Documentation for /proc/sys/kernel/*    kernel version 2.2.5

        (c) ,Rik van Riel <riel@nl.linux.org>

 

For general info and legal blurb, please look in README.

 

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This file contains documentation for the sysctl files in

/proc/sys/kernel/ and is valid for Linux kernel version 2.2.

 

The files in this directory can be used to tune and monitor

miscellaneous and general things in the operation of the Linux

kernel. Since some of the files _can_ be used to screw up your

system, it is advisable to read both documentation and source

before actually making adjustments.

 

Currently, these files might (depending on your configuration)

show up in /proc/sys/kernel:

- acct

- ctrl-alt-del

- dentry-state

- domainname

- hostname

- htab-reclaim                [ PPC only ]

- java-appletviewer           [ binfmt_java, obsolete ]

- java-interpreter            [ binfmt_java, obsolete ]

- l2cr                        [ PPC only ]

- modprobe                    ==> Documentation/kmod.txt

- osrelease

- ostype

- panic

- powersave-nap               [ PPC only ]

- printk

- real-root-dev               ==> Documentation/initrd.txt

- reboot-cmd                  [ SPARC only ]

- rtsig-nr

- rtsig-max

- sg-big-buff                 [ generic SCSI device (sg) ]

- shmmax                      [ sysv ipc ]

- version

- zero-paged                  [ PPC only ]

 

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acct:

 

highwater lowwater frequency

 

If BSD-style process accounting is enabled these values control

its behaviour. If free space on filesystem where the log lives

goes below <lowwater>% accounting suspends. If free space gets

above <highwater>% accounting resumes. <Frequency> determines

how often do we check the amount of free space (value is in

seconds). Default:

4 2 30

That is, suspend accounting if there left <= 2% free; resume it

if we got >=4%; consider information about amount of free space

valid for 30 seconds.

 

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ctrl-alt-del:

 

When the value in this file is 0, ctrl-alt-del is trapped and

sent to the init(1) program to handle a graceful restart.

When, however, the value is > 0, Linux's reaction to a Vulcan

Nerve Pinch (tm) will be an immediate reboot, without even

syncing its dirty buffers.

 

Note: when a program (like dosemu) has the keyboard in 'raw'

mode, the ctrl-alt-del is intercepted by the program before it

ever reaches the kernel tty layer, and it's up to the program

to decide what to do with it.

 

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domainname & hostname:

 

These files can be controlled to set the domainname and

hostname of your box. For the classic darkstar.frop.org

a simple:

# echo "darkstar" > /proc/sys/kernel/hostname

# echo "frop.org" > /proc/sys/kernel/domainname

would suffice to set your hostname and domainname.

 

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htab-reclaim: (PPC only)

 

Setting this to a non-zero value, the PowerPC htab

(see Documentation/powerpc/ppc_htab.txt) is pruned

each time the system hits the idle loop.

 

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l2cr: (PPC only)

 

This flag controls the L2 cache of G3 processor boards. If

0, the cache is disabled. Enabled if nonzero.

 

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osrelease, ostype & version:

 

# cat osrelease

2.1.88

# cat ostype

Linux

# cat version

#5 Wed Feb 25 21:49:24 MET

 

The files osrelease and ostype should be clear enough. Version

needs a little more clarification however. The '#5' means that

this is the fifth kernel built from this source base and the

date behind it indicates the time the kernel was built.

The only way to tune these values is to rebuild the kernel :-)

 

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panic:

 

The value in this file represents the number of seconds the

kernel waits before rebooting on a panic. When you use the

software watchdog, the recommended setting is 60.

 

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powersave-nap: (PPC only)

 

If set, Linux-PPC will use the 'nap' mode of powersaving,

otherwise the 'doze' mode will be used.

 

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printk:

 

The four values in printk denote: console_loglevel,

default_message_loglevel, minimum_console_level and

default_console_loglevel respectively.

 

These values influence printk() behavior when printing or

logging error messages. See 'man 2 syslog' for more info on

the different loglevels.

 

- console_loglevel: messages with a higher priority than

  this will be printed to the console

- default_message_level: messages without an explicit priority

  will be printed with this priority

- minimum_console_loglevel: minimum (highest) value to which

  console_loglevel can be set

- default_console_loglevel: default value for console_loglevel

 

Note: a quick look in linux/kernel/printk.c will reveal that

these variables aren't put inside a structure, so their order

in-core isn't formally guaranteed and garbage values _might_

occur when the compiler changes. (???)

 

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reboot-cmd: (Sparc only)

 

??? This seems to be a way to give an argument to the Sparc

ROM/Flash boot loader. Maybe to tell it what to do after

rebooting. ???

 

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rtsig-max & rtsig-nr:

 

The file rtsig-max can be used to tune the maximum number

of POSIX realtime (queued) signals that can be outstanding

in the system.

 

Rtsig-nr shows the number of RT signals currently queued.

 

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sg-big-buff:

 

This file shows the size of the generic SCSI (sg) buffer.

You can't tune it just yet, but you could change it on

compile time by editing include/scsi/sg.h and changing

the value of SG_BIG_BUFF.

 

There shouldn't be any reason to change this value. If

you can come up with one, you probably know what you

are doing anyway :)

 

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shmmax:

 

This value can be used to query and set the run time limit

on the maximum shared memory segment size that can be created.

Shared memory segments up to 1Gb are now supported in the

kernel.  This value defaults to SHMMAX.

 

==============================================================

 

zero-paged: (PPC only)

 

When enabled (non-zero), Linux-PPC will pre-zero pages in

the idle loop, possibly speeding up get_free_pages. Since

this only affects what the idle loop is doing, you should

enable this and see if anything changes.