Virtual-memory-manager

This document addresses how RAM and paging space are used . The information applies to AIX Versions 4.3 and above.

The Virtual Memory Manager (VMM) services memory requests from the system and its applications. Virtual-memory segments are partitioned in 4096 byte units called pages; each page is either located in physical memory (RAM) or stored on disk in paging space until it is needed. AIX uses virtual memory in order to address more memory than is physically available in the system. The management of memory pages in RAM or on disk in paging space 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:

1. manage the allocation of page frames
2. resolve references to virtual-memory pages that are not currently in RAM or stored on disk paging space.

In order to accomplish its tasks, 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 re-assigned to the free list. The page replacement algorithm takes into account the existence of persistent vs. working segments, repaging, and VMM thresholds.

The VMM maintains a list of free page frames that it uses to satisfy page faults. The free list is made up of unallocated page frames. To enhance performance, AIX tries to use all of RAM all the time, except for a small amount which it maintains on the free list. To maintain this small amount of unallocated pages the VMM will use Page Outs and Page Ins (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 re-assigned are selected via the VMM's page replacement algorithm.

AIX distinguishes between different types of memory segments, and to understand the Virtual Memory Manager, it is important to understand the difference between working and persistent segments.

A persistent segment has a permanent storage location on disk. Files containing non-modifiable data or read-only executable programs are mapped to persistent segments. When a Journaled File System (either JFS or JFS2 ) 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 this new data.

Working segments are transitory, exist only during their use by a process, and have no permanent disk storage location. Process stack and data regions as well as shared library text are mapped to working segments. Pages containing working segment data require disk storage locations in paging space when they cannot be kept in real memory. When a program exits, all of its working pages are placed back on the free list.

In AIX 4.3.2 and later, working pages in RAM that can be modified and paged out are assigned a corresponding slot in paging space in a deferred manner. That is, the space is allocated on disk immediately before the pages are paged out. Prior to AIX 4.3.2 the corresponding slots in paging space were allocated when the working pages were touched (referenced) in memory by AIX . This is referred to as the late location scheme. On even earlier versions of AIX, early paging space allocation was done when pages were initially read into memory, even before they were referenced. The current deffered allocation scheme lets the utilization of the page space ( % used) more accurately represent paging space usage. It also significantly reduces the paging space requirements of the system.

Since persistent pages already exist somewhere else on a system disk, paging space never needs to be allocated for persistent pages.

When a process references a virtual memory page that is on disk (because it either has been paged out or has yet to be read in ) the referenced page must be paged in and, if the memory is already nearly full, this may cause one or more pages to be paged out to make room. This happens when the available free page frames (on the free list) is low. The VMM uses the page-replacement algorithm to steal page frames that have not been recently referenced, and thus would be unlikely to be referenced in the near future.

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 be referenced in the near future by currently running processes. The result is that pages that will soon be referenced still get paged out and then paged in again later. When this happens, continuous paging in and paging out may occur. This condition is called 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.

avm stands for "Active Virtual Memory" and not "Available Memory". The avm value in VMSTAT indicates the average number of 4K virtual memory pages that have been accessed but not necessarily paged out. With the previous policy of late page space allocation, avm was equivalent to "paging space blocks" since the VMM would allocate one paging space disk block for each working page that was accessed. With the current deferred allocation policy, the page space disk blocks are allocated at the time the pages are actually paged out. The avm number will grow as more processes get started and/or existing processes allocate more working storage. Likewise, the avm number will shrink when processes (not threads) terminate or are released by the disclaim ( ) system call. They are not released by the free ( ) system call.

fre is the average number of 4K pages that are currently on the free list. When an application terminates, all of its working pages are immediately returned to the free list. Its persistent pages (files), however, remain in RAM and are not added back to the free list until they are stolen by the VMM for use by other programs. Persistent pages are also freed if the corresponding file is deleted.

For these reasons, the fre value may not indicate all the real memory that can be readily available for use by processes. If a page frame is needed, then persistent pages previously referenced by terminated applications are among the first to be handed over to another program.

The minimum number of pages that the Virtual Memory Manager keeps on the free list is determined by the minfree parameter of vmtune. If the number of pages on the free list drops below minfree, the Virtual Memory Manager will steal pages until the free list has been restored to the maxfree value.

The svmon command can be used to determine roughly how much memory the system is using.

NOTE: perfagent.tools must be installed in order to use svmon. Check to see if this is installed, by executing the following command:

     $ lslpp -1 perfagent.tools

If you are at AIX Version 4.3.0 or higher, then this file can be found on the AIX BASE Operating System media.

As root, type svmon or svmon -G.

Example :

$ [4.3.3 or later] svmon
               size      inuse       free        pin    virtual
memory       130904     129388       1516       7600      35349
pg space      65536       5048
               work       pers       clnt
pin            7600          0          0
in use        38073      91315          0
$ [4.3.2] svmon
        m e m o r y           i n  u s e         p i n        p g  s p a c e
  size inuse  free   pin   work  pers clnt   work pers clnt     size   inuse
 32768 30873  1895  2187  21691  9182    0   2187    0    0    65536   354

Memory:

SIZE    Total size of memory in 4K pages

INUSE    Number of pages in RAM that are in use by a process plus the number of persistent pages that belonged to a terminated process and are still resident in RAM. This value is the total size of memory minus the number of pages on the free list.

FREE    Number of pages on free list.

PIN     Number of pages pinned in RAM (a pinned page is a page that is always resident in RAM and cannot by paged out)

VIRTUAL    Number of pages allocated in the system virtual space (version 4.3.3 or later)

Pgspace:

SIZE    Total size of paging space in 4K pages

INUSE    Total number of allocated slots. (see "Paging space allocation and Virtual Memory" above)

Pin:

WORK    Number of working pages pinned in RAM

PERS    Number of persistent pages pinned in RAM

CLNT    Number of client pages pinned in RAM

In use:

WORK    Number of working pages in RAM

PERS    Number of persistent pages in RAM

CLNT    Number of client pages in RAM (client page is a remote file page)

To find out how much memory a single process is using, enter:

     $ svmon -P  

To find out how much memory the top 5 processes are using, enter:

     $ [4.3.3 or later] svmon -Put 5
     $ [4.3.2] svmon -Pau 5

To find out how much memory every process is using, enter:

$ [4.3.3 or later] svmon -Pu | more
$ [4.3.2] svmon -Pau | more

Example : [4.3.3]

$ svmon -P 19088
--------------------------------------------------------------------------
     Pid Command        Inuse      Pin     Pgsp  Virtual   64-bit    Mthrd
   19088 java            7759     1402     1161     4416        N        Y
  Vsid     Esid Type Description         Inuse   Pin Pgsp Virtual Addr Range
  e01c        d work shared library text  3510     0   23    33   0..65535
     0        0 work kernel seg           1869  1396 1103  3175   0..21740 :
                                                                    65348..65535
  7c2d        2 work process private      1031     1   31  1042   0..2724 :
                                                                    65307..65535
  9bb3        - pers /dev/hd2:35149        437     0    -     -   0..2404
  93b2        - pers /dev/hd2:2394         428     0    -     -   0..590
  83b0        - pers /dev/hd2:2393         231     0    -     -   0..338
  7cae        f work shared library data   109     0    2    87   0..1575
  9893        - work                        77     5    2    79   0..49746
  7baf        - pers /dev/hd2:2392          41     0    -     -   0..53
  abb5        - pers /dev/hd2:35119         10     0    -     -   0..144
  ...

To determine the number of working pages used in all of virtual memory that are unique to this process' private stack and data, look for the following items:

• Type:   work
• Description:   process private or blank

  In the preceding example, these values would be 1031+ 77.

The svmon output may also list several shared segments. For a complete picture, determine which segments are unique to an individual process and which are shared with other programs.The Vsid (virtual segment id) and Esid (effective segment register ids in 4.3.3 or greater) numbers will be identical for each shared segment.

Since each page in memory is 4KB in size, multiply the Inuse values by 4096 to get the number of bytes in memory that the process is using. Divide the number of pages by 256 in order to get megabytes.