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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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proc_sys_kernel(5) File Formats Manual proc_sys_kernel(5)
/proc/sys/kernel/ - control a range of kernel parameters
/proc/sys/kernel/
This directory contains files controlling a range of
kernel parameters, as described below.
/proc/sys/kernel/acct
This file contains three numbers: highwater, lowwater, and
frequency. If BSD-style process accounting is enabled,
these values control its behavior. If free space on
filesystem where the log lives goes below lowwater
percent, accounting suspends. If free space gets above
highwater percent, accounting resumes. frequency
determines how often the kernel checks the amount of free
space (value is in seconds). Default values are 4, 2, and
30. That is, suspend accounting if 2% or less space is
free; resume it if 4% or more space is free; consider
information about amount of free space valid for 30
seconds.
/proc/sys/kernel/auto_msgmni (Linux 2.6.27 to Linux 3.18)
From Linux 2.6.27 to Linux 3.18, this file was used to
control recomputing of the value in
/proc/sys/kernel/msgmni upon the addition or removal of
memory or upon IPC namespace creation/removal. Echoing
"1" into this file enabled msgmni automatic recomputing
(and triggered a recomputation of msgmni based on the
current amount of available memory and number of IPC
namespaces). Echoing "0" disabled automatic recomputing.
(Automatic recomputing was also disabled if a value was
explicitly assigned to /proc/sys/kernel/msgmni.) The
default value in auto_msgmni was 1.
Since Linux 3.19, the content of this file has no effect
(because msgmni defaults to near the maximum value
possible), and reads from this file always return the
value "0".
/proc/sys/kernel/cap_last_cap (since Linux 3.2)
See capabilities(7).
/proc/sys/kernel/cap-bound (from Linux 2.2 to Linux 2.6.24)
This file holds the value of the kernel capability
bounding set (expressed as a signed decimal number). This
set is ANDed against the capabilities permitted to a
process during execve(2). Starting with Linux 2.6.25, the
system-wide capability bounding set disappeared, and was
replaced by a per-thread bounding set; see
capabilities(7).
/proc/sys/kernel/core_pattern
See core(5).
/proc/sys/kernel/core_pipe_limit
See core(5).
/proc/sys/kernel/core_uses_pid
See core(5).
/proc/sys/kernel/ctrl-alt-del
This file controls the handling of Ctrl-Alt-Del from the
keyboard. 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 the value is greater than zero,
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.
/proc/sys/kernel/dmesg_restrict (since Linux 2.6.37)
The value in this file determines who can see kernel
syslog contents. A value of 0 in this file imposes no
restrictions. If the value is 1, only privileged users
can read the kernel syslog. (See syslog(2) for more
details.) Since Linux 3.4, only users with the
CAP_SYS_ADMIN capability may change the value in this
file.
/proc/sys/kernel/domainname and /proc/sys/kernel/hostname
can be used to set the NIS/YP domainname and the hostname
of your box in exactly the same way as the commands
domainname(1) and hostname(1), that is:
# 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.
/proc/sys/kernel/hotplug
This file contains the pathname for the hotplug policy
agent. The default value in this file is /sbin/hotplug.
/proc/sys/kernel/htab-reclaim (before Linux 2.4.9.2)
(PowerPC only) If this file is set to a nonzero value, the
PowerPC htab (see kernel file
Documentation/powerpc/ppc_htab.txt) is pruned each time
the system hits the idle loop.
/proc/sys/kernel/keys/
This directory contains various files that define
parameters and limits for the key-management facility.
These files are described in keyrings(7).
/proc/sys/kernel/kptr_restrict (since Linux 2.6.38)
The value in this file determines whether kernel addresses
are exposed via /proc files and other interfaces. A value
of 0 in this file imposes no restrictions. If the value
is 1, kernel pointers printed using the %pK format
specifier will be replaced with zeros unless the user has
the CAP_SYSLOG capability. If the value is 2, kernel
pointers printed using the %pK format specifier will be
replaced with zeros regardless of the user's capabilities.
The initial default value for this file was 1, but the
default was changed to 0 in Linux 2.6.39. Since Linux
3.4, only users with the CAP_SYS_ADMIN capability can
change the value in this file.
/proc/sys/kernel/l2cr
(PowerPC only) This file contains a flag that controls the
L2 cache of G3 processor boards. If 0, the cache is
disabled. Enabled if nonzero.
/proc/sys/kernel/modprobe
This file contains the pathname for the kernel module
loader. The default value is /sbin/modprobe. The file is
present only if the kernel is built with the
CONFIG_MODULES (CONFIG_KMOD in Linux 2.6.26 and earlier)
option enabled. It is described by the Linux kernel
source file Documentation/kmod.txt (present only in Linux
2.4 and earlier).
/proc/sys/kernel/modules_disabled (since Linux 2.6.31)
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. The file is present only if
the kernel is built with the CONFIG_MODULES option
enabled.
/proc/sys/kernel/msgmax (since Linux 2.2)
This file defines a system-wide limit specifying the
maximum number of bytes in a single message written on a
System V message queue.
/proc/sys/kernel/msgmni (since Linux 2.4)
This file defines the system-wide limit on the number of
message queue identifiers. See also
/proc/sys/kernel/auto_msgmni.
/proc/sys/kernel/msgmnb (since Linux 2.2)
This file defines a system-wide parameter used to
initialize the msg_qbytes setting for subsequently created
message queues. The msg_qbytes setting specifies the
maximum number of bytes that may be written to the message
queue.
/proc/sys/kernel/ngroups_max (since Linux 2.6.4)
This is a read-only file that displays the upper limit on
the number of a process's group memberships.
/proc/sys/kernel/ns_last_pid (since Linux 3.3)
See pid_namespaces(7).
/proc/sys/kernel/ostype and /proc/sys/kernel/osrelease
These files give substrings of /proc/version.
/proc/sys/kernel/overflowgid and /proc/sys/kernel/overflowuid
These files duplicate the files /proc/sys/fs/overflowgid
and /proc/sys/fs/overflowuid.
/proc/sys/kernel/panic
This file gives read/write access to the kernel variable
panic_timeout. If this is zero, the kernel will loop on a
panic; if nonzero, it indicates that the kernel should
autoreboot after this number of seconds. When you use the
software watchdog device driver, the recommended setting
is 60.
/proc/sys/kernel/panic_on_oops (since Linux 2.5.68)
This file controls the kernel's behavior when an oops or
BUG is encountered. If this file contains 0, then the
system tries to continue operation. If it contains 1,
then the system delays a few seconds (to give klogd time
to record the oops output) and then panics. If the
/proc/sys/kernel/panic file is also nonzero, then the
machine will be rebooted.
/proc/sys/kernel/pid_max (since Linux 2.5.34)
This file specifies the value at which PIDs wrap around
(i.e., the value in this file is one greater than the
maximum PID). PIDs greater than this value are not
allocated; thus, the value in this file also acts as a
system-wide limit on the total number of processes and
threads. The default value for this file, 32768, results
in the same range of PIDs as on earlier kernels. On
32-bit platforms, 32768 is the maximum value for pid_max.
On 64-bit systems, pid_max can be set to any value up to
2^22 (PID_MAX_LIMIT, approximately 4 million).
/proc/sys/kernel/powersave-nap (PowerPC only)
This file contains a flag. If set, Linux-PPC will use the
"nap" mode of powersaving, otherwise the "doze" mode will
be used.
/proc/sys/kernel/printk
See syslog(2).
/proc/sys/kernel/pty (since Linux 2.6.4)
This directory contains two files relating to the number
of UNIX 98 pseudoterminals (see pts(4)) on the system.
/proc/sys/kernel/pty/max
This file defines the maximum number of pseudoterminals.
/proc/sys/kernel/pty/nr
This read-only file indicates how many pseudoterminals are
currently in use.
/proc/sys/kernel/random/
This directory contains various parameters controlling the
operation of the file /dev/random. See random(4) for
further information.
/proc/sys/kernel/random/uuid (since Linux 2.4)
Each read from this read-only file returns a randomly
generated 128-bit UUID, as a string in the standard UUID
format.
/proc/sys/kernel/randomize_va_space (since Linux 2.6.12)
Select the address space layout randomization (ASLR)
policy for the system (on architectures that support
ASLR). Three values are supported for this file:
0 Turn ASLR off. This is the default for
architectures that don't support ASLR, and when the
kernel is booted with the norandmaps parameter.
1 Make the addresses of mmap(2) allocations, the
stack, and the VDSO page randomized. Among other
things, this means that shared libraries will be
loaded at randomized addresses. The text segment
of PIE-linked binaries will also be loaded at a
randomized address. This value is the default if
the kernel was configured with CONFIG_COMPAT_BRK.
2 (Since Linux 2.6.25) Also support heap
randomization. This value is the default if the
kernel was not configured with CONFIG_COMPAT_BRK.
/proc/sys/kernel/real-root-dev
This file is documented in the Linux kernel source file
Documentation/admin-guide/initrd.rst (or
Documentation/initrd.txt before Linux 4.10).
/proc/sys/kernel/reboot-cmd (Sparc only)
This file 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?
/proc/sys/kernel/rtsig-max
(Up to and including Linux 2.6.7; see setrlimit(2)) This
file can be used to tune the maximum number of POSIX real-
time (queued) signals that can be outstanding in the
system.
/proc/sys/kernel/rtsig-nr
(Up to and including Linux 2.6.7.) This file shows the
number of POSIX real-time signals currently queued.
/proc/pid/sched_autogroup_enabled (since Linux 2.6.38)
See sched(7).
/proc/sys/kernel/sched_child_runs_first (since Linux 2.6.23)
If this file contains the value zero, then, after a
fork(2), the parent is first scheduled on the CPU. If the
file contains a nonzero value, then the child is scheduled
first on the CPU. (Of course, on a multiprocessor system,
the parent and the child might both immediately be
scheduled on a CPU.)
/proc/sys/kernel/sched_rr_timeslice_ms (since Linux 3.9)
See sched_rr_get_interval(2).
/proc/sys/kernel/sched_rt_period_us (since Linux 2.6.25)
See sched(7).
/proc/sys/kernel/sched_rt_runtime_us (since Linux 2.6.25)
See sched(7).
/proc/sys/kernel/seccomp/ (since Linux 4.14)
This directory provides additional seccomp information and
configuration. See seccomp(2) for further details.
/proc/sys/kernel/sem (since Linux 2.4)
This file contains 4 numbers defining limits for System V
IPC semaphores. These fields are, in order:
SEMMSL The maximum semaphores per semaphore set.
SEMMNS A system-wide limit on the number of semaphores in
all semaphore sets.
SEMOPM The maximum number of operations that may be
specified in a semop(2) call.
SEMMNI A system-wide limit on the maximum number of
semaphore identifiers.
/proc/sys/kernel/sg-big-buff
This file shows the size of the generic SCSI device (sg)
buffer. You can't tune it just yet, but you could change
it at compile time by editing include/scsi/sg.h and
changing the value of SG_BIG_BUFF. However, there
shouldn't be any reason to change this value.
/proc/sys/kernel/shm_rmid_forced (since Linux 3.1)
If this file is set to 1, all System V shared memory
segments will be marked for destruction as soon as the
number of attached processes falls to zero; in other
words, it is no longer possible to create shared memory
segments that exist independently of any attached process.
The effect is as though a shmctl(2) IPC_RMID is performed
on all existing segments as well as all segments created
in the future (until this file is reset to 0). Note that
existing segments that are attached to no process will be
immediately destroyed when this file is set to 1. Setting
this option will also destroy segments that were created,
but never attached, upon termination of the process that
created the segment with shmget(2).
Setting this file to 1 provides a way of ensuring that all
System V shared memory segments are counted against the
resource usage and resource limits (see the description of
RLIMIT_AS in getrlimit(2)) of at least one process.
Because setting this file to 1 produces behavior that is
nonstandard and could also break existing applications,
the default value in this file is 0. Set this file to 1
only if you have a good understanding of the semantics of
the applications using System V shared memory on your
system.
/proc/sys/kernel/shmall (since Linux 2.2)
This file contains the system-wide limit on the total
number of pages of System V shared memory.
/proc/sys/kernel/shmmax (since Linux 2.2)
This file can be used to query and set the run-time limit
on the maximum (System V IPC) shared memory segment size
that can be created. Shared memory segments up to 1 GB
are now supported in the kernel. This value defaults to
SHMMAX.
/proc/sys/kernel/shmmni (since Linux 2.4)
This file specifies the system-wide maximum number of
System V shared memory segments that can be created.
/proc/sys/kernel/sysctl_writes_strict (since Linux 3.16)
The value in this file determines how the file offset
affects the behavior of updating entries in files under
/proc/sys. The file has three possible values:
-1 This provides legacy handling, with no printk
warnings. Each write(2) must fully contain the value
to be written, and multiple writes on the same file
descriptor will overwrite the entire value, regardless
of the file position.
0 (default) This provides the same behavior as for -1,
but printk warnings are written for processes that
perform writes when the file offset is not 0.
1 Respect the file offset when writing strings into
/proc/sys files. Multiple writes will append to the
value buffer. Anything written beyond the maximum
length of the value buffer will be ignored. Writes to
numeric /proc/sys entries must always be at file
offset 0 and the value must be fully contained in the
buffer provided to write(2).
/proc/sys/kernel/sysrq
This file controls the functions allowed to be invoked by
the SysRq key. By default, the file contains 1 meaning
that every possible SysRq request is allowed (in older
kernel versions, SysRq was disabled by default, and you
were required to specifically enable it at run-time, but
this is not the case any more). Possible values in this
file are:
0 Disable sysrq completely
1 Enable all functions of sysrq
> 1 Bit mask of allowed sysrq functions, as follows:
2 Enable control of console logging level
4 Enable control of keyboard (SAK, unraw)
8 Enable debugging dumps of processes etc.
16 Enable sync command
32 Enable remount read-only
64 Enable signaling of processes (term, kill, oom-
kill)
128 Allow reboot/poweroff
256 Allow nicing of all real-time tasks
This file is present only if the CONFIG_MAGIC_SYSRQ kernel
configuration option is enabled. For further details see
the Linux kernel source file
Documentation/admin-guide/sysrq.rst (or
Documentation/sysrq.txt before Linux 4.10).
/proc/sys/kernel/version
This file contains a string such as:
#5 Wed Feb 25 21:49:24 MET 1998
The "#5" means that this is the fifth kernel built from
this source base and the date following it indicates the
time the kernel was built.
/proc/sys/kernel/threads-max (since Linux 2.3.11)
This file specifies the system-wide limit on the number of
threads (tasks) that can be created on the system.
Since Linux 4.1, the value that can be written to
threads-max is bounded. The minimum value that can be
written is 20. The maximum value that can be written is
given by the constant FUTEX_TID_MASK (0x3fffffff). If a
value outside of this range is written to threads-max, the
error EINVAL occurs.
The value written is checked against the available RAM
pages. If the thread structures would occupy too much
(more than 1/8th) of the available RAM pages, threads-max
is reduced accordingly.
/proc/sys/kernel/yama/ptrace_scope (since Linux 3.5)
See ptrace(2).
/proc/sys/kernel/zero-paged (PowerPC only)
This file contains a flag. When enabled (nonzero), Linux-
PPC will pre-zero pages in the idle loop, possibly
speeding up get_free_pages.
proc(5), proc_sys(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 proc_sys_kernel(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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