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Tune AIX for Optimal Performance

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Lots of great tools are available to help you evaluate and manage performance.


When looking at performance issues, I tend to group them into the following categories:

  • CPU bound
  • Memory bound
  • Disk access
  • Network
  • Process monitoring


This article looks at some of the popular tools available to gather information. Based on that information, you can then implement performance-related changes if required.


Please note: When considering performance tuning, you are strongly advised to gather evidence of the performance loss or examine the area in which you want to make gains before making tuning changes.


Always note any changes you make so that you can back out if the change you made has caused more issues or has not improved performance. Also, be aware of the "little picture" syndrome; it's no good trying to improve, say, disk operations if you have a slow network. The users will notice little difference.


In this article, I will try to be generic when using the tools and thus not specific to certain applications; this helps broaden scope of the article. Let's look at the most common tools that are available to show or gather system performance-related information.


Without a doubt (in my opinion), the best tool to gather or record system information is nmon. Nmon comes with additional tools for presentation. Particularly helpful is the spreadsheet converter, which presents the metrics data in graphical format on your PC using Excel or some other similar spreadsheet product. Download nmon here.


Nmon is typically run from AIX's cron scheduler and commonly runs through a 24-hour cycle. This approach allows the systems administrator to go back in time and view the graphical charts. Nmon will report on the following:

  • CPU
  • Memory
  • Network
  • File systems
  • Disks
  • Top-running processes
  • Paging spaces
  • Summaries


An example of one of the many reports nmon generates is shown in Figure 1, nmon_cpu. This report shows the CPU usages and the applications/processes that were running during a specific time span. The interval for this demonstration represents the nightly batch processing window. We can see that, between 10:00 p.m. (22:00) and midnight (24:00), CPU usage peaked.



Figure 1: The nmon_cpu report shows CPU usage. (Click images to enlarge.)


To run nmon for a 24-hour cycle (well, nearly 24 hours), you need to know the format of the cycles. It is best to have each daily capture be just short of 24 hours in order to avoid overlap of performance information from one day to the next. The most common parameters are these:

  • -f means output to spreadsheet format
  • -t includes top-running processors
  • -s is interval in seconds
  • -c is the number of counts between intervals
  • -m is the destination directory for the output file, which would be in the format <hostname<date>.nmon


To run for nearly 24 hours (in this demonstration, it's 23.75 hours), I could use the following:

  • -s would be 900 (900 seconds = 15 minutes)
  • -c would be 95 counts


Using the above numbers, we come up with 23.75 hours, like so:

15 mins * 95 / 60 = 23.75 hours


So, the following crontab entry would generate a daily 24-hour system report. Cron will execute this every day at 9:00 a.m. (09:00).

00 09 * * * /usr/bin/nmon -f -t -s 900 -c 95 -m /opt/dump/nmon


The tool that will generally be the first port of call for any system admins when experiencing performance issues is topas. Though topas can record and graph, it's more commonly used in real-time mode to see what's happening on the system at a particular point in time. Topas will show everything you need to know to make a quick decision on where the impact is. To navigate through the different real-time reports, be sure to read the man page. Most operations are carried out using keystrokes from the keyboard. Topas will report on these issues:


  • Memory
  • Paging space
  • NFS
  • CPU
  • Network
  • Workloads
  • Disks
  • Processors using resources


Figure 2 shows a topas screenshot. In this example, it's displaying in real time the top processes that are using the paging space. Displaying this screen is accomplished by hitting : P on the keyboard and then using the arrow keys to move to the PAGESPACE column. To highlight top CPU processors, use the arrow key to move to the CPU% column.



Figure 2: In this screenshot, topas shows the processes that are using the paging space.


The system activity reporter sar command is the stable diet of system admins in gathering command-line-readable information about the system. It's typically run from cron but is occasionally run directly from the command line. Sar will, by default, report on user activity, percentage of CPU activity, the time processors were idle, and the number of CPUs used, though sar can report on many different system activities. The following sar example will generate statistics three times running at five-second intervals:


# sar 5 3

AIX rs6000 1 7 00C23BED4C00    11/17/11

System configuration: lcpu=4 ent=1.00 mode=Uncapped

13:21:35    %usr    %sys    %wio   %idle   physc   %entc

13:21:40      30      59       0      11    1.24   124.3

13:21:45      24      64       1      11    1.09   108.9

13:21:50      36      53       1      10    1.63   163.0

Average       31      58       0      11    1.32   132.0

Check the Loads

One of the oldest and most commonly used commands is the uptime command. It's a favorite because its output is used in many scripts to report on load averages. It's commonly used in alert email scripts that are run on the system and are sent when load averages hit a certain level, like 60 or 70, to forewarn system admins of a potential issue. The output shown below produces the total time the system has been up in a day and the number of users connected. The load average is displayed in intervals of 1, 5, and 15 minutes:


$ uptime
03:37PM up 179 days,  4:38, 27 users, load average: 3.34, 2.39, 2.26


Mpstat will report on all CPU devices, which is particularly useful when investigating CPU bottlenecks. In the following output, mpstat will run once at a 5-second interval:


# mpstat 5 1

System configuration: lcpu=4 ent=1.0 mode=Uncapped

cpu  min  maj  mpc  int   cs  ics   rq  mig lpa sysc us sy wt id   pc  %ec  lcs

  0 4940    0    1  632  685  268    0  320 100 263924 42 55  0  4 0.57 35.1  277

  1  990    0    3 1387 2234  805    0  684 100 130290 28 47  0 25 0.27 16.6  649

  2 3943    0    2  531  663  223    0  389 100 276520 44 54  0  3 0.57 34.9  270

  3 1298    0    2 1856 2742  846    0  752 100 82141 31 40  0 29 0.22 13.4  650

ALL 11171    0    8 4406 6324 2142    0 2145 100 752875 39 51  0 10 1.63 163.1 1846


Of particular interest is the column maj; this is the total major page faults (i.e., when AIX tries to get a page that is not resident in real memory). In the above output, there is none, so all looks good. A quick way to see how busy your CPUs are is to use this command:


# mpstat -s

System configuration: lcpu=4 ent=1.0 mode=Uncapped

   Proc0  Proc2
 39.20%   38.60%
cpu0    cpu1    cpu2  cpu3
20.54% 18.66% 20.00% 18.61%


In the above output, we can see that we have two physical CPUs split into two logical CPUs. The percentages of the spread show how busy they are.


To look at memory consumption, svmon is the command line tool for you. Svmon will report on general memory usage across the system. It can be run at intervals to gather information over a period. AIX differs from other UNIX operating systems in that it will use all the available memory from the start, so if you see the system using nearly all the memory, this is normal. To display the global memory spread that is in use on a system, with the usage in GB, use this code:

 # svmon -G -O unit=GB

Unit: GB


               size       inuse        free         pin     virtual  available

memory         8.50        8.48        0.02        2.14        13.9       1.04

pg space       17.1        7.24

               work        pers        clnt       other

pin            1.85           0           0        0.29

in use         6.94           0        1.54


The above output reports on size of real memory, total memory currently in use, free memory, memory that is pinned (that is, it will stay in memory and not be swapped out to paging space), and  virtual memory. The report also displays information on workloads and persistent and client memory.

To find out how much memory user ukinst2 is using, use this code:


# svmon -U | head -n 3; svmon -U -O unit=GB |grep ukinst2

User                                 Inuse      Pin     Pgsp  Virtual

ukinst2                              1.16     0.03     0.03     0.50


In the above output, we can determine that user ukinst2 is using 1.16 GB of  real memory, 30MB is used by the paging space, 30MB is pinned in memory, and 50MB is currently in virtual memory.


You can also use svmon -U to produce a list of all users and their respective memory use.

There is also the vmstat command, which provides reports similar to those of svmon but without as much detail.


If the system is suffering from extreme disk activity, you need to identify the source. The lvmstat command will provide information on hotspots contained in your Volume Groups, or Logical Volumes. First, enable logging using lvmstat on the required Volume Group—in this example, appsvg:


# lvmstat -v appsvg -e


Next, you could run a report on all Logical Volumes contained in that Volume Group, running at 2-second intervals, producing five reports:


# lvmstat -v appsvg 2 5

Logical Volume       iocnt   Kb_read   Kb_wrtn      Kbps

  fslv05             19460       280   4165952   1091.84

  fslv04               291        20     68480     17.95

  loglv00               16         0        64      0.02

  fslv03                 0         0         0      0.00

  fslv02                 0         0         0      0.00

Logical Volume       iocnt   Kb_read   Kb_wrtn      Kbps

  fslv04               416        16    104448  52232.00

Logical Volume       iocnt   Kb_read   Kb_wrtn      Kbps

  fslv04               404        12    101376  50694.00

Logical Volume       iocnt   Kb_read   Kb_wrtn      Kbps

  fslv04               390        12     98304  49158.00

Logical Volume       iocnt   Kb_read   Kb_wrtn      Kbps

  fslv04               386        12     97280  48646.00


In the above output, we can determine that the Logical Volume fslv04 is getting a high write hit rate, which will require further investigation. To produce a running output of the Logical Volume fslv04, you could use this command:


# lvmstat -l fslv04 2

Log_part  mirror#  iocnt   Kb_read   Kb_wrtn      Kbps

      16       1     165        60     32764      7.76

       1       1     162        24     32648      7.72

       4       1     157        48     32768      7.75

      32       1     131         0     32532      7.69

      10       1     130         0     32768      7.74


To disable the logging on the Volume Group appsvg, use this:


# lvmstat -l appsvg -d

Multi-Threading Enabled

Be sure to have simultaneous multi-threading processors enabled on your AIX box to increase process run times. If your AIX box does linear heavy number-crunching, there will probably be no gains. To see if it's active, use this:


# smtctl

This system is SMT capable.

This system supports up to 2 SMT threads per processor.

SMT is currently enabled.

SMT boot mode is not set.

SMT threads are bound to the same physical processor.

proc0 has 2 SMT threads.

Bind processor 0 is bound with proc0

Bind processor 1 is bound with proc0


Note that the above output states "SMT is currently enabled." This means it's active. To enable it if it is not active, use this:


 # smtctl -m on



There will be occasions when you need to get a process completed, but it's in contention with other processes during peak load times on the system. One method available is to bind the process to a dedicated CPU. Using the bindprocessor command, display your current processors:


# bindprocessor -q

The available processors are:  0 1


In the above output, we have two processors. Let's now bind a DB2 instance to processor 1. First, get the process ID of the db2instance, which in this example is db2sysc, which is owned by aixdev:


# ps -ef |grep db2sysc

  aixdev 4325606 7209196   0 17:49:38      -  0:00 db2sysc 0

Next, bind process ID 4325606 to processor 1, like so:


# bindprocessor 4325606 1


To confirm it's bound, the BND column should have a value of 1 for the process ID of 4325606, which it has:


# ps -emo THREAD| grep db2sysc

  USER     PID    PPID       TID ST  CP PRI SC    WCHAN        F     TT BND COMMAND

aixdev 4325606 7209196         - A    0  60  1        -    40001      - 1     db2sysc 0


After load-processing has completed, you can unbind the process and confirm it is unbound:


# bindprocessor -u 4325606

# ps -emo THREAD| grep db2sysc

  USER     PID    PPID       TID ST  CP PRI SC    WCHAN        F     TT BND COMMAND

  aixdev 4325606 7209196         - A    0  60  1        -    40001    -   - d b2sysc 0

Sparsely Does It

Coming across sparse files is a common occurrence when dealing with database systems, especially Oracle. Sparse files have indexes that are not mapped to a data block, causing null data; thus, it's difficult to gauge the size, though the du command can report it correctly. You may think the file is, say, 1GB in size when in fact it could be 10 or 20GB in size. Sparse files can cause headaches, as they fill up file-system space and tie up file-system resources if there are a lot of them. These files can be moved around, but be sure you use the correct tool like gnu tar or  backup. If you don't, the copy will go on forever and the file system will run out of space. To identify a sparse file, use the fileplace command:


# fileplace myfile

File: myfile Size: 52299 bytes Vol: /dev/hd3
Blk Size: 4096 Frag Size: 4096 Nfrags: 1

Logical Extent
00060386       1 frags    4096 Bytes, 100.0%
 unallocated   12 frags    49152 Bytes 0.0%


Notice in the above output of the file myfile the unallocated 4K 12 blocks; this is indicative of a sparse file. Normal files will have no unallocated blocks. Now that you know how to determine a sparse file, you can monitor or act on it.

Thanks for the Memory

When an application is in use and requires more memory, if there is not enough in the system to give it more, then portions will be paged into the paging space (see the Paging Space section below). You can avoid this paging by locking the application into memory, also known as pinning. The application to be pinned will require further configuration for this, so use the following vmo command to view current attributes set:


# vmo -a


To enable pinning, use this command:


# vmo –p –o v_pinshm=1


In normal operations, AIX will move page frames that contain files and programs around in order to free up memory. To help AIX know what percentage to reach before it does this, three settings can be used:

  • maxperm
  • minperm
  • maxclient


If the amount of pages is above the maxclient and maxperm, then pages will be moved. If the amount of pages is between minperm and maxclient, pages are moved based on past history page movement. There is a direct memory relationship between maxclient and maxperm; it should either be the same as or less than the value of maxperm.


 # vmo -a |grep -w maxperm%

              maxperm% = 90

# vmo -a |grep -w maxclient%

            maxclient% = 90


The minperm% should ideally be set quite low:


# vmo -a |grep -w minperm%

              minperm% = 5

Paging Space

The paging space, sometimes called swap space, is used when an application or process cannot be accommodated fully into the current memory allocation. When this happens, memory pages are moved out to the paging space. (Actually, there's a lot more to it than that, but that that topic is out of scope for this article.) So how much paging space should you have? Well, that depends on the type of applications you're running. As a rule of thumb, I create the paging space size to be 1.5 times the amount of the memory size. By default, AIX will use the paging device to hold the dump file if the system crashes. This is the case if you have a system with a small amount of memory. Boxes with large amounts of memory have separate dump devices for their core dumps; however, you can create your own dump devices no matter how small your memory is.


Some system admins have very little paging space configured to ensure everything stays in memory. This approach is generally used when performance is a high priority and no paging of the application should happen.


A newly installed AIX box with 2GB of memory would as a rule have 512MB for the paging space:


# lsps -s

Total Paging Space   Percent Used

      512MB               23%


To view full details of the paging space, use this command:


# lsps -a

Page Space      Physical Volume   Volume Group Size      %Used Active  Auto  Type Chksum

hd6             hdisk0            rootvg       512MB     23     yes   yes    lv   0


Paging space usage should be reviewed constantly. If you're running out of paging space, increase the paging space device. If you're still running out of paging space, create a device and place it on different disks.

Async It

If you have database systems like Oracle, DB2, or Sybase, be sure to enable async I/O as some database systems won't run without it. Async I/O allows other processing to continue before it has committed a transaction. A maximum of 10 processes running per CPU is generally considered the standard. To see if async I/O is running, use this command:


# pstat -A| grep aio

67 s   430f1    2008a  unbound      RR  27   0            aioserver

 70 s   46011    3a076  unbound      RR  27   0           aioserver

104 s   680ff    28088  unbound      RR  27   0           aioserver


In AIX version 6 and up, aio is dynamic and can be viewed via the ioo command.

Increase Bandwidth

To increase network bandwidth to your box, you can create an etherchannel. This involves using two or more network cards and binding them up to create an etherchannel interface. Doing so can double the capacity of the data going through the network card. Also consider enabling jumbo frames, which allows bigger frames through your card than the standard 1500 frames allowed. Extra work needs to be done on the switch side for both of the above changes to work.


The performance tools I covered in this article allow the system admin to gather information from the system on the current (and past) workloads. Using this information, anyone should be able to find what the potential performance issues might be and, more importantly, where the source of the problem is.


David Tansley

David Tansley has over 10 years experience as a pSeries AIX administrator. When not working, he enjoys playing badminton and watching Formula 1. But his favorite social activity is touring on his GSA motorbike with his wife and fellow riders.



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