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|
Documentation for /proc/sys/kernel/* kernel version 2.2.10
(c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
(c) 2009, Shen Feng<shen@cn.fujitsu.com>
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
- hung_task_panic
- hung_task_check_count
- hung_task_timeout_secs
- hung_task_warnings
- kexec_load_disabled
- kptr_restrict
- kstack_depth_to_print [ X86 only ]
- l2cr [ PPC only ]
- modprobe ==> Documentation/debugging-modules.txt
- modules_disabled
- msg_next_id [ sysv ipc ]
- msgmax
- msgmnb
- msgmni
- nmi_watchdog
- osrelease
- ostype
- overflowgid
- overflowuid
- panic
- panic_on_oops
- panic_on_stackoverflow
- panic_on_unrecovered_nmi
- panic_on_warn
- 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
- sem_next_id [ sysv ipc ]
- sg-big-buff [ generic SCSI device (sg) ]
- shm_next_id [ sysv ipc ]
- shm_rmid_forced
- shmall
- shmmax [ sysv ipc ]
- shmmni
- softlockup_all_cpu_backtrace
- soft_watchdog
- stop-a [ SPARC only ]
- sysrq ==> Documentation/sysrq.txt
- sysctl_writes_strict
- tainted
- threads-max
- unknown_nmi_panic
- watchdog
- watchdog_thresh
- 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.
==============================================================
acpi_video_flags:
flags
See Doc*/kernel/power/video.txt, it allows mode of video boot to be
set during run time.
==============================================================
auto_msgmni:
This variable has no effect and may be removed in future kernel
releases. Reading it always returns 0.
Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni
upon memory add/remove or upon ipc namespace creation/removal.
Echoing "1" into this file enabled msgmni automatic recomputing.
Echoing "0" turned it off. auto_msgmni default value was 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:
%<NUL> '%' is dropped
%% output one '%'
%p pid
%P global pid (init PID namespace)
%i tid
%I global tid (init PID namespace)
%u uid
%g gid
%d dump mode, matches PR_SET_DUMPABLE and
/proc/sys/fs/suid_dumpable
%s signal number
%t UNIX time of dump
%h hostname
%e executable filename (may be shortened)
%E executable path
%<OTHER> 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/<crashing pid>/). 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".
==============================================================
hung_task_panic:
Controls the kernel's behavior when a hung task is detected.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
0: continue operation. This is the default behavior.
1: panic immediately.
==============================================================
hung_task_check_count:
The upper bound on the number of tasks that are checked.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
==============================================================
hung_task_timeout_secs:
Check interval. When a task in D state did not get scheduled
for more than this value report a warning.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
0: means infinite timeout - no checking done.
Possible values to set are in range {0..LONG_MAX/HZ}.
==============================================================
hung_task_warnings:
The maximum number of warnings to report. During a check interval
if a hung task is detected, this value is decreased by 1.
When this value reaches 0, no more warnings will be reported.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
-1: report an infinite number of warnings.
==============================================================
kexec_load_disabled:
A toggle indicating if the kexec_load syscall has been disabled. This
value defaults to 0 (false: kexec_load enabled), but can be set to 1
(true: kexec_load disabled). Once true, kexec can no longer be used, and
the toggle cannot be set back to false. This allows a kexec image to be
loaded before disabling the syscall, allowing a system to set up (and
later use) an image without it being altered. Generally used together
with the "modules_disabled" sysctl.
==============================================================
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), the default, there are no restrictions.
When kptr_restrict is set to (1), kernel pointers printed using the %pK
format specifier will be replaced with 0's unless the user has CAP_SYSLOG
and effective user and group ids are equal to the real ids. This is
because %pK checks are done at read() time rather than open() time, so
if permissions are elevated between the open() and the read() (e.g via
a setuid binary) then %pK will not leak kernel pointers to unprivileged
users. Note, this is a temporary solution only. The correct long-term
solution is to do the permission checks at open() time. Consider removing
world read permissions from files that use %pK, and using dmesg_restrict
to protect against uses of %pK in dmesg(8) if leaking kernel pointer
values to unprivileged users is a concern.
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. Generally used with the "kexec_load_disabled" toggle.
==============================================================
msg_next_id, sem_next_id, and shm_next_id:
These three toggles allows to specify desired id for next allocated IPC
object: message, semaphore or shared memory respectively.
By default they are equal to -1, which means generic allocation logic.
Possible values to set are in range {0..INT_MAX}.
Notes:
1) kernel doesn't guarantee, that new object will have desired id. So,
it's up to userspace, how to handle an object with "wrong" id.
2) Toggle with non-default value will be set back to -1 by kernel after
successful IPC object allocation.
==============================================================
nmi_watchdog:
This parameter can be used to control the NMI watchdog
(i.e. the hard lockup detector) on x86 systems.
0 - disable the hard lockup detector
1 - enable the hard lockup detector
The hard lockup detector monitors each CPU for its ability to respond to
timer interrupts. The mechanism utilizes CPU performance counter registers
that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
The NMI watchdog is disabled by default if the kernel is running as a guest
in a KVM virtual machine. This default can be overridden by adding
nmi_watchdog=1
to the guest kernel command line (see Documentation/kernel-parameters.txt).
==============================================================
numa_balancing
Enables/disables automatic page fault based NUMA memory
balancing. Memory is moved automatically to nodes
that access it often.
Enables/disables automatic NUMA memory balancing. On NUMA machines, there
is a performance penalty if remote memory is accessed by a CPU. When this
feature is enabled the kernel samples what task thread is accessing memory
by periodically unmapping pages and later trapping a page fault. At the
time of the page fault, it is determined if the data being accessed should
be migrated to a local memory node.
The unmapping of pages and trapping faults incur additional overhead that
ideally is offset by improved memory locality but there is no universal
guarantee. If the target workload is already bound to NUMA nodes then this
feature should be disabled. Otherwise, if the system overhead from the
feature is too high then the rate the kernel samples for NUMA hinting
faults may be controlled by the numa_balancing_scan_period_min_ms,
numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
numa_balancing_scan_size_mb, and numa_balancing_settle_count sysctls.
==============================================================
numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms,
numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
Automatic NUMA balancing scans tasks address space and unmaps pages to
detect if pages are properly placed or if the data should be migrated to a
memory node local to where the task is running. Every "scan delay" the task
scans the next "scan size" number of pages in its address space. When the
end of the address space is reached the scanner restarts from the beginning.
In combination, the "scan delay" and "scan size" determine the scan rate.
When "scan delay" decreases, the scan rate increases. The scan delay and
hence the scan rate of every task is adaptive and depends on historical
behaviour. If pages are properly placed then the scan delay increases,
otherwise the scan delay decreases. The "scan size" is not adaptive but
the higher the "scan size", the higher the scan rate.
Higher scan rates incur higher system overhead as page faults must be
trapped and potentially data must be migrated. However, the higher the scan
rate, the more quickly a tasks memory is migrated to a local node if the
workload pattern changes and minimises performance impact due to remote
memory accesses. These sysctls control the thresholds for scan delays and
the number of pages scanned.
numa_balancing_scan_period_min_ms is the minimum time in milliseconds to
scan a tasks virtual memory. It effectively controls the maximum scanning
rate for each task.
numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
when it initially forks.
numa_balancing_scan_period_max_ms is the maximum time in milliseconds to
scan a tasks virtual memory. It effectively controls the minimum scanning
rate for each task.
numa_balancing_scan_size_mb is how many megabytes worth of pages are
scanned for a given scan.
==============================================================
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.
==============================================================
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.
==============================================================
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_warn:
Calls panic() in the WARN() path when set to 1. This is useful to avoid
a kernel rebuild when attempting to kdump at the location of a WARN().
0: only WARN(), default behaviour.
1: call panic() after printing out WARN() location.
==============================================================
perf_cpu_time_max_percent:
Hints to the kernel how much CPU time it should be allowed to
use to handle perf sampling events. If the perf subsystem
is informed that its samples are exceeding this limit, it
will drop its sampling frequency to attempt to reduce its CPU
usage.
Some perf sampling happens in NMIs. If these samples
unexpectedly take too long to execute, the NMIs can become
stacked up next to each other so much that nothing else is
allowed to execute.
0: disable the mechanism. Do not monitor or correct perf's
sampling rate no matter how CPU time it takes.
1-100: attempt to throttle perf's sample rate to this
percentage of CPU. Note: the kernel calculates an
"expected" length of each sample event. 100 here means
100% of that expected length. Even if this is set to
100, you may still see sample throttling if this
length is exceeded. Set to 0 if you truly do not care
how much CPU is consumed.
==============================================================
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 - Additionall
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