5.4. Virtual Memory Management

The Paged Memory Management scheme gives rise to the notion of demand paging using virtual memory. The Virtual Memory Management system maintains a copy of the memory for all programs on secondary storage, such as a hard drive. In fact, many pages for a process may only reside in virtual memory. Loading only the page frames that are needed to run a program can make it faster to load a program. Some memory frames for a program may never be needed while the program runs.

If a current copy of a frame of physical memory is held on disk in virtual memory, then that frame may be removed from physical memory to free up needed memory. When a process references memory that is not loaded in physical memory, the Memory Management Unit of the CPU issues a page fault trap.

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The tasks of the Virtual Memory Management system include:

1. Maintaining the virtual memory with a reasonably current copy of memory.
2. Keeping the page tables current.
3. Responding to page faults by loading the requested memory frames into physical memory.

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The performance of a virtual memory management system depends on the total number of page faults, which depend on:

The page replacement algorithm

Select which frames are to be replaced when a page fault occurs. (Page Replacement Algoritms)

The frame allocation algorithm

Decide how many frames to allocate to each process. (Frame Allocation)

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Contents:

• 5.4.1. Virtual Memory is not Process Swapping
• 5.4.2. The Virtual Memory Address Space
• 5.4.3. Page Faults
  • 5.4.3.1. Handling of a Page Fault
• 5.4.4. Page Replacement Algoritms
  • 5.4.4.1. Static Replacement Algorithms
  • 5.4.4.2. LRU Approximation
• 5.4.5. Frame Allocation
  • 5.4.5.1. Frames Needed by a Process
  • 5.4.5.2. Dynamic Page Allocation
  • 5.4.5.3. The Working Set Model
• 5.4.6. Segmented Virtual Memory with Demand Paging