Chapter 18: Virtual Memory Management

Overview

Virtual memory is a technique that allows processes to use (or at least, appear to use) more memory than is actually available on a computer. A process can reference any address within a 4GB virtual address range. The virtual memory management subsystem of OpenVMS is responsible for allocating system resources in such a way that a process is unaware that the memory it uses is not "real."

Several processes together, or even one process, may have memory requirements far exceeding the physical memory available on the computer. OpenVMS conceptually divides physical memory into small units called pages that are allocated to processes on an as-needed basis. A combination of techniques known as paging and swapping is used to ensure that memory is always available, or will become available.

Note 

Other operating systems may use the terms paging and swapping interchangeably. Under OpenVMS, each has a separate meaning.

Paging assigns physical memory pages to processes, associating them with a specific page of the process virtual address space through complex page tables. When a process references a memory address, the memory-management hardware locates the physical memory page assigned to hold that virtual address. When memory is scarce, pages may be taken away from processes for other uses. When this occurs, OpenVMS ensures that the current contents of the pages can be reconstructed later when they are needed again.

Swapping is the technique of removing entire processes from physical memory to free nearly all pages associated with that process (the only data left in memory are necessary to locate and inswap (reload) the process when it is needed). OpenVMS uses a variety of other methods to reclaim memory before resorting to swapping [1]. When selecting a process for outswapping (removal from memory), OpenVMS locates a process that has been idle for some time and has been trimmed (had its number of physical pages reduced to a minimal level).

[1]Some experienced system managers intentionally tune their systems to encourage early swapping to leave more pages for those process that remain in memory.



Getting Started with OpenVMS(c) A Guide for New Users
Getting Started with OpenVMS: A Guide for New Users (HP Technologies)
ISBN: 1555582796
EAN: 2147483647
Year: 2005
Pages: 215

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