proc_sys_kernel - Man Page

control a range of kernel parameters



This directory contains files controlling a range of kernel parameters, as described below.


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).


See core(5).


See core(5).


See core(5).


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 has the hostname "darkstar" and DNS (Internet Domain Name Server) domainname "", 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.


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

(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.


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.


(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.


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.


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.


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.


This file defines the maximum number of pseudoterminals.


This read-only file indicates how many pseudoterminals are currently in use.


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:


Turn ASLR off. This is the default for architectures that don't support ASLR, and when the kernel is booted with the norandmaps parameter.


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.


(Since Linux 2.6.25) Also support heap randomization. This value is the default if the kernel was not configured with CONFIG_COMPAT_BRK.


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?


(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.


(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:


The maximum semaphores per semaphore set.


A system-wide limit on the number of semaphores in all semaphore sets.


The maximum number of operations that may be specified in a semop(2) call.


A system-wide limit on the maximum number of semaphore identifiers.


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:


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.


(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.


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).


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:


Disable sysrq completely


Enable all functions of sysrq

> 1

Bit mask of allowed sysrq functions, as follows:


Enable control of console logging level


Enable control of keyboard (SAK, unraw)


Enable debugging dumps of processes etc.


Enable sync command


Enable remount read-only


Enable signaling of processes (term, kill, oom-kill)


Allow reboot/poweroff


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).


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.

See Also

proc(5), proc_sys(5)


2023-09-30 Linux man-pages 6.7