Documentation/sysctl/kernel.txt

Documentation for /proc/sys/kernel/*	kernel version 2.2.10
	(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

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:
- acct
- core_pattern
- core_uses_pid
- ctrl-alt-del
- dentry-state
- domainname
- hostname
- hotplug
- java-appletviewer           [ binfmt_java, obsolete ]
- java-interpreter            [ binfmt_java, obsolete ]
- 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 ]
- suid_dumpable
- sysrq                       ==> Documentation/sysrq.txt
- tainted
- threads-max
- version

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

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.

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

core_pattern:

core_pattern is used to specify a core dumpfile pattern name.
. max length 64 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:
	%<NUL>	'%' is dropped
	%%	output one '%'
	%p	pid
	%u	uid
	%g	gid
	%s	signal number
	%t	UNIX time of dump
	%h	hostname
	%e	executable filename
	%<OTHER> both are dropped

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

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.

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

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: delay a few seconds (to give klogd time to record the oops output) and
   then panic.  If the `panic' sysctl is also non-zero then the machine will
   be rebooted.

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

pid_max:

PID allocation wrap value.  When the kenrel'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.

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

suid_dumpable:

This value can be used to query and set the core dump mode for setuid
or otherwise protected/tainted binaries. The modes are

0 - (default) - traditional behaviour. Any process which has changed
	privilege levels or is execute only will not be dumped
1 - (debug) - all processes dump core when possible. The core dump is
	owned by the current user and no security is applied. This is
	intended for system debugging situations only. Ptrace is unchecked.
2 - (suidsafe) - any binary which normally would not be dumped is dumped
	readable by root only. This allows the end user to remove
	such a dump but not access it directly. For security reasons
	core dumps in this mode will not overwrite one another or
	other files. This mode is appropriate when adminstrators are
	attempting to debug problems in a normal environment.

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

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.