[OS] Memory Management

Memory management

• ideally programmers want memory that is
  • fast
  • large
  • nonvolatile
• Memory hierarchy
  • small amount of fast, expensive memory - cache
  • some medium speed, medium price main memory
  • gigabytes of slow, cheap disk storage
• Memory manager
  • part of the operating sys that manages the memory hierachy

Basic Memory Management

Monoprogramming without swapping or paging

• an operating sys with one user process
  • Only one process at a time is running
• Three simple ways of organizing memory
  • 하나의 user process와 운영체제
  • (a) mainframes, minicomputers
  • (b) palmtop computers, embeded systems
  • (c) personal computers

Multiprogramming with fixed partitions

• Fixed Memory partitions
  • separate input queues for each partition
    • partition마다 input queue 단점 :수용할 수 있는 가장작은 partition에 가는데 partition 3에는 갈 수 없고 다른 곳에 배당되기 때문에 partition 3는 이용되지 않을 수 있다.
  • single input queue -> 사이즈만 맞으면 먼저 도착한 작은 job들에게 유리하다.

Relocation and protection

Multiprogramming의 두가지 문제점이다. 메모리안에 여러 프로세스가 존재하기 때문.

• Cannot be sure where program will be loaded in memory.
  • Relocation
    • address locations of variables. code routines cannot be absolute
  • Protection
    - must keep a program out of other processes partitions
• Relocation and protection
  • Modify the instruction as the program is loaded into memory
    • Linker should tell which program words are addresses to be relocated
      • Protection code
  • User base and limit register
    • address locs added to base value to map to physical addr
      • address locations larger than limit value is an error

Excutable file이 look에 loading되면 주소가 partition에 배당되면 ㅣㅐㅐㅏ + 주소가 되어야 함?? 무슨말일까...

Relocation 해결법
1. program memory 로딩될 때, call look + 주소번지 이런식으로 instruction을 변경한다.
2. base reg & limit register 사용, 로딩시 inst 바꾸지 않고 그냥 로딩 call 주소 번지 base register에 (loading된 파티션 값)저장된 값 (주소)을 더해준다.

Protection 해결법
1. limit register 사용, program partition 1에 배당, 주소 100k partition 사이즈가 limit register로 저장하고, call 100k와 limit register 사이즈를 비교하여 다른 메모리를 침범하지 않게 한다.
단점 :loading은 빠르나 비교하기 때문에 수행이 느리다.

Swapping & Virtual memory

Swapping	Virtual memory
Bring in each process in its entirety, running it for a while, then put it back on dist	Allows programs to run even when they are only partially in main memory

Swapping

Multiprogramming with Variable partitions

• Memory allocation changes as
  • processes come into memory
  • leave memory
• Shaded regions are unused memory
• Memory compaction
  • Combine multiple holes into one big one by moving all the processes downward.
• Swapping has room for allocating space for growing data segment
• also for stack & data segment

Keeping track of memory usage

메모리 동적으로 할당. 메모리 사용을 keep track하는 방법 2가지.

• Bitmap
• Free list

Bit Maps

• Part of memory with 5 processes, 3 holes
  • tick marks show allocation units
  • shaded regions are free
• Size of the allocation unit
  • The smaller the allocation unit, the larger the bitmap
  • The larger the allocation unit, the more memory is wasted
• Searching a bitmap for a run of a given length is a slow op.
  process 새로 생성되면 메모리 새로 할당. alloc unit 7이되는 것을 찾아야한다. 0이 7개 연속되는 것 찾는것은 처음 부터 찾으면 O(n)의 시간 복잡도를 갖는다.
  (b) 할당되어 있으면 bitmap - 1 할당 x - 0

Linked Lists

• Allocating memory for a newly created process
  • First fit
    • Scans along the list until it finds a hole that is big enough
  • Best fit
    • Searches the entire list and takes the smallest hole available
  • Worst fit
    • Takes the largest hole available

Virtual Memory

• Programs that are too big to fit in the available memory
• Solution

  • Overlay (program 조각낸다.)

    • A programmer splits the program into pieces, called overlays.
      • when the first overlay is done, it calls another overlay, which is swapped into the memory from the disk.

  • Virtual memory (Os가 알아서 함)

    • Computer handles the job
    • paging **

paging

• Virtual address
  • Program generated address
    • MOV REG, 1000
  • Forms the virtual address space
  • Virtual address space is divided up into units called pages and the corresponding units in the physical memory are called page frames (매우 중요)

• The relation between virtual addressees and physical memory addresses given by page table.
• Page falut
  • When the MMU notices that the page is unmapped, it causes the CPU to trap to the operating system.
  • The OS picks a little-used page frame and writes its cotents back to the disk
  • It then fetches the page just referenced into the page frame just freed, changes the map, and restarts the trapped instruction.

virtual memory가 physical memory의 어디를 참조하고 있는지 저장하고 있다.