The AIX® virtual memory manager (AIX VMM) is a page-based virtual memory manager.
A page is a fixed-size block of data.
A page might be resident in memory (that is, mapped into a location in physical memory), or a page might be resident on
a disk (that is, paged out of physical memory into paging space or a file system).
AIX maps pages into real memory based on demand.
When an application references a page that is not mapped into real memory, the system generates a page fault.
To resolve the page fault, the AIX kernel loads the referenced page to a location in real memory.
If the referenced page is a new page (that is, a page in a data heap of the process that has never been previously referenced), "loading" the referenced page simply means filling a real memory location with zeros (that is, providing a zero-filled page).
If the referenced page is a pre-existing page (that is, a page in a file or a previously paged out page), loading the referenced page involves reading the page from the disk (paging space or disk file system) into a location in real memory.
Once a page is loaded into real memory, it is marked as unmodified.
If a process or the kernel modifies the page, the state of the page changes to modified.
This allows AIX to keep track of whether a page has been modified after it was loaded into memory.
As the system adds more pages into real memory, the number of empty locations in real memory that can contain pages decreases.
When the number of free page frames gets to a low value, the AIX kernel must empty out some locations in real memory for reuse of new pages.
This process is otherwise known as page replacement.
The AIX VMM has background daemons responsible for doing page replacement. A page replacement daemon is referred to as lrud (shows up as lrud in the output of ps -k).
lrud daemons are responsible for scanning in memory pages and evicting pages in order to empty locations in real memory. When a page
replacement daemon determines that it wants to evict a specific page, the page replacement daemon does one of two things:
# If the page is modified, the page replacement daemon writes the page out to a secondary storage location.
# If the page is unmodified, the page replacement daemon can simply mark the physical memory block as free, and the physical memory block can
then be re-used for another page.
The page replacement daemons target different types of pages for eviction based on system memory usage and tunable parameters.
Fundamentally, there are two types of pages on AIX:
• Working storage pages (Computational pages)
• Permanent storage pages (Non-computational pages)
Working storage pages are pages that contain volatile data (in other words, data that is not preserved across a reboot).
* Process data
* Stack
* Shared memory
* Kernel data
When modified working storage pages need to be paged out (moved from memory to the disk), they are written to paging space. Working storage pages are never written to a file system.
When a process exits, the system releases all of its private working storage pages.
Permanent storage pages are pages that contain permanent data (that is, data that is preserved across a reboot). This permanent data is just file data. So, permanent storage pages are basically just pieces of files cached in memory.
When a modified permanent storage page needs to be paged out (moved from memory to disk), it is written to a file system.
As mentioned earlier, an unmodified permanent storage page can just be released without being written to the file
system, since the file system contains a pristine copy of the data.
You can divide permanent storage pages into two sub-types:
• Client pages
• Non-client pages
When you first open a file, the AIX kernel creates an internal VMM object to represent the file. It marks it as non-computational, meaning all files start out as non-computational.
As a program does reads and writes to the file, the AIX kernel caches the file's data in memory as non-computational permanent storage pages.
If the file is closed, the AIX kernel continues to cache the file data in memory (in permanent storage pages). The kernel continues to cache the file for performance;
for example, if another process comes along later and uses the same file, the file data is still in memory, and the AIX kernel does not have to read the file data in from disk.
A page is a fixed-size block of data.
A page might be resident in memory (that is, mapped into a location in physical memory), or a page might be resident on
a disk (that is, paged out of physical memory into paging space or a file system).
AIX maps pages into real memory based on demand.
When an application references a page that is not mapped into real memory, the system generates a page fault.
To resolve the page fault, the AIX kernel loads the referenced page to a location in real memory.
If the referenced page is a new page (that is, a page in a data heap of the process that has never been previously referenced), "loading" the referenced page simply means filling a real memory location with zeros (that is, providing a zero-filled page).
If the referenced page is a pre-existing page (that is, a page in a file or a previously paged out page), loading the referenced page involves reading the page from the disk (paging space or disk file system) into a location in real memory.
Once a page is loaded into real memory, it is marked as unmodified.
If a process or the kernel modifies the page, the state of the page changes to modified.
This allows AIX to keep track of whether a page has been modified after it was loaded into memory.
As the system adds more pages into real memory, the number of empty locations in real memory that can contain pages decreases.
When the number of free page frames gets to a low value, the AIX kernel must empty out some locations in real memory for reuse of new pages.
This process is otherwise known as page replacement.
The AIX VMM has background daemons responsible for doing page replacement. A page replacement daemon is referred to as lrud (shows up as lrud in the output of ps -k).
lrud daemons are responsible for scanning in memory pages and evicting pages in order to empty locations in real memory. When a page
replacement daemon determines that it wants to evict a specific page, the page replacement daemon does one of two things:
# If the page is modified, the page replacement daemon writes the page out to a secondary storage location.
# If the page is unmodified, the page replacement daemon can simply mark the physical memory block as free, and the physical memory block can
then be re-used for another page.
The page replacement daemons target different types of pages for eviction based on system memory usage and tunable parameters.
Fundamentally, there are two types of pages on AIX:
• Working storage pages (Computational pages)
• Permanent storage pages (Non-computational pages)
Working storage pages are pages that contain volatile data (in other words, data that is not preserved across a reboot).
* Process data
* Stack
* Shared memory
* Kernel data
When modified working storage pages need to be paged out (moved from memory to the disk), they are written to paging space. Working storage pages are never written to a file system.
When a process exits, the system releases all of its private working storage pages.
Permanent storage pages are pages that contain permanent data (that is, data that is preserved across a reboot). This permanent data is just file data. So, permanent storage pages are basically just pieces of files cached in memory.
When a modified permanent storage page needs to be paged out (moved from memory to disk), it is written to a file system.
As mentioned earlier, an unmodified permanent storage page can just be released without being written to the file
system, since the file system contains a pristine copy of the data.
You can divide permanent storage pages into two sub-types:
• Client pages
• Non-client pages
When you first open a file, the AIX kernel creates an internal VMM object to represent the file. It marks it as non-computational, meaning all files start out as non-computational.
As a program does reads and writes to the file, the AIX kernel caches the file's data in memory as non-computational permanent storage pages.
If the file is closed, the AIX kernel continues to cache the file data in memory (in permanent storage pages). The kernel continues to cache the file for performance;
for example, if another process comes along later and uses the same file, the file data is still in memory, and the AIX kernel does not have to read the file data in from disk.
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