Operating System Swapping and Dynamic Linking

Dynamic Linking

• Uses shared libraries when compiling programs
• Useful for libraries

Swapping

• Mechanism where a process is swapped out of memory to a backing store
• 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: variant in swapping policy
• Implemented in higher priority and low priority processes

Context Switch Time

• Involves storing the context or state of a process so that it can be reloaded when execution is resumed
• Example: transfer rate of a 10MB file to and from memory is 1/4 sec or 250 milliseconds
• Swap time: 258ms, swapping in and out: 516ms

Swapping on Mobile Systems

• Not typically supported due to:
  • Flash memory based
  • Small amount of space
  • Limited number of write cycles
  • Poor throughput between flash memory and CPU on mobile platform
• Instead, use other methods to free memory:
  • iOS asks apps to voluntarily relinquish allocated memory
  • Android terminates apps if low free memory, but first writes application state to flash for fast restart

Contiguous Memory Allocation

• Memory must accommodate user processes and the operating system
• 2 partitions of contiguous memory: resident OS and user process
• Lower memory: where the operating system is located
• High memory: for user processes
• Relocation registers used to protect user processes from each other and from changing operating-system code and data
• Base register contains value of smallest physical address
• Limit register contains range of logical addresses
• MMU maps logical address dynamically

Memory Mapping and Protection

• Hardware support for relocation and limit registers

Multiple Partition Allocation

• Multiple-partition allocation
• Degree of multiprogramming is limited by the number of partitions
• Variable-partition sizes for efficiency (sized to a given process’ needs)
• Hole: block of available memory; holes of various size are scattered throughout memory
• When a process arrives, it is allocated memory from a hole large enough to accommodate it
• Process exiting frees its partition, adjacent free partitions combined
• Operating system maintains information about:
  • Allocated partitions
  • Free partitions (hole)

Static and Dynamic Memory Allocation

• Static Memory Allocation: declaring variables before runtime
• Examples:
  • Main program, procedure, function, method
  • Object, local variables, global variables
  • Common block, stack, symbol table, arrays

Segmentation

• Logical View of Segmentation:
  • User space
  • Physical memory space
• Segmentation:
  • Main process
  • Segment table
  • Memory
  • Segment size and address

Paging

• Memory-management scheme that permits the physical address space of a process to be noncontiguous
• Avoids external fragmentation
• Uses frames and pages
• Breaking physical memory into fixed-sized blocks called frames (size is a power of 2, between 512 bytes and 16 Mbytes)
• Breaking logical memory into blocks of the same size called pages
• Keeps track of all free frames

Paging: Address Translation Scheme

• Division of an address from the CPU:
  • Page number (p) – used as an index into a page table which contains base address of each page in the physical memory
  • Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit

Paging Hardware

• Hardware support for Paging

Paging Model of Logical and Physical Memory

• Example: logical memory has 4-byte page size and physical memory has 32 bytes (8 pages)

Free Frames

• Implementation of free frames before and after allocation

Page Table - Implementation

• PTBR (Page-Table Base Register) – points to the page table
• PTLR (Page Table Length Register) – specifies the size of the page table
• TLB (Translation Look-aside Buffers) - memory cache used to lessen the time to access a user memory location
• Also called Associative Memory
• Two Memory Accesses for data / instructions:
  • Page table
  • Data / instruction

Internal Fragmentation on Paging

• Example: page size = 2,048 (size = 2n), size of process = 58,800 bytes, no. of pages = 28 and excess bytes = 1,456

Learn about swapping mechanism in operating systems, including swapping policies and backing store. Also, understand dynamic linking and its application in shared libraries.