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计算机内存管理

OS memory

内存层级

memory hierarchy

  • Register
  • Cache(L0-L4)
  • RAM
  • Hard Disk

操作系统内存管理

  • Relocation caused by Swapping & Compaction
  • Protection 进程地址空间隔离
  • Organization
    • Physical
    • Logical
  • Sharing

内存交换

当可用内存不足时，操作系统为了保证当前运行的程序正常执行，会将某些程序从内存中暂时移除放到二级存储(硬盘等)内

现代操作系统基本都支持内存交换

Linux 有单独的 swap partition Windows和macOS都是用Swap File(windows: Swapfile.sys macOS: /private/var/vm/swapfile*)

地址空间(Address Space)

每个进程独立的虚拟内存地址空间

  • Low Memory (操作系统占用)
  • High Memory (用户进程)

virtual address space

编程语言内存模型

memory layout of c program

stack memory space

虚拟内存(Virtual Memory)

对物理内存的抽象，可以大于实际可用物理内存。现代操作系统基本都提供虚拟内存功能，CPU在执行代码时通过MMU(Memory Management Unit)将虚拟内存地址转换成物理内存地址。

virtual memory

  • 内存分配更简单高效

  • 比实际可用物理内存更大的内存空间

  • 减少外部碎片，更高的内存利用率

  • 内存交换效率更高

  • Allocating memory is easy and cheap

  • Any free page is ok, OS can take first one out of list it keeps

  • Eliminates external fragmentation

  • Data (page frames) can be scattered all over PM

  • Pages are mapped appropriately anyway

  • Allows demand paging and prepaging

  • More efficient swapping

  • No need for considerations about fragmentation

  • Just swap out page least likely to be used

连续内存分配

给进程分配一块连续的内存

Partitions were used mainly on batch systems

  • Single Partition Embedded system

  • Multiple Partition 需要隔离

  • 分割(Partition)

    • Equal-Size Partition
      • internal fragment
    • Unequal-Size Partition
    • Dynamic Partition
      • external fragment
    • Buddy System

  • 放置(Placement Algorithm)

    • Best fit,choose smallest hole
    • First fit,choose first block from beginning
    • Next fit,choose first block from last placement
    • Worst fit,choose largest hole

  • 压缩(compaction)

非连续内存分配

将物理内存切分成很多小块(frames)，把虚拟内存也切分成很多小块(segements/pages)，然后进行映射。就不用给程序分配一整块连续的物理内存了。

分段(Segmentation)

divide virtual address space into variable-sized frames

segments

分页(Page)

page table

divide virtual address space into equal-sized frames

paging is implemented by integrating the hardware and operating system

  • page 512b(扇区大小)~8192b
  • page table
    • 分层(hierarchy)
    • 哈希表(hashed)
    • 翻转(inverted), use physical frame to track page
  • frame 物理内存切片 大小同page
    • frame table

置换算法

  • FIFO
  • Optimal Page Algorithm
  • Page Buffering Algorithm
  • LRU(least recent use)
  • LFU(least frequent use)
  • MFU(most frequent use)

分段分页对比

  • Segmentation requires more complicated hardware for address translation
  • Segmentation suffers from external fragmentation
  • Paging only yield a small internal fragmentation
  • Segmentation is visible to the programmer whereas paging is transparent
  • Segmentation can be viewed as commodity offered to the programmer to organize logically a program into segments and using different kinds of protection (ex: execute-only for code but read-write for data)

参考资料