Oracle Solaris Tunable Parameters Reference Manual

Tuning the Solaris Kernel

The following table describes the different ways tunable parameters can be applied.

Apply Tunable Parameters in These Ways 

For More Information 

Modify the /etc/system file

/etc/system File

Use the kernel debugger (kmdb)

kmdb Command

Use the modular debugger (mdb)

mdb Command

Use the ndd command to set TCP/IP parameters

Chapter 4, Internet Protocol Suite Tunable Parameters

Modify the /etc/default files

Tuning NCA Parameters

/etc/system File

The /etc/system file provides a static mechanism for adjusting the values of kernel parameters. Values specified in this file are read at boot time and are applied. Any changes that are 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 parameters were applied in two phases:

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

Starting 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 the /etc/system file before modifying it so that you can easily recover from incorrect value. For example:

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

If a value specified in the /etc/system file 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 Return key 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, type the name of the good /etc/system file or /dev/null:

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

If /dev/null is specified, this path causes the system to attempt to read from /dev/null for its configuration information. Because this file 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.

kmdb Command

kmdb is a interactive kernel debugger with the same general syntax as mdb. An advantage of interactive kernel debugger is that you can set breakpoints. When a breakpoint is reached, you can examine data or step through the execution of kernel code.

kmdb can be loaded and unloaded on demand. You do not have to reboot the system to perform interactive kernel debugging, as was the case with kadb.

For more information, see kmdb(1).

mdb Command

Starting with the Solaris 8 release is the modular debugger, mdb, is unique among 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. mdb is the recommended post-mortem debugger for the kernel.

For more information, see mdb(1).

Example—Using mdb to Change a Value

To change the value of the integer parameter maxusers from 495 to 512, 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 parameter is to be set to.

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

When using either kmdb or mdb debugger, the module name prefix is not required. 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 debugger (assuming the UFS module is loaded). The ufs: prefix is required when set in the /etc/system file.