Understanding Linux Kernel Parameters

 Documentation for /proc/sys/kernel/* kernel version 2.2.10
  (c) 1998, 1999,  Rik van Riel 
  (c) 2009,        Shen Feng
 
 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
 - acpi_video_flags
 - auto_msgmni
 - bootloader_type      [ X86 only ]
 - bootloader_version      [ X86 only ]
 - callhome       [ S390 only ]
 - cap_last_cap
 - core_pattern
 - core_pipe_limit
 - core_uses_pid
 - ctrl-alt-del
 - dmesg_restrict
 - domainname
 - hostname
 - hotplug
 - kptr_restrict
 - kstack_depth_to_print       [ X86 only ]
 - l2cr                        [ PPC only ]
 - modprobe                    ==> Documentation/debugging-modules.txt
 - modules_disabled
 - msgmax
 - msgmnb
 - msgmni
 - nmi_watchdog
 - osrelease
 - ostype
 - overflowgid
 - overflowuid
 - panic
 - panic_on_oops
 - panic_on_unrecovered_nmi
 - panic_on_stackoverflow
 - pid_max
 - powersave-nap               [ PPC only ]
 - printk
 - printk_delay
 - printk_ratelimit
 - printk_ratelimit_burst
 - randomize_va_space
 - real-root-dev               ==> Documentation/initrd.txt
 - reboot-cmd                  [ SPARC only ]
 - rtsig-max
 - rtsig-nr
 - sem
 - sg-big-buff                 [ generic SCSI device (sg) ]
 - shm_rmid_forced
 - shmall
 - shmmax                      [ sysv ipc ]
 - shmmni
 - softlockup_thresh
 - stop-a                      [ SPARC only ]
 - sysrq                       ==> Documentation/sysrq.txt
 - tainted
 - threads-max
 - unknown_nmi_panic
 - 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 % 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.
 
 ==============================================================
 
 acpi_video_flags:
 
 flags
 
 See Doc*/kernel/power/video.txt, it allows mode of video boot to be
 set during run time.
 
 ==============================================================
 
 auto_msgmni:
 
 Enables/Disables automatic recomputing of msgmni upon memory add/remove
 or upon ipc namespace creation/removal (see the msgmni description
 above). Echoing "1" into this file enables msgmni automatic recomputing.
 Echoing "0" turns it off. auto_msgmni default value is 1.
 
 
 ==============================================================
 
 bootloader_type:
 
 x86 bootloader identification
 
 This gives the bootloader type number as indicated by the bootloader,
 shifted left by 4, and OR'd with the low four bits of the bootloader
 version.  The reason for this encoding is that this used to match the
 type_of_loader field in the kernel header; the encoding is kept for
 backwards compatibility.  That is, if the full bootloader type number
 is 0x15 and the full version number is 0x234, this file will contain
 the value 340 = 0x154.
 
 See the type_of_loader and ext_loader_type fields in
 Documentation/x86/boot.txt for additional information.
 
 ==============================================================
 
 bootloader_version:
 
 x86 bootloader version
 
 The complete bootloader version number.  In the example above, this
 file will contain the value 564 = 0x234.
 
 See the type_of_loader and ext_loader_ver fields in
 Documentation/x86/boot.txt for additional information.
 
 ==============================================================
 
 callhome:
 
 Controls the kernel's callhome behavior in case of a kernel panic.
 
 The s390 hardware allows an operating system to send a notification
 to a service organization (callhome) in case of an operating system panic.
 
 When the value in this file is 0 (which is the default behavior)
 nothing happens in case of a kernel panic. If this value is set to "1"
 the complete kernel oops message is send to the IBM customer service
 organization in case the mainframe the Linux operating system is running
 on has a service contract with IBM.
 
 ==============================================================
 
 cap_last_cap
 
 Highest valid capability of the running kernel.  Exports
 CAP_LAST_CAP from the kernel.
 
 ==============================================================
 
 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 (may be shortened)
  %E executable path
  % 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_pipe_limit:
 
 This sysctl is only applicable when core_pattern is configured to pipe
 core files to a user space helper (when the first character of
 core_pattern is a '|', see above).  When collecting cores via a pipe
 to an application, it is occasionally useful for the collecting
 application to gather data about the crashing process from its
 /proc/pid directory.  In order to do this safely, the kernel must wait
 for the collecting process to exit, so as not to remove the crashing
 processes proc files prematurely.  This in turn creates the
 possibility that a misbehaving userspace collecting process can block
 the reaping of a crashed process simply by never exiting.  This sysctl
 defends against that.  It defines how many concurrent crashing
 processes may be piped to user space applications in parallel.  If
 this value is exceeded, then those crashing processes above that value
 are noted via the kernel log and their cores are skipped.  0 is a
 special value, indicating that unlimited processes may be captured in
 parallel, but that no waiting will take place (i.e. the collecting
 process is not guaranteed access to /proc//).  This
 value defaults to 0.
 
 ==============================================================
 
 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.
 
 ==============================================================
 
 dmesg_restrict:
 
 This toggle indicates whether unprivileged users are prevented
 from using dmesg(8) to view messages from the kernel's log buffer.
 When dmesg_restrict is set to (0) there are no restrictions. When
 dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
 dmesg(8).
 
 The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
 default value of dmesg_restrict.
 
 ==============================================================
 
 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".
 
 ==============================================================
 
 kptr_restrict:
 
 This toggle indicates whether restrictions are placed on
 exposing kernel addresses via /proc and other interfaces.  When
 kptr_restrict is set to (0), there are no restrictions.  When
 kptr_restrict is set to (1), the default, kernel pointers
 printed using the %pK format specifier will be replaced with 0's
 unless the user has CAP_SYSLOG.  When kptr_restrict is set to
 (2), kernel pointers printed using %pK will be replaced with 0's
 regardless of privileges.
 
 ==============================================================
 
 kstack_depth_to_print: (X86 only)
 
 Controls the number of words to print when dumping the raw
 kernel stack.
 
 ==============================================================
 
 l2cr: (PPC only)
 
 This flag controls the L2 cache of G3 processor boards. If
 0, the cache is disabled. Enabled if nonzero.
 
 ==============================================================
 
 modules_disabled:
 
 A toggle value indicating if modules are allowed to be loaded
 in an otherwise modular kernel.  This toggle defaults to off
 (0), but can be set true (1).  Once true, modules can be
 neither loaded nor unloaded, and the toggle cannot be set back
 to false.
 
 ==============================================================
 
 nmi_watchdog:
 
 Enables/Disables the NMI watchdog on x86 systems. When the value is
 non-zero the NMI watchdog is enabled and will continuously test all
 online cpus to determine whether or not they are still functioning
 properly. Currently, passing "nmi_watchdog=" parameter at boot time is
 required for this function to work.
 
 If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
 parameter), the NMI watchdog shares registers with oprofile. By
 disabling the NMI watchdog, oprofile may have more registers to
 utilize.
 
 ==============================================================
 
 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_unrecovered_nmi:
 
 The default Linux behaviour on an NMI of either memory or unknown is
 to continue operation. For many environments such as scientific
 computing it is preferable that the box is taken out and the error
 dealt with than an uncorrected parity/ECC error get propagated.
 
 A small number of systems do generate NMI's for bizarre random reasons
 such as power management so the default is off. That sysctl works like
 the existing panic controls already in that directory.
 
 ==============================================================
 
 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.
 
 ==============================================================
 
 panic_on_stackoverflow:
 
 Controls the kernel's behavior when detecting the overflows of
 kernel, IRQ and exception stacks except a user stack.
 This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
 
 0: try to continue operation.
 
 1: panic immediately.
 
 ==============================================================
 
 
 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.
 
 ==============================================================
 
 ns_last_pid:
 
 The last pid allocated in the current (the one task using this sysctl
 lives in) pid namespace. When selecting a pid for a next task on fork
 kernel tries to allocate a number starting from this one.
 
 ==============================================================
 
 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_loglevel: 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_delay:
 
 Delay each printk message in printk_delay milliseconds
 
 Value from 0 - 10000 is allowed.
 
 ==============================================================
 
 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.
 
 ==============================================================
 
 randomize_va_space:
 
 This option can be used to select the type of process address
 space randomization that is used in the system, for architectures
 that support this feature.
 
 0 - Turn the process address space randomization off.  This is the
     default for architectures that do not support this feature anyways,
     and kernels that are booted with the "norandmaps" parameter.
 
 1 - Make the addresses of mmap base, stack and VDSO page randomized.
     This, among other things, implies that shared libraries will be
     loaded to random addresses.  Also for PIE-linked binaries, the
     location of code start is randomized.  This is the default if the
     CONFIG_COMPAT_BRK option is enabled.
 
 2 - Additionally enable heap randomization.  This is the default if
     CONFIG_COMPAT_BRK is disabled.
 
     There are a few legacy applications out there (such as some ancient
     versions of libc.so.5 from 1996) that assume that brk area starts
     just after the end of the code+bss.  These applications break when
     start of the brk area is randomized.  There are however no known
     non-legacy applications that would be broken this way, so for most
     systems it is safe to choose full randomization.
 
     Systems with ancient and/or broken binaries should be configured
     with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
     address space randomization.
 
 ==============================================================
 
 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.
 
 ==============================================================
 
 shm_rmid_forced:
 
 Linux lets you set resource limits, including how much memory one
 process can consume, via setrlimit(2).  Unfortunately, shared memory
 segments are allowed to exist without association with any process, and
 thus might not be counted against any resource limits.  If enabled,
 shared memory segments are automatically destroyed when their attach
 count becomes zero after a detach or a process termination.  It will
 also destroy segments that were created, but never attached to, on exit
 from the process.  The only use left for IPC_RMID is to immediately
 destroy an unattached segment.  Of course, this breaks the way things are
 defined, so some applications might stop working.  Note that this
 feature will do you no good unless you also configure your resource
 limits (in particular, RLIMIT_AS and RLIMIT_NPROC).  Most systems don't
 need this.
 
 Note that if you change this from 0 to 1, already created segments
 without users and with a dead originative process will be destroyed.
 
 ==============================================================
 
 softlockup_thresh:
 
 This value can be used to lower the softlockup tolerance threshold.  The
 default threshold is 60 seconds.  If a cpu is locked up for 60 seconds,
 the kernel complains.  Valid values are 1-60 seconds.  Setting this
 tunable to zero will disable the softlockup detection altogether.
 
 ==============================================================
 
 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.
    8 - A module was forcibly unloaded from the system by rmmod -f.
   16 - A hardware machine check error occurred on the system.
   32 - A bad page was discovered on the system.
   64 - The user has asked that the system be marked "tainted".  This
        could be because they are running software that directly modifies
        the hardware, or for other reasons.
  128 - The system has died.
  256 - The ACPI DSDT has been overridden with one supplied by the user
         instead of using the one provided by the hardware.
  512 - A kernel warning has occurred.
 1024 - A module from drivers/staging was loaded.
 2048 - The system is working around a severe firmware bug.
 4096 - An out-of-tree module has been loaded.
 
 ==============================================================
 
 unknown_nmi_panic:
 
 The value in this file affects behavior of handling NMI. When the
 value is non-zero, unknown NMI is trapped and then panic occurs. At
 that time, kernel debugging information is displayed on console.
 
 NMI switch that most IA32 servers have fires unknown NMI up, for
 example.  If a system hangs up, try pressing the NMI switch.