Introduction
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).
One unique aspect of the AIX VMM is the management of cached file data. The AIX
VMM integrates cached file data with the management of other types of virtual
memory (for example, process data, process stack, and so forth). It caches the
file data as pages, just like virtual memory for processes.
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. You can also refer to the number of empty
locations as the number of free page frames. 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 (for example, paging space or file system disk). The
physical memory block that contains the page is marked as free and ready for
reuse for additional pages.
- 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. In this case, the page replacement daemon does not have to
write the page out to disk, because the in-memory version of the page is
unmodified, and thus is identical to the copy of the page that resides on the
disk (in paging space or on a disk file system).
The page replacement daemons target different types of pages for eviction based
on system memory usage and tunable parameters. The remainder of this article
provides details on how the page replacement daemons target pages for eviction.
Page types
Fundamentally, there are two types of pages on AIX:
- Working storage pages
- Permanent storage pages
Working storage
Working storage pages are pages that contain volatile data (in other
words, data that is not preserved across a reboot). On other platforms, working
storage memory is sometimes referred to as anonymous memory. Examples of
virtual memory regions that consist of working storage pages are:
- 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. Thus,
the system releases the working storage pages for the data of a process and stack
when the process exits. The working storage pages for shared memory regions are
not released until the shared memory region is deleted.
Permanent storage
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.
For example, if an application is reading a file, the file data is cached in
memory in permanent storage pages. These permanent storage pages are unmodified,
meaning that the pages have not been modified in memory. So, the in-memory
permanent storage pages are equivalent to the file data on the disk. When AIX
needs to free up memory, it can just "release" these pages
without having to write anything to disk. If the application had been doing writes
to the file instead of reads, the permanent storage pages would be
"modified," and AIX would have to flush the pages to disk before
releasing the pages.
You can divide permanent storage pages into two sub-types:
- Client pages
- Non-client pages
Non-client pages are pages containing cached Journaled File System (JFS)
file data. Non-client pages are sometimes referred to as persistent pages. Client
pages are pages containing cached data for all other file systems (for example,
JFS2 and Network File System (NFS)).
Page classification
In order to help optimize which pages are selected for replacement by the page
replacement daemons, AIX classifies pages into one of two types:
- Computational pages
- Non-computational pages
Computational pages are pages used for the text, data, stack, and shared memory
of a process. Non-computational pages are pages containing file data for files
that are being read and written.
How pages get classified
All working storage pages are computational. A working storage page is never
marked as non-computational.
Depending on how you use the permanent storage pages, the pages can be
computational or non-computational. If a file contains executable text for a
process, the system treats the file as computational and marks all of the
permanent storage pages in the file as computational. If the file does not contain
executable text, the system treats the file as non-computational file and marks
all of the pages in the file as non-computational.
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.
When a page fault is taken on a file due to an instruction fetch, only the
non-computational file transitions to the computational state. When a process page
faults on a file (meaning the process references a part of the file that is not
currently cached in memory in a permanent storage page), the process generates a
page fault. If the page fault is due to an instruction fetch (meaning the process
was trying to load an instruction from the page to execute), the kernel marks the
file as computational. This involves marking all pages in the file as
computational. A file is either completely computational or non-computational.
Once a file has been marked as computational, it remains marked as a
computational file until the file is deleted (or the system is rebooted). Thus, a
file remains marked as computational even after it is moved or renamed.
Page replacement
The AIX page replacement daemons scan memory a page at a time to find pages to
evict in order to free up memory. The page replacement daemons must choose pages
carefully to minimize the performance impact of paging on the system, and the page
replacement daemons target pages of different classes based on tunable parameter
settings and system conditions.
There are a number of tunable parameters that you can use to control how AIX
selects pages to replace.
minperm and maxperm
The two most basic page replacement tunable parameters are
minperm and maxperm. These
tunable parameters are used to indicate how much memory the AIX kernel should use
to cache non-computational pages. The maxperm tunable parameter indicates the
maximum amount of memory that should be used to cache non-computational pages.
By default, maxperm is an
"un-strict" limit, meaning that the limit can be exceeded.
Making maxperm an un-strict limit allows more
non-computational files to be cached in memory when there is available free
memory. The maxperm limit can be made a
"strict" limit by setting the
strict_maxperm tunable parameter to 1. When
maxperm is a strict-limit, the kernel does not allow the
number of non-computational pages to exceed maxperm,
even if there is free memory available. Thus, the disadvantage with making
maxperm a strict limit is that the number of
non-computational pages cannot grow beyond maxperm and
consume more memory when there is free memory on the system.
The minperm limit indicates the target minimum amount
of memory that should be used for non-computational pages.
The number of non-computational pages is referred to as
numperm: The vmstat –v
command displays the numperm value for a system as a
percentage of a system’s real memory.
Figure 1 below gives an overview of how these tunable
parameters work under different system conditions:
Figure 1. minperm and maxperm limits
When the number of non-computational pages (numperm)
is greater than or equal to maxperm, the AIX page
replacement daemons strictly target non-computational pages (for example, cached
files that are not executables).
When the number of non-computational pages (numperm)
is less than or equal to minperm, the AIX page
replacement daemons target both computational and non-computational pages. In this
case, AIX scans both classes of pages and evicts the least recently used pages.
When the number of non-computational pages (numperm)
is between minperm and
maxperm, the lru_file_repage
tunable parameter controls what kind of pages the AIX page replacement daemons
should steal (see Figure 2).
lru_file_repage
Figure 2. lru_file_repage tunable parameter
When numperm is between
minperm and maxperm, the AIX
page replacement daemons determine what type of pages to target based on their
internal re-paging table when the lru_file_repage
tunable parameter is set to 1.
The AIX kernel maintains a re-paging table in order to identify pages that are
paged out and then quickly paged back in. When the kernel pages a page out and
then pages it back in, it usually indicates that there is strong demand for the
page and that the page should stay in memory. The kernel maintains an indication
of how many times it re-pages computational pages and how many times it re-pages
non-computational pages. The AIX kernel can then use this information to determine
which class of pages is being re-paged more heavily (thus, indicating which class
of pages is experiencing higher demand). When the
lru_file_repage tunable parameter is set to 1, the AIX
kernel uses this re-paging information to determine whether to target just
non-computational pages or target both computational and non-computational pages.
If the rate of re-paging computational pages is higher than that of
non-computational pages, the AIX kernel just targets non-computational pages
(since there appears to be stronger demand for computational pages). If the rate
of re-paging non-computational pages is higher than that of computational pages,
the AIX kernel targets both computational as well as non-computational pages.
In most customer environments, it is most optimal to just have the kernel always
target non-computational pages, because paging computational pages (for example, a
process’s stack, data, and so forth) usually has a much higher performance cost on
a process than paging non-computational pages (that is, data file cache). Thus,
the lru_file_repage tunable parameter can be set to 0.
In this case, the AIX kernel always targets non-computational pages when
numperm is between minperm
and maxperm.
maxclient
In addition to the minperm and
maxperm tunable parameters, there is also a
maxclient tunable parameter. The
maxclient tunable parameter specifies a limit on the
maximum amount of memory that should be used to cache non-computational client
pages. Because all non-computational client pages are a subset of the total number
of non-computational permanent storage pages, the
maxclient limit must always be less than or equal to
the maxperm limit.
The number of non-computational client pages is referred to as
numclient. The vmstat –v
command displays the numclient value for a system as a
percentage of a system’s real memory.
By default, the maxclient limit is a strict limit.
This means that the AIX kernel does not allow the non-computational client file
cache to exceed the maxclient limit (that is, the AIX
kernel does not allow numclient to exceed
maxclient). When numclient
reaches the maxclient limit, the AIX kernel starts page
replacement in a special, client-only mode. In this client-only mode, the AIX page
replacement daemons strictly target client pages.
Monitoring a system's
memory usage
AIX provides several tools for providing information about counts of the
different pages on the system.
vmstat command
The vmstat command reports information about a
system’s memory usage and statistics about VMM operations like page replacement.
The -v option specified with the
vmstat command displays the percentage of real memory
being used for different classification of pages (see
Listing
1):
Listing 1. vmstat -v
command
# vmstat -v
4980736 memory pages
739175 lruable pages
432957 free pages
1 memory pools
84650 pinned pages
80.0 maxpin percentage
20.0 minperm percentage <<- system’s minperm% setting
80.0 maxperm percentage <<- system’s maxperm%
setting
2.2 numperm percentage << % of memory containing
non-comp. pages
16529 file pages <<- # of non-comp. pages
0.0 compressed percentage
0 compressed pages
2.2 numclient percentage <<- % of memory containing
non-comp. client pages
80.0 maxclient percentage <<- system’s maxclient%
setting
16503 client pages <<- # of client pages
0 remote pageouts scheduled
0 pending disk I/Os blocked with no pbuf
0 paging space I/Os blocked with no psbuf
2484 filesystem I/Os blocked with no fsbuf
0 client filesystem I/Os blocked with no fsbuf
0 external pager filesystem I/Os blocked with no fsbuf
0 Virtualized Partition Memory Page Faults
0.00 Time resolving virtualized partition memory page faults
|
So, in the above example, there are 16529 non-computational file pages mapped
into memory. These non-computational pages consume 2.2 percent of memory. Of these
16529 non-computational file pages, 16503 of them are client pages.
The vmstat output does not provide information about
computational file pages. Information about computational file pages can be
gathered from the svmon command.
svmon command
Another command that can be used to display information about a system’s memory
usage is the svmon command. The
svmon command supports a number of different options
for providing very detailed information about a system’s memory usage.
The -G option to the svmon
command isplays information about how much memory is being used for different
types of pages (see Listing
2):
Listing 2. -G option
to svmon command
# svmon -G
size inuse free pin virtual
memory 786432 209710 576722 133537 188426
pg space 131072 1121
work pers clnt
pin 133537 0 0
in use 188426 0 21284
PageSize PoolSize inuse pgsp pin virtual
s 4 KB - 103966 1121 68929 82682
m 64 KB - 6609 0 4038 6609
|
To understand how a system’s real memory is being used,
svmon displays three columns:
work—working storagepers—persistent storage
(Persistent storage pages are non-client pages—that is, JFS
pages.)clnt—client storage
For each page type, svmon displays two rows:
inuse—number of 4K pages mapped
into memorypin—number of 4K pages mapped into
memory and pinned (pin is a subset of inuse)
So, in the above example, there are 188426 working storage pages mapped into
memory. Of those 188426 working storage pages, 133537 of them are pinned (that is,
can’t be paged out).
There are no persistent storage pages (because there are no JFS filesystems in
use on the system). There are 21284 client storage pages, and none of them are
pinned.
The svmon command does not display the number of
permanent storage pages, but it can be calculated from the
svmon output. As mentioned earlier, the number of
permanent storage pages is the sum of the number of persistent storage pages and
the number of client storage pages. So, in the above example, there are a total of
21284 permanent storage pages on the system:
0 persistent storage pages + 21284 client storage pages = 21284 permanent storage pages
|
The type of information reported by svmon is slightly
different than vmstat. svmon
reports information about the number of in-memory pages of different
types—working, persistent (that is, non-client), and client.
svmon does not report information about computational
versus non-computational. svmon just reports the total
number of in-memory pages of each page type.
In contrast, vmstat reports information about
non-computational versus computational pages.
To illustrate this difference, consider the above example of
svmon output. Some of the 21284 client pages will be
computational, and the rest of the 21284 client pages will be non-computational.
To determine the breakdown of these client pages between computational and
non-computational, use the vmstat command to determine
how many of the 21284 client pages are non-computational.
Displaying and setting
tunable parameters
The vmo command is used to interact with VMM tunable
parameters. The vmo command can be used to display
information about tunable parameters as well as to set the values for tunable
parameters.
To display the current values of all VMM tunable parameters, run the
vmo command with the –L
option:
To display the current values of select VMM tunable parameters, use the
–L option to list names of tunable parameters. For
example, the following command snapshot shows output when listing the current
values of the minperm%,
maxperm%, maxclient%, and
lru_file_repage tunable parameters (see
Listing
3):
Listing 3. Tunable parameters
# vmo -L minperm% -L maxperm% -L maxclient% -L lru_file_repage
NAME CUR DEF BOOT MIN MAX UNIT TYPE
DEPENDENCIES
--------------------------------------------------------------------------------
lru_file_repage 1 1 1 0 1 boolean D
--------------------------------------------------------------------------------
maxclient% 80 80 80 1 100 % memory D
maxperm%
minperm%
--------------------------------------------------------------------------------
maxperm% 80 80 80 1 100 % memory D
minperm%
maxclient%
--------------------------------------------------------------------------------
minperm% 20 20 20 1 100 % memory D
maxperm%
maxclient%
|
Table 1. Tunable parameters: Column descriptions
| Column |
Description |
| CUR |
This column lists the current values of the tunable parameters. |
| DEF |
This column lists the default values. |
| BOOT |
This column lists the values of the tunable parameters at the time the
system was booted. |
| MIN |
This column lists the minimum values of the tunable parameters. |
| MAX |
This column lists the maximum values of the tunable parameters. |
| UNIT |
This column lists the unit in which the tunable parameter is specified. |
The vmo command supports changing the value of a
tunable parameter immediately or to defer changing the value of a tunable
parameter until the system is rebooted. To change the above tunable parameters and
have the changes take affect immediately and on all subsequent reboots, specify
the -p option. Here is an example (see
Listing
4):
Listing 4. -p option
# vmo -p -o lru_file_repage=0 -o maxclient%=90 -o maxperm%=90 -o minperm%=3
Setting minperm% to 3 in nextboot file
Setting maxperm% to 90 in nextboot file
Setting maxclient% to 90 in nextboot file
Setting lru_file_repage to 0 in nextboot file
Setting minperm% to 3
Setting maxperm% to 90
Setting maxclient% to 90
Setting lru_file_repage to 0
|
Suggested tunable
parameter settings
The vast majority of workloads benefit from the VMM page replacement daemons
targeting non-computational pages. Thus, the following suggested tunable
parameters provide the best performance for the majority of workloads (see
Listing
5):
Listing 5. Tunable parameters with best
performance
lru_file_repage = 0
maxperm = 90%
maxclient = 90%
minperm = 3%
strict_maxclient = 1 (default)
strict_maxperm = 0 (default)
|
These tunable parameters can be set with the vmo
command (see Listing
6):
Listing 6. Tunable parameters set with the
vmo command
# vmo –p –o lru_file_repage=0 –o maxclient%=90 –o maxperm%=90 –o minperm%=3
# vmo –p –o strict_maxclient=1 –o strict_maxperm=0
|
The settings can be viewed with the vmo –L command.
These tunable parameter settings apply to AIX Version 5.2 and AIX Version 5.3.
To set these tunable parameters on AIX Version 5.2, AIX Version 5.2 TL6 or later
is required. To set the above tunable parameters on AIX Version 5.3, AIX Version
5.3 TL1 or later is required.
The above tunable parameters settings are the default settings for AIX Version
6.1.
Conclusion
The AIX VMM classifies pages based on use. You can use system tunable parameters
to control the behavior of the AIX page replacement daemons and control how AIX
treats different classes of pages page replacement. Tuning the AIX VMM can result
in significant performance improvements for workloads.
Resources
Learn
- See
VMM
page replacement tuning
for more information.
- IBM Redbooks:
The AIX 5L Practical Performance Tools and Tuning Guide is a comprehensive guide
about AIX performance monitoring and tuning tools.
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David
is an AIX kernel architect. His responsibilities include designing and
developing new technology for the AIX operating system. His background
is in kernel development. You can contact him at dhepkin@us.ibm.com.
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