proc_sys_kernel - control a range of kernel parameters - man(5) - [phpMan]

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proc_sys_kernel(5) File Formats Manual proc_sys_kernel(5)

NAME
/proc/sys/kernel/ - control a range of kernel parameters

DESCRIPTION
/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 val‐
ues 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 re‐
computing (and triggered a recomputation of msgmni based on the current amount of
available memory and number of IPC namespaces). Echoing "0" disabled automatic recom‐
puting. (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_MOD‐
ULES (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 ear‐
lier).

/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 sin‐
gle 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 archi‐
tectures 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 random‐
ized. 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 de‐
fault 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/ini‐
trd.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 sys‐
tem.

/proc/sys/kernel/rtsig-nr
(Up to and including Linux 2.6.7.) This file shows the number of POSIX real-time sig‐
nals 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 sched‐
uled 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 sec‐
comp(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 de‐
struction 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 cre‐
ated, 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 seg‐
ments 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 Sys‐
tem 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 writ‐
ten 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 en‐
abled. For further details see the Linux kernel source file Documentation/ad‐
min-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 struc‐
tures 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.

man > proc_sys_kernel(5)