Caché Installation Guide - InterSystems Documentation

A.1.2 How OpenVMS Measures Process Memory
OpenVMS Page Organization
OpenVMS measures the code and data area that a process requires in pages. Shared sections
are also measured in pages, sometimes called shared or global pages.
On OpenVMS systems, page size is not fixed. Normally, it is 8192 bytes. Each page is divided
into pagelets that are 512 bytes long.
A.2 OpenVMS Page Organization
Sometimes a process needs to access many more pages than can fit within the physical
memory allocated to the process. When this situation arises, OpenVMS stores the extra pages
on disk. The operating system keeps only the most active, or most recently used, pages in
memory. The total of all the pages of a process, whether they are stored in memory or on
disk, is called the virtual memory set of the process. The pages currently stored in memory
are called the physical working set of the process, often called working set. The status of each
page is kept in the process's working set list.
OpenVMS keeps track of free pages available to processes in a free page list. It also keeps a
modified page list, which tracks pages that have been changed by a process and must be
written to disk before being made available on the free page list.
Sometimes a process needs access to a page not currently in its physical working set. When
this happens, OpenVMS searches two places for that page. First, OpenVMS looks in the
modified and free page lists, because the page might still be available in physical memory.
If the page does not appear on either of these lists, OpenVMS then retrieves the page from
disk.
When OpenVMS must retrieve a page, the process has incurred a page fault. When the new
page is in the modified or free page list, the page fault is called a soft page fault. When the
page is only available on disk, the page fault is called a hard page fault. While OpenVMS
retrieves the page from disk, the process becomes inactive. It remains in a page fault wait
state until OpenVMS has successfully retrieved the necessary page. The length of time that
this procedure requires varies according to the size of the disk, its rotation speed, and the load
on the system.
When OpenVMS retrieves the new page, it places one of the current working set pages in
the modified or free page list. OpenVMS chooses a page that the process has not used recently.
Caché Installation Guide 53