Understanding Memory Management in OS: Address Spaces, Swapping, Segmentation, Study notes of Operating Systems

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Operating Systems

Memory Management

Lecture 9

Michael O’Boyle

1

Memory Management

• Background
• Logical/Virtual Address Space vs Physical

Address Space

• Swapping
• Contiguous Memory Allocation
• Segmentation

Background

• Program must be brought (from disk) into memory and placed

within a process for it to be run

• Main memory and registers are only storage CPU can access

directly

• Memory unit only sees a stream of addresses + read requests,

or address + data and write requests

• Register access in one CPU clock (or less)
• Main memory can take many cycles, causing a stall
• Cache sits between main memory and CPU registers
• Protection of memory required to ensure correct operation

Base and Limit Registers

• A pair of base and limit registers define the logical address

space

• CPU must check every memory access generated in user

mode to be sure it is between base and limit for that user

Virtual addresses for multiprogramming

• To make it easier to manage memory of multiple processes,

make processes use logical or virtual addresses

• Logical/virtual addresses are independent of location in physical memory data lives - OS determines location in physical memory
• Instructions issued by CPU reference logical/virtual

addresses

• e.g., pointers, arguments to load/store instructions, PC …
• Logical/virtual addresses are translated by hardware into

physical addresses (with some setup from OS)

7

Logical/Virtual Address Space

• The set of logical/virtual addresses a process can reference

is its address space

• many different possible mechanisms for translating logical/virtual addresses to physical addresses
• Program issues addresses in a logical/virtual address space
• must be translated to physical address space
• Think of the program as having a contiguous logical/virtual address space that starts at 0,
• and a contiguous physical address space that starts somewhere else
• Logical/virtual address space is the set of all logical

addresses generated by a program

• Physical address space is the set of all physical

addresses generated by a program

8

MMU as a relocation register

Swapping

• What if not enough memory to hold all processes?
• A process can be swapped temporarily
  • out of memory to a backing store,
  • brought back into memory for continued execution
  • Total physical memory space of processes can exceed physical memory
• Backing store – fast disk
  • large enough to accommodate copies of all memory images for all users;
  • must provide direct access to these memory images
• Roll out, roll in – swapping variant
  • used for priority-based scheduling algorithms;
  • lower-priority process is swapped out so higher-priority process can be loaded and executed
• Major part of swap time is transfer time;
  • total transfer time is directly proportional to the amount of memory swapped
• System maintains a ready queue
  • ready-to-run processes which have memory images on disk

Context Switch Time including Swapping

• If next processes to be put on CPU is not in memory,
  • need to swap out a process and swap in target process
• Context switch time can then be very high
• Can reduce cost
  • reduce size of – by knowing how much memory really being used
  • inform OS of memory use via request_memory() and release_memory()
• Other constraints as well on swapping
  • Pending I/O – can’t swap out as I/O would occur to wrong process
• Or always transfer I/O to kernel space, then to I/O device
  • Known as double buffering , adds overhead
• Standard swapping not used in modern operating systems
  • But modified version common
    • Swap only when free memory extremely low

Contiguous Allocation

• Main memory must support both OS and user processes
• Limited resource, must allocate efficiently
• Contiguous allocation is one early method
• Main memory usually into two partitions :
  • Resident operating system, usually held in low memory with interrupt vector
  • User processes then held in high memory
  • Each process contained in single contiguous section of memory

Hardware Support for Relocation and Limit Registers

Multiple-partition allocation

• Multiple-partition allocation
  • Degree of multiprogramming limited by number of partitions
  • Exam 2 approaches
    • Fixed partition
    • Variable partition

Mechanics of fixed partitions

19 partition 0 partition 1 partition 2 partition 3 0 2K 6K 8K 12K physical memory offset + Logical address P2’s base: 6K base register 2K <? no raise protection fault limit register yes

Old technique #2: Variable partitions

• Obvious next step: physical memory is broken up into

partitions dynamically – partitions are tailored to programs

• hardware requirements: base register, limit register
• physical address = logical address + base register
• Advantages
• no internal fragmentation
• simply allocate partition size to be just big enough for process (assuming we know what that is!)
• Problems
• external fragmentation
• as we load and unload jobs, holes are left scattered throughout physical memory 20