The Virtual Memory Manager (VMM) services memory requests from the system and its applications. Virtual-memory segments are partitioned in units called pages; each page is either located in real physical memory (RAM) or stored on disk until it is needed. AIX uses virtual memory to address more memory than is physically available in the system. The management of memory pages in RAM or on disk is handled by the VMM.
In AIX, virtual-memory segments are partitioned into 4096-byte units called pages. Real memory is divided into 4096-byte page frames. The VMM has two major functions:
To accomplish these functions, the VMM maintains a free list of available page frames. The VMM also uses a page-replacement algorithm to determine which virtual-memory pages currently in RAM will have their page frames reassigned to the free list. The page-replacement algorithm takes into account the existence of persistent versus working segments, repaging, and VMM thresholds.
The VMM maintains a list of free (unallocated) page frames that it uses to satisfy page faults. AIX tries to use all of RAM all of the time, except for a small amount which it maintains on the free list. To maintain this small amount of unallocated pages the VMM uses page outs and page steals to free up space and reassign those page frames to the free list. The virtual-memory pages whose page frames are to be reassigned are selected using the VMM's page-replacement algorithm.
AIX distinguishes between different types of memory segments. To understand the VMM, it is important to understand the difference between working and persistent segments. A persistent segment has a permanent storage location on disk. Files containing data or executable programs are mapped to persistent segments. When a JFS or JFS2 file is opened and accessed, the file data is copied into RAM. VMM parameters control when physical memory frames allocated to persistent pages should be overwritten and used to store other data.
Working segments are transitory and exist only during their use by a process. Working segments have no permanent disk storage location. Process stack and data regions are mapped to working segments and shared library text segments. Pages of working segments must also occupy disk storage locations when they cannot be kept in real memory. The disk paging space is used for this purpose. When a program exits, all of its working pages are placed back on the free list immediately.
Working pages in RAM that can be modified and paged out are assigned a corresponding slot in paging space. The allocated paging space is used only if the page needs to be paged out. However, an allocated page in paging space cannot be used by another page. It remains reserved for a particular page for as long as that page exists in virtual memory. Because persistent pages are paged out to the same location on disk from which they came, paging space does not need to be allocated for persistent pages residing in RAM.
The VMM has two modes for allocating paging space: early and late. Early allocation policy reserves paging space whenever a memory request for a working page is made. Late allocation policy only assigns paging space when the working page is actually paged out of memory, which significantly reduces the paging space requirements of the system.
When a process references a virtual-memory page that is on disk, because it either has been paged out or has never been read, the referenced page must be paged in, and this might cause one or more pages to be paged out if the number of available (free) page frames is low. The VMM attempts to steal page frames that have not been recently referenced and, therefore, are not likely to be referenced in the near future, using a page-replacement algorithm.
A successful page-replacement keeps the memory pages of all currently active processes in RAM, while the memory pages of inactive processes are paged out. However, when RAM is over-committed, it becomes difficult to choose pages for page out because, they will probably be referenced in the near future by currently running processes. The result is that pages that are likely to be referenced soon might still get paged out and then paged in again when actually referenced. When RAM is over-committed, continuous paging in and paging out, called thrashing, can occur. When a system is thrashing, the system spends most of its time paging in and paging out instead of executing useful instructions, and none of the active processes make any significant progress. The VMM has a memory load control algorithm that detects when the system is thrashing and then attempts to correct the condition.