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Documentation for /proc/sys/kernel/*    kernel version 2.2.10

        (c) 1998, 1999,  Rik van Riel 

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

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

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:

- acpi_video_flags

- acct

- core_pattern

- core_uses_pid

- ctrl-alt-del

- dentry-state

- domainname

- hostname

- hotplug

- java-appletviewer           [ binfmt_java, obsolete ]

- java-interpreter            [ binfmt_java, obsolete ]

- kstack_depth_to_print       [ X86 only ]

- l2cr                        [ PPC only ]

- modprobe                    ==> Documentation/kmod.txt

- msgmax

- msgmnb

- msgmni

- osrelease

- ostype

- overflowgid

- overflowuid

- panic

- pid_max

- powersave-nap               [ PPC only ]

- printk

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

- reboot-cmd                  [ SPARC only ]

- rtsig-max

- rtsig-nr

- sem

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

- shmall

- shmmax                      [ sysv ipc ]

- shmmni

- stop-a                      [ SPARC only ]

- sysrq                       ==> Documentation/sysrq.txt

- tainted

- threads-max

- version

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

acpi_video_flags:

flags

See Doc*/kernel/power/video.txt, it allows mode of video boot to be

set during run time.

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

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 % accounting suspends. If free space gets

above % accounting resumes.  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.

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

core_pattern:

core_pattern is used to specify a core dumpfile pattern name.

. max length 128 characters; default value is "core"

. core_pattern is used as a pattern template for the output filename;

  certain string patterns (beginning with '%') are substituted with

  their actual values.

. backward compatibility with core_uses_pid:

        If core_pattern does not include "%p" (default does not)

        and core_uses_pid is set, then .PID will be appended to

        the filename.

. corename format specifiers:

        %       '%' is dropped

        %%      output one '%'

        %p      pid

        %u      uid

        %g      gid

        %s      signal number

        %t      UNIX time of dump

        %h      hostname

        %e      executable filename

        % both are dropped

. If the first character of the pattern is a '|', the kernel will treat

  the rest of the pattern as a command to run.  The core dump will be

  written to the standard input of that program instead of to a file.

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

core_uses_pid:

The default coredump filename is "core".  By setting

core_uses_pid to 1, the coredump filename becomes core.PID.

If core_pattern does not include "%p" (default does not)

and core_uses_pid is set, then .PID will be appended to

the filename.

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

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.

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

domainname & hostname:

These files can be used to set the NIS/YP domainname and the

hostname of your box in exactly the same way as the commands

domainname and hostname, i.e.:

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

# echo "mydomain" > /proc/sys/kernel/domainname

has the same effect as

# hostname "darkstar"

# domainname "mydomain"

Note, however, that the classic darkstar.frop.org has the

hostname "darkstar" and DNS (Internet Domain Name Server)

domainname "frop.org", not to be confused with the NIS (Network

Information Service) or YP (Yellow Pages) domainname. These two

domain names are in general different. For a detailed discussion

see the hostname(1) man page.

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

hotplug:

Path for the hotplug policy agent.

Default value is "/sbin/hotplug".

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

l2cr: (PPC only)

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

0, the cache is disabled. Enabled if nonzero.

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

kstack_depth_to_print: (X86 only)

Controls the number of words to print when dumping the raw

kernel stack.

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

osrelease, ostype & version:

# cat osrelease

2.1.88

# cat ostype

Linux

# cat version

#5 Wed Feb 25 21:49:24 MET 1998

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 :-)

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

overflowgid & overflowuid:

if your architecture did not always support 32-bit UIDs (i.e. arm, i386,

m68k, sh, and sparc32), a fixed UID and GID will be returned to

applications that use the old 16-bit UID/GID system calls, if the actual

UID or GID would exceed 65535.

These sysctls allow you to change the value of the fixed UID and GID.

The default is 65534.

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

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.

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

panic_on_oops:

Controls the kernel's behaviour when an oops or BUG is encountered.

0: try to continue operation

1: panic immediately.  If the `panic' sysctl is also non-zero then the

   machine will be rebooted.

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

pid_max:

PID allocation wrap value.  When the kernel's next PID value

reaches this value, it wraps back to a minimum PID value.

PIDs of value pid_max or larger are not allocated.

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

powersave-nap: (PPC only)

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

otherwise the 'doze' mode will be used.

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

printk:

The four values in printk denote: console_loglevel,

default_message_loglevel, minimum_console_loglevel 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

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

printk_ratelimit:

Some warning messages are rate limited. printk_ratelimit specifies

the minimum length of time between these messages (in jiffies), by

default we allow one every 5 seconds.

A value of 0 will disable rate limiting.

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

printk_ratelimit_burst:

While long term we enforce one message per printk_ratelimit

seconds, we do allow a burst of messages to pass through.

printk_ratelimit_burst specifies the number of messages we can

send before ratelimiting kicks in.

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

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. ???

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

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.

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

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 :)

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

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.

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

softlockup_thresh:

This value can be used to lower the softlockup tolerance

threshold. The default threshold is 10s.  If a cpu is locked up

for 10s, the kernel complains.  Valid values are 1-60s.

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

tainted:

Non-zero if the kernel has been tainted.  Numeric values, which

can be ORed together:

  1 - A module with a non-GPL license has been loaded, this

      includes modules with no license.

      Set by modutils >= 2.4.9 and module-init-tools.

  2 - A module was force loaded by insmod -f.

      Set by modutils >= 2.4.9 and module-init-tools.

  4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.