3    Displaying and Modifying Kernel Subsystem Attributes

The operating system includes various subsystems that define or extend the kernel. Kernel subsystem attributes are used to set kernel variables, which control subsystem behavior or track subsystem statistics. Attributes are assigned default values at boot time. If the default values of some attributes are not appropriate for your system, you must modify these values to provide optimal performance.

To display and modify kernel subsystem attributes, follow these steps:

1. Determine the operating system support for an attribute (Section 3.1)

2. Display the attribute values (Section 3.2)

3. Modify the attribute values (Section 3.3)

3.1    Operating System Support for Attributes

To determine if your version of the operating system supports a particular kernel subsystem attribute, use the following method:

• Use the sysconfig -q subsystem [attribute] command.

  If you do not specify an attribute, the system displays all the subsystem attributes that can be modified with the sysconfig or sysconfigdb command. If the subsystem is not configured, the operating system displays a message similar to the following:

  framework error: subsystem 'inet' not found  
   
  

  If you specify an attribute, only the information specific to that attribute is displayed. For example:

  
  # sysconfig -q inet tcbhashsize 
  inet: 
  tcbhashsize = 32
  

  If the attribute is not supported or if it cannot be accessed by using sysconfig, the operating system displays a message similar to the following:

  inet: 
  tcbhashsize = unknown attribute
  

• Use the dbx p (print) command. If you cannot access the attribute, the operating system displays a message stating that the attribute is not defined or active. For example:

  # dbx -k /vmunix
  dbx version 5.1 
  Type 'help' for help. 
   
  (dbx) p tcp_keepalive_default 
  "tcp_keepalive_default" is not defined or not active 
  (dbx)
  

See sysconfig(8) and dbx(1) for more information.

3.2    Displaying Attribute Values

There are various methods that you can use to display the current value of a kernel subsystem attribute and other descriptive information. Use:

• The Kernel Tuner (dxkerneltuner) graphical user interface (GUI) to display permanent, current (run-time), minimum, and maximum values of attributes. Access the GUI through the Common Desktop Environment (CDE) Application Manager window; select the System_Admin icon, and then select the MonitoringTuning icon. You can then choose the subsystem whose attributes you want to display.

• The sysconfig -q subsystem [attribute] command to display the current (run-time) value of the specified attribute or, if an attribute is not specified, all the attributes for the specified subsystem:

  sysconfig -q subsystem [attribute]  
   
  

  For example:

  # sysconfig -q vm ubc_maxpercent 
  vm: 
  ubc_maxpercent = 100
  

• The sysconfig -Q subsystem [attribute] command to display the maximum and minimum values of the attributes for the specified subsystem. If you specify a particular attribute, the system displays information only for that attribute.

  The command output also includes information about the operations that you can perform on the attribute. The following list describes some of the sysconfig attributes:

  • C - The attribute can be modified when the subsystem is initially loaded; that is, the attribute supports boot time, permanent modifications.

  • R - The attribute can be tuned at run time; that is, you can modify the value that the system is currently using.

  • Q - The attribute's current value can be displayed (queried).

  For example:

  # sysconfig -Q vfs bufcache
  vfs: 
  bufcache -     type=INT op=CQ min_val=0 max_val=50
  

  The output of the previous command shows that the minimum value of the bufcache attribute is 0 and the maximum value is 50. The output also shows that you cannot modify the current (run-time) value.

The following example shows how to use the dbx p (print) command to display the current value of a kernel variable, instead of an attribute. For example:

# dbx -k /vmunix 
dbx version 5.1
Type 'help' for help. 
 
(dbx) p ipport_userreserved 
5000  
(dbx)

See dxkerneltuner(8), sysconfig(8), and dbx(1) for more information.

3.3    Modifying Attribute Values

There are various methods that you can use to modify attribute values. The method you use depends on the version of the operating system you are running and whether you want to modify the current (run-time) value of an attribute or modify an attribute's permanent value. You must be root to modify attribute values.

The following sections describe how to modify the current and permanent values.

3.3.1    Current Value

In some cases, you can modify the current (run-time) value of an attribute. This allows you to determine if modifying an attribute will improve your system performance without rebooting the system. Not all attributes can be tuned at run time, and the temporary modifications are lost when you reboot the system. Use the sysconfig -Q command to determine whether an attribute can be tuned at run time.

To modify an attribute's current (run-time) value, use one of the following methods:

• The Kernel Tuner (dxkerneltuner) GUI, if the attribute supports this operation. Access the GUI through the Common Desktop Environment (CDE) Application Manager window; select the System_Admin icon, and then select the MonitoringTuning icon. Choose the subsystem whose attribute you want to modify, and enter the new value in the Current Value field.

• The sysconfig -r command, if the attribute supports this operation. Use the following command syntax:

  sysconfig -r subsystem attribute=value
  

  For example:

  
  # sysconfig -r inet tcp_keepinit=30 
  tcp_keepinit: reconfigured
  

The following example shows how to use the dbx assign command to modify the current value of a kernel variable, instead of an attribute. However, modifications made with the dbx assign command are lost when you reboot the system. Use the following command syntax:

dbx assign attribute=value

For example:

# dbx -k /vmunix 
dbx version 5.1 
Type 'help' for help. 
 
(dbx) assign ipport_userreserved=60000 
60000 
(dbx)

See dxkerneltuner(8), sysconfig(8), and dbx(1) for more information.

3.3.2    Permanent Value

To modify an attribute's permanent (boot-time) value, the sysconfigtab file must contain the subsystem name, the attribute name, and the value of the attribute. Do not manually modify the sysconfigtab file. To make these modifications, use one of the following methods:

• The Kernel Tuner (dxkerneltuner) GUI. Access the GUI through the Common Desktop Environment (CDE) Application Manager window, select the System_Admin icon, and then select the MonitoringTuning icon. Choose the subsystem whose attribute you want to modify, and enter the new value in the Boot Time Value field.

• The sysconfigdb command. Use the following command syntax:

  sysconfigdb -a -f stanza_file subsystem
  

  The stanza_file is a specially formatted file that contains the name of the subsystem and a list of attributes and their values. This file is merged into the sysconfigtab file. See stanza(4) for more information.

  To use the new attribute value, you must invoke the sysconfig -r command if the attribute can be tuned at run time, or reboot the system.

In addition, you can use the dbx patch command to modify the value of a variable, as well as the on-disk /vmunix image value.

See dxkerneltuner(8), sysconfig(8), and sysconfigdb(8) for more information. See the System Administration manual for information about modifying the system configuration file.