DOCUMENT ID:  1249-02

SYNOPSIS:     General Description of System V Release 4 Paging

OS RELEASE:   2.x

PRODUCT:      Solaris x86

KEYWORDS:     paging memory cache system V


DESCRIPTION:

System V Release 4 Paging


SOLUTION:

Paging

In general, all disk I/O centers around the VM system's role as a cache
manager. 

All page-in requests come as a result of the cache manager's need to
fill its cache (main memory). 

Page-out requests come when a page is written back to disk.


PAGING IN

This is no specific page_in() routine, Instead, a set of routines
governs page-in policies. 

All of main memory can act as the page cache.

A page is searched for using page identity to see if it still has a page
structure. 

* page_find()

* page_lookup()

If it has a page structure, but its p_free bit is set, the
page_reclaim() routine attempts to reclaim it. 

PAGE REPLACEMENT

The system strategy for page replacement is as follows:

* Try to keep a minimum amount of memory free (minfree).

* If free memory drops below lotsfree (little free memory is left), then
  run the page daemon. 

* If free memory is below t_gpgslo, then the swapper looks for an LWP to
  swap out. 

A two-handed clock algorithm is used to search memory for the least
recently used pages. 


DATE APPROVED: 01/11/95