用户程序使用逻辑地址，永远也看不到物理地址地址绑定总结——逻辑地址和物理地址的关系⏹Logical and physical addresses are the same incompile-time and load-time address-bindingschemes;编译时和装载时地址绑定方案中，两种地址相同⏹Logical (virtual) and physical addresses differ inexecution-time address-binding scheme.执行时地址绑定方案中，两种地址不同。

由MMU 计算并接通物理地址线。

Windows、Linux等通用操作系统采用三种地址绑定时机，可以综合采用Paging分页Paging 分页图示⏹Physical address space of a process can be noncontig uous.物理地址可不连续⏹Page size 页多大合适？⏹Internal fragmentat ion内部碎片页帧逻辑空间逻辑页pagePage Table页表Address Translation Scheme地址转换方案⏹Address generated by CPU is divided into:☞Page number(p)页号–used as an indexinto a page table which contains baseaddress of each page in physical memory.Page offset(d)页偏移–combined with baseaddress to define the physical memoryaddress that is sent to the memory unit.Address Translation Architecture地址转换结构Paging Example The internaloperation of theMMU with 164KB pages.Free Frames空闲页帧Before allocation After allocationStructure of Page Table Entry页表项结构TLB 关联存储Speeding Up Paging分页的加速⏹Page table is kept in main memory.页表须放在内存☞If the virtual address space is large, the page table will be large.页表不能太大Page-table base register (PTBR) points to the page table.页表寄存器指向页表Page-table length register(PRLR) indicates size of the page table.页表长度寄存器保存页表的长度⏹The mapping from virtual address to physical address must befast.虚地址到物理地址的映射越快越好☞In this scheme every data/instruction access requires two memory accesses两次内存访问. 页表一次，数据/指令一次☞The two memory access problem can be solved by the use of a special fast-lookup快速查找hardware cache硬件缓存calledassociative memory关联存储or translation look-aside buffers(TLBs)⏹Address translation (A ´, A ´´)☞If A ´is in associative register , getframe # out.☞Otherwise get frame # from pagetable in memory Associative Memory关联存储Page #Frame #Paging Hardware With TLB 带有TLB的分页硬件Effective Access Time有效访问时间⏹Associative Lookup = εtime unit查关联存储的时间⏹Assume memory cycle time is 1 microsecond⏹Hit ratio命中率–percentage of times that a page numberis found in the associative registers; ration related tonumber of associative registers.⏹Hit ratio = α⏹Effective Access Time (EAT)EAT = (1 + ε) α+ (2 + ε)(1 –α)= 2 + ε–α内存保护Memory Protection内存保护⏹Memory protection implemented by associating protectionbit位with each frame.内存保护由每一页帧的相关保护位实现⏹Valid-invalid bit attached to each entry in the page table:页表每一项设立是否有效位☞“valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page.☞“invalid” indicates that the page is not in the process’ logical address space.Valid (v) or Invalid (i) Bit In A Page TablePage Table Structure 超大页表结构Page Table Structure页表结构⏹Hierarchical Page Tables分级页表⏹Hashed Page Tables散列页表⏹Inverted Page Tables反向页表Hierarchical Page Tables分级页表⏹Break up the logical address space into multiple pagetables.⏹ A simple technique is a two-level page table.Two-Level Paging Example两级页表举例⏹ A logical address (on 32-bit machine with 4K page size) is divided into:☞ a page number consisting of 20 bits.☞ a page offset consisting of 12 bits.⏹Since the page table is paged, the page number is further divided into:☞ a 10-bit page number.☞ a 10-bit page offset.⏹Thus, a logical address is as follows:where p i is an index into the outer page table, and p 2is the displacement within the page of the outer page table.page numberpage offset p ip 2d 101012Two-Level Page-Table ExampleTwo-Level Page-Table SchemeAddress-Translation Scheme地址转换方案Address-translation scheme for a two-level 32-bit paging architectureHashed Page Tables散列页表⏹Common in address spaces > 32 bits.常用于在地址空间> 32 位时⏹The virtual page number is hashed into a page table. Thispage table contains a chain of elements hashing to the same location.虚拟页号被散列到一个页表⏹Virtual page numbers are compared in this chainsearching for a match. If a match is found, thecorresponding physical frame is extracted.在链表中搜索相应的虚拟页号，如果找到，即可确定其对应的物理页帧号Hashed Page TableInverted Page Table反向页表⏹One entry for each real page of memory.一物理页一表项⏹Entry consists of the virtual address of the page stored inthat real memory location, with information about theprocess that owns that page.表项内包含虚地址页号及其所属的进程信息⏹Decreases memory needed to store each page table, butincreases time needed to search the table when a page reference occurs.减少了存储空间，但增加了查找时间⏹Use hash table to limit the search to one —or at most afew —page-table entries.利用hash表减少查找次数Inverted Page Table ArchitectureShared Pages共享页⏹Shared code 共享代码☞One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, windowsystems).多个进程共享同一份代码☞Shared code must appear in same location in the logical address space of all processes共享代码在所有进程中需要位于相同逻辑地址.⏹Private code and data 私有的代码和数据☞Each process keeps a separate copy of the code and data.每个进程保有一份独立的代码和数据☞The pages for the private code and data can appear anywhere in the logical address space.代码和数据页可出现进程虚拟地址空间的任何地方Shared Pages ExampleSegmentation分段Segmentation分段⏹Memory-management scheme that supports user view ofmemory用户角度的内存.⏹ A program is a collection of segments. A segment is alogical unit such as:main program,procedure,function,method,object,local variables, global variables,common block,stack,symbol table, arraysUser’s View of a Program 用户眼中的程序ELF。