Virtual Memory Two - Operating Systems - Lecture Slides, Slides of Computer Science

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Operating Systems

Lecture 23:

Virtual Memory

Execution Behavior

Breakthrough in

Memory Management

• If:
  • Memory references are logical addresses dynamically translated into physical addresses at run time (processes may be swapped in and out of main memory occupying different regions at different times)
  • A process may be broken up into pieces that do not need to be located contiguously in main memory
• Then it is not necessary to have all pages of a process in main memory during execution. - If the next instruction, and the next data location are in memory, then execution can proceed

Chapter 9: Virtual Memory

Implications of

Virtual Memory

• More processes may be maintained in main memory - Only load in some of the pieces of each process - With so many processes in main memory, it is very likely a process will be in the Ready state at any particular time
• A process may be larger than all of main memory

Execution of a Process

• Operating system brings into main memory a few pieces of the program - Resident set: portion of process that is in main memory
• An interrupt is generated when an address is needed that is not in main memory
• Operating system places the process in a blocking state

Support Needed for

Virtual Memory

• Hardware must support paging and/or segmentation
• Operating system must be able to manage the movement of pages and/or segments between secondary memory and main memory - What if no room in main memory? Which frame to replace?

Page Table

Entries Formats

Replacement Policy

• When all of the frames in main memory are occupied and it is necessary to bring in a new page, the replacement policy determines which page currently in memory is to be replaced.
• Subject to other design issues such as:
  • Number of frames per process
  • Local or global allocation

But…

• Which page is replaced?
• Page removed should be the page least likely to be referenced in the near future - How is that determined? - Principle of locality again
• Most policies predict the future behavior on the basis of past behavior

Basic Replacement

Algorithms

• There are certain basic algorithms that are used for the selection of a page to replace, they include - Optimal - Least recently used (LRU) - First-in-first-out (FIFO) - Clock

Optimal Page Replacement

Policy

• Knows the future, minimizes page faults
• Theoretical limit, not a practical algorithm

LRU Example

• The LRU policy does nearly as well as the optimal policy. - In this example, there are four page faults

First-in, first-out (FIFO)

• Treats page frames allocated to a process as a circular buffer
• Pages are removed in round-robin style
  • Simplest replacement policy to implement
• Page that has been in memory the longest is replaced - But, these pages may be needed again very soon if it hasn’t truly fallen out of use