Solaris Tunable Parameters Reference Manual

Tuning the Solaris Kernel

The table below describes the different ways tuning parameters can be applied.

Apply Tuning Parameters in These Ways 

For More Information 

Modifying the /etc/system file

/etc/system File

Using the kernel debugger (kadb)


Using the modular debugger (mdb)


Using the ndd command to set TCP/IP parameters

Chapter 4, TCP/IP Tunable Parameters

Modifying the /etc/default files

Overview of Tuning NCA Parameters

/etc/system File

The /etc/system file provides a static mechanism for adjusting the values of kernel variables. Values specified in this file are read at boot time and are applied. Any changes made to the file are not applied to the operating system until the system is rebooted.

Prior to the Solaris 8 release, /etc/system entries that set the values of system variables were applied in two phases:

The second phase sometimes caused confusion to users and administrators by setting variables to values that seem to be impermissible or assigning values to variables (for example, max_nprocs) that have a value overridden during the initial configuration.

In the Solaris 8 release, one pass is made to set all the values before the configuration parameters are calculated.

Example—Setting a Parameter in /etc/system

The following /etc/system entry sets the number of read-ahead blocks that are read for file systems mounted using NFS version 2 software.

set nfs:nfs_nra=4

Recovering From an Incorrect Value

Make a copy of /etc/system before modifying it so you can easily recover from incorrect value:

# cp /etc/system /etc/system.good

If a value entered in /etc/system causes the system to become unbootable, you can recover with the following command:

ok boot -a

This command causes the system to ask for the name of various files used in the boot process. Press the carriage return to accept the default values until the name of the /etc/system file is requested. When the Name of system file [/etc/system]: prompt is displayed, enter the name of the good /etc/system file or /dev/null:

Name of system file [/etc/system]: /etc/system.good

If /dev/null is entered, this path causes the system to attempt to read from /dev/null for its configuration information and because it is empty, the system uses the default values. After the system is booted, the /etc/system file can be corrected.

For more information on system recovery, see System Administration Guide: Basic Administration.


kadb is a bootable kernel debugger with the same general syntax as adb. For the exceptions, see kadb(1M). One advantage of kadb is that the user can set breakpoints and when the breakpoint is reached, examine data or step through the execution of kernel code.

If the system is booted with the kadb -d command, values for variables in the core kernel can be set, but values for loadable modules would have to be set when the module was actually loaded.

For a brief tutorial on using the kadb command, see “Debugging” in Writing Device Drivers.


Starting with the Solaris 8 release is the modular debugger, mdb(1), which is unique among available Solaris debuggers because it is easily extensible. A programming API is available that allows compilation of modules to perform desired tasks within the context of the debugger.

mdb also includes a number of desirable usability features including command-line editing, command history, built-in output pager, syntax checking, and command pipelining. This is the recommended post-mortem debugger for the kernel.

Example—Using mdb to Change a Value

To change the value of the integer variable maxusers from 5 to 6, do the following:

# mdb -kw
Loading modules: [ unix krtld genunix ip logindmux ptm nfs ipc lofs ]
> maxusers/D
maxusers:       495
> maxusers/W 200
maxusers:       0x1ef           =       0x200
> $q

Replace maxusers with the actual address of the item to be changed as well as the value the variable is to be set to.

For more information on using the modular debugger, see the Solaris Modular Debugger Guide.

When using kadb or mdb, the module name prefix is not required because after a module is loaded, its symbols form a common name space with the core kernel symbols and any other previously loaded module symbols.

For example, ufs:ufs_WRITES would be accessed as ufs_WRITES in each of the debuggers (assuming the UFS module is loaded), but would require the ufs: prefix when set in the /etc/system file. Including the module name prefix kadb results in an undefined symbol message.