proc - process information pseudo-file system


The proc file system is a pseudo-file system which is used as an interface to kernel data structures. It is commonly mounted at /proc. Most of it is read-only, but some files allow kernel variables to be changed.

The following outline gives a quick tour through the /proc hierarchy.

There is a numerical subdirectory for each running process; the subdirectory is named by the process ID. Each such subdirectory contains the following pseudo-files and directories.
/proc/[pid]/auxv (since 2.6.0-test7)
This contains the contents of the ELF interpreter information passed to the process at exec time. The format is one unsigned long ID plus one unsigned long value for each entry. The last entry contains two zeros.
This holds the complete command line for the process, unless the process is a zombie. In the latter case, there is nothing in this file: that is, a read on this file will return 0 characters. The command-line arguments appear in this file as a set of null-separated strings, with a further null byte ('\0') after the last string.
/proc/[pid]/coredump_filter (since kernel 2.6.23)
See core(5).
/proc/[pid]/cpuset (since kernel 2.6.12)
See cpuset(7).
This is a symbolic link to the current working directory of the process. To find out the current working directory of process 20, for instance, you can do this:

$ cd /proc/20/cwd; /bin/pwd
Note that the pwd command is often a shell built-in, and might not work properly. In bash(1), you may use pwd -P.
In a multithreaded process, the contents of this symbolic link are not available if the main thread has already terminated (typically by calling pthread_exit(3)).
This file contains the environment for the process. The entries are separated by null bytes ('\0'), and there may be a null byte at the end. Thus, to print out the environment of process 1, you would do:

$ (cat /proc/1/environ; echo) | tr '\000' '\n'
Under Linux 2.2 and later, this file is a symbolic link containing the actual pathname of the executed command. This symbolic link can be dereferenced normally; attempting to open it will open the executable. You can even type /proc/[pid]/exe to run another copy of the same executable as is being run by process [pid]. In a multithreaded process, the contents of this symbolic link are not available if the main thread has already terminated (typically by calling pthread_exit(3)).
Under Linux 2.0 and earlier /proc/[pid]/exe is a pointer to the binary which was executed, and appears as a symbolic link. A readlink(2) call on this file under Linux 2.0 returns a string in the format:

For example, [0301]:1502 would be inode 1502 on device major 03 (IDE, MFM, etc. drives) minor 01 (first partition on the first drive).
find(1) with the -inum option can be used to locate the file.
This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor, and which is a symbolic link to the actual file. Thus, 0 is standard input, 1 standard output, 2 standard error, etc.
In a multithreaded process, the contents of this directory are not available if the main thread has already terminated (typically by calling pthread_exit(3)).
Programs that will take a filename as a command-line argument, but will not take input from standard input if no argument is supplied, or that write to a file named as a command-line argument, but will not send their output to standard output if no argument is supplied, can nevertheless be made to use standard input or standard out using /proc/[pid]/fd. For example, assuming that -i is the flag designating an input file and -o is the flag designating an output file:

$ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...
and you have a working filter.
/proc/self/fd/N is approximately the same as /dev/fd/N in some Unix and Unix-like systems. Most Linux MAKEDEV scripts symbolically link /dev/fd to /proc/self/fd, in fact.
Most systems provide symbolic links /dev/stdin, /dev/stdout, and /dev/stderr, which respectively link to the files 0, 1, and 2 in /proc/self/fd. Thus the example command above could be written as:

$ foobar -i /dev/stdin -o /dev/stdout ...
/proc/[pid]/fdinfo/ (since kernel 2.6.22)
This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor. The contents of each file can be read to obtain information about the corresponding file descriptor, for example:

$ cat /proc/12015/fdinfo/4 pos: 1000
flags: 01002002
The pos field is a decimal number showing the current file offset. The flags field is an octal number that displays the file access mode and file status flags (see open(2)).
The files in this directory are readable only by the owner of the process.
/proc/[pid]/limits (since kernel 2.6.24)
This file displays the soft limit, hard limit, and units of measurement for each of the process's resource limits (see getrlimit(2)). The file is protected to only allow reading by the real UID of the process.
A file containing the currently mapped memory regions and their access permissions.
The format is:
address perms offset dev inode pathname
08048000-08056000 r-xp 00000000 03:0c 64593 /usr/sbin/gpm
08056000-08058000 rw-p 0000d000 03:0c 64593 /usr/sbin/gpm
08058000-0805b000 rwxp 00000000 00:00 0
40000000-40013000 r-xp 00000000 03:0c 4165 /lib/
40013000-40015000 rw-p 00012000 03:0c 4165 /lib/
4001f000-40135000 r-xp 00000000 03:0c 45494 /lib/ 40135000-4013e000 rw-p 00115000 03:0c 45494 /lib/ 4013e000-40142000 rw-p 00000000 00:00 0
bffff000-c0000000 rwxp 00000000 00:00 0
where "address" is the address space in the process that it occupies, "perms" is a set of permissions:

r = read
w = write
x = execute
s = shared
p = private (copy on write)
"offset" is the offset into the file/whatever, "dev" is the device (major:minor), and "inode" is the inode on that device. 0 indicates that no inode is associated with the memory region, as the case would be with BSS (uninitialized data).
Under Linux 2.0 there is no field giving pathname.
This file can be used to access the pages of a process's memory through open(2), read(2), and lseek(2).
/proc/[pid]/mountinfo (since Linux 2.6.26)
This file contains information about mount points. It contains lines of the form:
36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue (1)(2)(3) (4) (5) (6) (7) (8) (9) (10) (11)
The numbers in parentheses are labels for the descriptions below:
(1) mount ID: unique identifier of the mount (may be reused
after umount(2)).
(2) parent ID: ID of parent mount (or of self for the top of
the mount tree).
(3) major:minor: value of st_dev for files on file system (see
(4) root: root of the mount within the file system.
(5) mount point: mount point relative to the process's root.
(6) mount options: per-mount options.
(7) optional fields: zero or more fields of the form
(8) separator: marks the end of the optional fields.
(9) file system type: name of file system in the form
(10) mount source: file system-specific information or "none".
(11) super options: per-super block options.
Parsers should ignore all unrecognized optional fields. Currently the possible optional fields are:

shared:X mount is shared in peer group X
master:X mount is slave to peer group X
propagate_from:X mount is slave and receives propagation
from peer group X (*)
unbindable mount is unbindable
(*) X is the closest dominant peer group under the process's root. If X is the immediate master of the mount, or if there is no dominant peer group under the same root, then only the "master:X" field is present and not the "propagate_from:X" field.
For more information on mount propagation see: Documentation/filesystems/sharedsubtree.txt in the kernel source tree.
/proc/[pid]/mounts (since Linux 2.4.19)
This is a list of all the file systems currently mounted in the process's mount namespace. The format of this file is documented in fstab(5). Since kernel version 2.6.15, this file is pollable: after opening the file for reading, a change in this file (i.e., a file system mount or unmount) causes select(2) to mark the file descriptor as readable, and poll(2) and epoll_wait(2) mark the file as having an error condition.
/proc/[pid]/mountstats (since Linux 2.6.17)
This file exports information (statistics, configuration information) about the mount points in the process's name space. Lines in this file have the form:
device /dev/sda7 mounted on /home with fstype ext3 [statistics] ( 1 ) ( 2 ) (3 ) (4)
The fields in each line are:
(1) The name of the mounted device (or "nodevice" if there is
no corresponding device).
(2) The mount point within the file system tree.
(3) The file system type.
(4) Optional statistics and configuration information. Cur
rently (as at Linux 2.6.26), only NFS file systems export information via this field.
This file is only readable by the owner of the process.
/proc/[pid]/numa_maps (since Linux 2.6.14)
See numa(7).
/proc/[pid]/oom_adj (since Linux 2.6.11)
This file can be used to adjust the score used to select which process should be killed in an out-of-memory (OOM) situation. The kernel uses this value for a bit-shift operation of the process's oom_score value: valid values are in the range -16 to +15, plus the special value -17, which disables OOM-killing altogether for this process. A positive score increases the likelihood of this process being killed by the OOM-killer; a negative score decreases the likelihood. The default value for this file is 0; a new process inherits its parent's oom_adj setting. A process must be privileged (CAP_SYS_RESOURCE) to update this file.
/proc/[pid]/oom_score (since Linux 2.6.11)
This file displays the current score that the kernel gives to this process for the purpose of selecting a process for the OOMkiller. A higher score means that the process is more likely to be selected by the OOM-killer. The basis for this score is the amount of memory used by the process, with increases (+) or decreases (-) for factors including:
* whether the process creates a lot of children using fork(2)
* whether the process has been running a long time, or has used
a lot of CPU time (-);
* whether the process has a low nice value (i.e., > 0) (+);
* whether the process is privileged (-); and
* whether the process is making direct hardware access (-).
The oom_score also reflects the bit-shift adjustment specified by the oom_adj setting for the process.
Unix and Linux support the idea of a per-process root of the file system, set by the chroot(2) system call. This file is a symbolic link that points to the process's root directory, and behaves as exe, fd/*, etc. do.
In a multithreaded process, the contents of this symbolic link are not available if the main thread has already terminated (typically by calling pthread_exit(3)).
/proc/[pid]/smaps (since Linux 2.6.14)
This file shows memory consumption for each of the process's mappings. For each of mappings there is a series of lines such as the following:

08048000-080bc000 r-xp 00000000 03:02 13130 /bin/bash
Size: 464 kB
Rss: 424 kB
Shared_Clean: 424 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
The first of these lines shows the same information as is displayed for the mapping in /proc/[pid]/maps. The remaining lines show the size of the mapping, the amount of the mapping that is currently resident in RAM, the number of clean and dirty shared pages in the mapping, and the number of clean and dirty private pages in the mapping.
This file is only present if the CONFIG_MMU kernel configuration option is enabled.
Status information about the process. This is used by ps(1). It is defined in /usr/src/linux/fs/proc/array.c.
The fields, in order, with their proper scanf(3) format specifiers, are:
pid %d The process ID.
comm %s The filename of the executable, in parentheses.
This is visible whether or not the executable is swapped out.
state %c One character from the string "RSDZTW" where R is
running, S is sleeping in an interruptible wait, D is waiting in uninterruptible disk sleep, Z is zombie, T is traced or stopped (on a signal), and W is paging.
ppid %d The PID of the parent.
pgrp %d The process group ID of the process.
session %d The session ID of the process.
tty_nr %d The controlling terminal of the process. (The minor
device number is contained in the combination of bits 31 to 20 and 7 to 0; the major device number is in bits 15 to 8.)
tpgid %d The ID of the foreground process group of the con
trolling terminal of the process.
flags %u (%lu before Linux 2.6.22)
The kernel flags word of the process. For bit meanings, see the PF_* defines in <linux/sched.h>. Details depend on the kernel version.
minflt %lu The number of minor faults the process has made
which have not required loading a memory page from disk.
cminflt %lu The number of minor faults that the process's
waited-for children have made.
majflt %lu The number of major faults the process has made
which have required loading a memory page from disk.
cmajflt %lu The number of major faults that the process's
waited-for children have made.
utime %lu Amount of time that this process has been scheduled
in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK). This includes guest time, guest_time (time spent running a virtual CPU, see below), so that applications that are not aware of the guest time field do not lose that time from their calculations.
stime %lu Amount of time that this process has been scheduled
in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK).
cutime %ld Amount of time that this process's waited-for chil
dren have been scheduled in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK). (See also times(2).) This includes guest time, cguest_time (time spent running a virtual CPU, see below).
cstime %ld Amount of time that this process's waited-for chil
dren have been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK).
priority %ld
(Explanation for Linux 2.6) For processes running a real-time scheduling policy (policy below; see sched_setscheduler(2)), this is the negated scheduling priority, minus one; that is, a number in the range -2 to -100, corresponding to real-time priorities 1 to 99. For processes running under a nonreal-time scheduling policy, this is the raw nice value (setpriority(2)) as represented in the kernel. The kernel stores nice values as numbers in the range 0 (high) to 39 (low), corresponding to the user-visible nice range of -20 to 19.
Before Linux 2.6, this was a scaled value based on the scheduler weighting given to this process.
nice %ld The nice value (see setpriority(2)), a value in the
range 19 (low priority) to -20 (high priority).
num_threads %ld
Number of threads in this process (since Linux 2.6). Before kernel 2.6, this field was hard coded to 0 as a placeholder for an earlier removed field.
itrealvalue %ld
The time in jiffies before the next SIGALRM is sent to the process due to an interval timer. Since kernel 2.6.17, this field is no longer maintained, and is hard coded as 0.
starttime %llu (was %lu before Linux 2.6)
The time in jiffies the process started after system boot.
vsize %lu Virtual memory size in bytes.
rss %ld Resident Set Size: number of pages the process has
in real memory. This is just the pages which count towards text, data, or stack space. This does not include pages which have not been demand-loaded in, or which are swapped out.
rsslim %lu Current soft limit in bytes on the rss of the
process; see the description of RLIMIT_RSS in getpriority(2).
startcode %lu
The address above which program text can run.
endcode %lu The address below which program text can run.
startstack %lu
The address of the start (i.e., bottom) of the stack.
kstkesp %lu The current value of ESP (stack pointer), as found
in the kernel stack page for the process.
kstkeip %lu The current EIP (instruction pointer).
signal %lu The bitmap of pending signals, displayed as a deci
mal number. Obsolete, because it does not provide information on real-time signals; use /proc/[pid]/status instead.
blocked %lu The bitmap of blocked signals, displayed as a deci
mal number. Obsolete, because it does not provide information on real-time signals; use /proc/[pid]/status instead.
sigignore %lu
The bitmap of ignored signals, displayed as a decimal number. Obsolete, because it does not provide information on real-time signals; use /proc/[pid]/status instead.
sigcatch %lu
The bitmap of caught signals, displayed as a decimal number. Obsolete, because it does not provide information on real-time signals; use /proc/[pid]/status instead.
wchan %lu This is the "channel" in which the process is wait
ing. It is the address of a system call, and can be looked up in a namelist if you need a textual name. (If you have an up-to-date /etc/psdatabase, then try ps -l to see the WCHAN field in action.)
nswap %lu Number of pages swapped (not maintained).
cnswap %lu Cumulative nswap for child processes (not main
exit_signal %d (since Linux 2.1.22)
Signal to be sent to parent when we die.
processor %d (since Linux 2.2.8)
CPU number last executed on.
rt_priority %u (since Linux 2.5.19; was %lu before Linux 2.6.22)
Real-time scheduling priority, a number in the range 1 to 99 for processes scheduled under a real-time policy, or 0, for non-real-time processes (see sched_setscheduler(2)).
policy %u (since Linux 2.5.19; was %lu before Linux 2.6.22)
Scheduling policy (see sched_setscheduler(2)). Decode using the SCHED_* constants in linux/sched.h.
delayacct_blkio_ticks %llu (since Linux 2.6.18)
Aggregated block I/O delays, measured in clock ticks (centiseconds).
guest_time %lu (since Linux 2.6.24)
Guest time of the process (time spent running a virtual CPU for a guest operating system), measured in clock ticks (divide by sysconf(_SC_CLK_TCK).
cguest_time %ld (since Linux 2.6.24)
Guest time of the process's children, measured in clock ticks (divide by sysconf(_SC_CLK_TCK).
Provides information about memory usage, measured in pages. The columns are:

size total program size
(same as VmSize in /proc/[pid]/status)
resident resident set size
(same as VmRSS in /proc/[pid]/status)
share shared pages (from shared mappings)
text text (code)
lib library (unused in Linux 2.6)
data data + stack
dt dirty pages (unused in Linux 2.6)
Provides much of the information in /proc/[pid]/stat and /proc/[pid]/statm in a format that's easier for humans to parse. Here's an example:

$ cat /proc/$$/status
Name: bash
State: S (sleeping)
Tgid: 3515
Pid: 3515
PPid: 3452
TracerPid: 0
Uid: 1000 1000 1000 1000
Gid: 100 100 100 100
FDSize: 256
Groups: 16 33 100
VmPeak: 9136 kB
VmSize: 7896 kB
VmLck: 0 kB
VmHWM: 7572 kB
VmRSS: 6316 kB
VmData: 5224 kB
VmStk: 88 kB
VmExe: 572 kB
VmLib: 1708 kB
VmPTE: 20 kB
Threads: 1
SigQ: 0/3067
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000010000
SigIgn: 0000000000384004
SigCgt: 000000004b813efb
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: ffffffffffffffff
Cpus_allowed: 00000001
Cpus_allowed_list: 0
Mems_allowed: 1
Mems_allowed_list: 0
voluntary_ctxt_switches: 150
nonvoluntary_ctxt_switches: 545
The fields are as follows:
* Name: Command run by this process.
* State: Current state of the process. One of "R (running)", "S
(sleeping)", "D (disk sleep)", "T (stopped)", "T (tracing stop)", "Z (zombie)", or "X (dead)".
* Tgid: Thread group ID (i.e., Process ID).
* Pid: Thread ID (see gettid(2)).
* TracerPid: PID of process tracing this process (0 if not being
* Uid, Gid: Real, effective, saved set, and file system UIDs
* FDSize: Number of file descriptor slots currently allocated.
* Groups: Supplementary group list.
* VmPeak: Peak virtual memory size.
* VmSize: Virtual memory size.
* VmLck: Locked memory size.
* VmHWM: Peak resident set size ("high water mark").
* VmRSS: Resident set size.
* VmData, VmStk, VmExe: Size of data, stack, and text segments.
* VmLib: Shared library code size.
* VmPTE: Page table entries size (since Linux 2.6.10).
* Threads: Number of threads in process containing this thread.
* SigPnd, ShdPnd: Number of signals pending for thread and for
process as a whole (see pthreads(7) and signal(7)).
* SigBlk, SigIgn, SigCgt: Masks indicating signals being
blocked, ignored, and caught (see signal(7)).
* CapInh, CapPrm, CapEff: Masks of capabilities enabled in
inheritable, permitted, and effective sets (see capabilities(7)).
* CapBnd: Capability Bounding set (since kernel 2.6.26, see
* Cpus_allowed: Mask of CPUs on which this process may run
(since Linux 2.6.24, see cpuset(7)).
* Cpus_allowed_list: Same as previous, but in "list format"
(since Linux 2.6.26, see cpuset(7)).
* Mems_allowed: Mask of memory nodes allowed to this process
(since Linux 2.6.24, see cpuset(7)).
* Mems_allowed_list: Same as previous, but in "list format"
(since Linux 2.6.26, see cpuset(7)).
* voluntary_context_switches, nonvoluntary_context_switches:
Number of voluntary and involuntary context switches (since Linux 2.6.23).
/proc/[pid]/task (since Linux 2.6.0-test6)
This is a directory that contains one subdirectory for each thread in the process. The name of each subdirectory is the numerical thread ID ([tid]) of the thread (see gettid(2)). Within each of these subdirectories, there is a set of files with the same names and contents as under the /proc/[pid] directories. For attributes that are shared by all threads, the contents for each of the files under the task/[tid] subdirectories will be the same as in the corresponding file in the parent /proc/[pid] directory (e.g., in a multithreaded process, all of the task/[tid]/cwd files will have the same value as the /proc/[pid]/cwd file in the parent directory, since all of the threads in a process share a working directory). For attributes that are distinct for each thread, the corresponding files under task/[tid] may have different values (e.g., various fields in each of the task/[tid]/status files may be different for each thread).
In a multithreaded process, the contents of the /proc/[pid]/task directory are not available if the main thread has already terminated (typically by calling pthread_exit(3)).
Advanced power management version and battery information when CONFIG_APM is defined at kernel compilation time.
Contains subdirectories for installed busses.
Subdirectory for PCMCIA devices when CONFIG_PCMCIA is set at kernel compilation time.
Contains various bus subdirectories and pseudo-files containing information about PCI busses, installed devices, and device drivers. Some of these files are not ASCII.
Information about PCI devices. They may be accessed through lspci(8) and setpci(8).
Arguments passed to the Linux kernel at boot time. Often done via a boot manager such as lilo(8) or grub(8).
/proc/config.gz (since Linux 2.6)
This file exposes the configuration options that were used to build the currently running kernel, in the same format as they would be shown in the .config file that resulted when configuring the kernel (using make xconfig, make config, or similar). The file contents are compressed; view or search them using zcat(1), zgrep(1), etc. As long as no changes have been made to the following file, the contents of /proc/config.gz are the same as those provided by :

cat /lib/modules/$(uname -r)/build/.config
/proc/config.gz is only provided if the kernel is configured with CONFIG_IKCONFIG_PROC.
This is a collection of CPU and system architecture dependent items, for each supported architecture a different list. Two common entries are processor which gives CPU number and bogomips; a system constant that is calculated during kernel initialization. SMP machines have information for each CPU.
Text listing of major numbers and device groups. This can be used by MAKEDEV scripts for consistency with the kernel.
/proc/diskstats (since Linux 2.5.69)
This file contains disk I/O statistics for each disk device. See the kernel source file Documentation/iostats.txt for further information.
This is a list of the registered ISA DMA (direct memory access) channels in use.
Empty subdirectory.
List of the execution domains (ABI personalities).
Frame buffer information when CONFIG_FB is defined during kernel compilation.
A text listing of the file systems which are supported by the kernel, namely file systems which were compiled into the kernel or whose kernel modules are currently loaded. (See also filesystems(5).) If a file system is marked with "nodev", this means that it does not require a block device to be mounted (e.g., virtual file system, network file system).
Incidentally, this file may be used by mount(8) when no file system is specified and it didn't manage to determine the file system type. Then file systems contained in this file are tried (excepted those that are marked with "nodev").
Empty subdirectory.
This directory exists on systems with the IDE bus. There are directories for each IDE channel and attached device. Files include:

cache buffer size in KB
capacity number of sectors
driver driver version
geometry physical and logical geometry
identify in hexadecimal
media media type
model manufacturer's model number
settings drive settings
smart_thresholds in hexadecimal
smart_values in hexadecimal
The hdparm(8) utility provides access to this information in a friendly format.
This is used to record the number of interrupts per CPU per IO device. Since Linux 2.6.24, for the i386 and x86_64 architectures, at least, this also includes interrupts internal to the system (that is, not associated with a device as such), such as NMI (nonmaskable interrupt), LOC (local timer interrupt), and for SMP systems, TLB (TLB flush interrupt), RES (rescheduling interrupt), CAL (remote function call interrupt), and possibly others. Very easy to read formatting, done in ASCII.
I/O memory map in Linux 2.4.
This is a list of currently registered Input-Output port regions that are in use.
/proc/kallsyms (since Linux 2.5.71)
This holds the kernel exported symbol definitions used by the modules(X) tools to dynamically link and bind loadable modules. In Linux 2.5.47 and earlier, a similar file with slightly different syntax was named ksyms.
This file represents the physical memory of the system and is stored in the ELF core file format. With this pseudo-file, and an unstripped kernel (/usr/src/linux/vmlinux) binary, GDB can be used to examine the current state of any kernel data structures.
The total length of the file is the size of physical memory (RAM) plus 4KB.
This file can be used instead of the syslog(2) system call to read kernel messages. A process must have superuser privileges to read this file, and only one process should read this file. This file should not be read if a syslog process is running which uses the syslog(2) system call facility to log kernel messages.
Information in this file is retrieved with the dmesg(1) program.
/proc/ksyms (Linux 1.1.23-2.5.47)
See /proc/kallsyms.
The first three fields in this file are load average figures giving the number of jobs in the run queue (state R) or waiting for disk I/O (state D) averaged over 1, 5, and 15 minutes. They are the same as the load average numbers given by uptime(1) and other programs. The fourth field consists of two numbers separated by a slash (/). The first of these is the number of currently executing kernel scheduling entities (processes, threads); this will be less than or equal to the number of CPUs. The value after the slash is the number of kernel scheduling entities that currently exist on the system. The fifth field is the PID of the process that was most recently created on the system.
This file shows current file locks (flock(2) and fcntl(2)) and leases (fcntl(2)).
/proc/malloc (only up to and including Linux 2.2)
This file is only present if CONFIG_DEBUG_MALLOC was defined during compilation.
This file reports statistics about memory usage on the system. It is used by free(1) to report the amount of free and used memory (both physical and swap) on the system as well as the shared memory and buffers used by the kernel.
A text list of the modules that have been loaded by the system. See also lsmod(8).
Before kernel 2.4.19, this file was a list of all the file systems currently mounted on the system. With the introduction of per-process mount namespaces in Linux 2.4.19, this file became a link to /proc/self/mounts, which lists the mount points of the process's own mount namespace. The format of this file is documented in fstab(5).
Memory Type Range Registers. See the kernel source file Documentation/mtrr.txt for details.
various net pseudo-files, all of which give the status of some part of the networking layer. These files contain ASCII structures and are, therefore, readable with cat(1). However, the standard netstat(8) suite provides much cleaner access to these files.
This holds an ASCII readable dump of the kernel ARP table used for address resolutions. It will show both dynamically learned and preprogrammed ARP entries. The format is:
IP address HW type Flags HW address Mask Device 0x1 0x2 00:50:BF:25:68:F3 * eth0 0x1 0xc 00:00:00:00:00:00 * eth0

Here "IP address" is the IPv4 address of the machine and the "HW type" is the hardware type of the address from RFC 826. The flags are the internal flags of the ARP structure (as defined in /usr/include/linux/if_arp.h) and the "HW address" is the data link layer mapping for that IP address if it is known.
The dev pseudo-file contains network device status information. This gives the number of received and sent packets, the number of errors and collisions and other basic statistics. These are used by the ifconfig(8) program to report device status. The format is:
Inter-| Receive | Transmit
face |bytes packets errs drop fifo frame compressed multicast|bytes packets errs drop fifo colls carrier compressed
lo: 2776770 11307 0 0 0 0 0 0 2776770 11307 0 0 0 0 0 0
eth0: 1215645 2751 0 0 0 0 0 0 1782404 4324 0 0 0 427 0 0 ppp0: 1622270 5552 1 0 0 0 0 0 354130 5669 0 0 0 0 0 0 tap0: 7714 81 0 0 0 0 0 0 7714 81 0 0 0 0 0 0

Defined in /usr/src/linux/net/core/dev_mcast.c:
indx interface_name dmi_u dmi_g dmi_address
2 eth0 1 0 01005e000001
3 eth1 1 0 01005e000001
4 eth2 1 0 01005e000001
Internet Group Management Protocol. Defined in /usr/src/linux/net/core/igmp.c.
This file uses the same format as the arp file and contains the current reverse mapping database used to provide rarp(8) reverse address lookup services. If RARP is not configured into the kernel, this file will not be present.
Holds a dump of the RAW socket table. Much of the information is not of use apart from debugging. The "sl" value is the kernel hash slot for the socket, the "local_address" is the local address and protocol number pair. "St" is the internal status of the socket. The "tx_queue" and "rx_queue" are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields are not used by RAW. The "uid" field holds the effective UID of the creator of the socket.
This file holds the ASCII data needed for the IP, ICMP, TCP, and UDP management information bases for an SNMP agent.
Holds a dump of the TCP socket table. Much of the information is not of use apart from debugging. The "sl" value is the kernel hash slot for the socket, the "local_address" is the local address and port number pair. The "rem_address" is the remote address and port number pair (if connected). "St" is the internal status of the socket. The "tx_queue" and "rx_queue" are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields hold internal information of the kernel socket state and are only useful for debugging. The "uid" field holds the effective UID of the creator of the socket.
Holds a dump of the UDP socket table. Much of the information is not of use apart from debugging. The "sl" value is the kernel hash slot for the socket, the "local_address" is the local address and port number pair. The "rem_address" is the remote address and port number pair (if connected). "St" is the internal status of the socket. The "tx_queue" and "rx_queue" are the outgoing and incoming data queue in terms of kernel memory usage. The "tr", "tm->when", and "rexmits" fields are not used by UDP. The "uid" field holds the effective UID of the creator of the socket. The format is:
sl local_address rem_address st tx_queue rx_queue tr rexmits tm->when uid
1: 01642C89:0201 0C642C89:03FF 01 00000000:00000001 01:000071BA 00000000 0
1: 00000000:0801 00000000:0000 0A 00000000:00000000 00:00000000 6F000100 0
1: 00000000:0201 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0

Lists the Unix domain sockets present within the system and their status. The format is:
Num RefCount Protocol Flags Type St Path
0: 00000002 00000000 00000000 0001 03
1: 00000001 00000000 00010000 0001 01 /dev/printer
Here "Num" is the kernel table slot number, "RefCount" is the number of users of the socket, "Protocol" is currently always 0, "Flags" represent the internal kernel flags holding the status of the socket. Currently, type is always "1" (Unix domain datagram sockets are not yet supported in the kernel). "St" is the internal state of the socket and Path is the bound path (if any) of the socket.
Contains major and minor numbers of each partition as well as number of blocks and partition name.
This is a listing of all PCI devices found during kernel initialization and their configuration.
This file has been deprecated in favor of a new /proc interface for PCI (/proc/bus/pci). It became optional in Linux 2.2 (available with CONFIG_PCI_OLD_PROC set at kernel compilation). It became once more nonoptionally enabled in Linux 2.4. Next, it was deprecated in Linux 2.6 (still available with CONFIG_PCI_LEGACY_PROC set), and finally removed altogether since Linux 2.6.17.
A directory with the scsi mid-level pseudo-file and various SCSI low-level driver directories, which contain a file for each SCSI host in this system, all of which give the status of some part of the SCSI IO subsystem. These files contain ASCII structures and are, therefore, readable with cat(1).
You can also write to some of the files to reconfigure the subsystem or switch certain features on or off.
This is a listing of all SCSI devices known to the kernel. The listing is similar to the one seen during bootup. scsi currently supports only the add-single-device command which allows root to add a hotplugged device to the list of known devices.
The command

echo 'scsi add-single-device 1 0 5 0' > /proc/scsi/scsi
will cause host scsi1 to scan on SCSI channel 0 for a device on ID 5 LUN 0. If there is already a device known on this address or the address is invalid, an error will be returned.
[drivername] can currently be NCR53c7xx, aha152x, aha1542, aha1740, aic7xxx, buslogic, eata_dma, eata_pio, fdomain, in2000, pas16, qlogic, scsi_debug, seagate, t128, u15-24f, ultrastore, or wd7000. These directories show up for all drivers that registered at least one SCSI HBA. Every directory contains one file per registered host. Every host-file is named after the number the host was assigned during initialization.
Reading these files will usually show driver and host configuration, statistics, etc.
Writing to these files allows different things on different hosts. For example, with the latency and nolatency commands, root can switch on and off command latency measurement code in the eata_dma driver. With the lockup and unlock commands, root can control bus lockups simulated by the scsi_debug driver.
This directory refers to the process accessing the /proc file system, and is identical to the /proc directory named by the process ID of the same process.
Information about kernel caches. Since Linux 2.6.16 this file is only present if the CONFIG_SLAB kernel configuration option is enabled. The columns in /proc/slabinfo are:

See slabinfo(5) for details.
kernel/system statistics. Varies with architecture. Common entries include:
cpu 3357 0 4313 1362393
The amount of time, measured in units of USER_HZ (1/100ths of a second on most architectures, use sysconf(_SC_CLK_TCK) to obtain the right value), that the system spent in user mode, user mode with low priority (nice), system mode, and the idle task, respectively. The last value should be USER_HZ times the second entry in the uptime pseudo-file.
In Linux 2.6 this line includes three additional columns: iowait - time waiting for I/O to complete (since 2.5.41);
irq - time servicing interrupts (since 2.6.0-test4);
softirq - time servicing softirqs (since 2.6.0-test4).
Since Linux 2.6.11, there is an eighth column, steal stolen time, which is the time spent in other operating systems when running in a virtualized environment
Since Linux 2.6.24, there is a ninth column, guest, which is the time spent running a virtual CPU for guest operating systems under the control of the Linux kernel.
page 5741 1808
The number of pages the system paged in and the number that were paged out (from disk).
swap 1 0
The number of swap pages that have been brought in and out.
intr 1462898
This line shows counts of interrupts serviced since boot time, for each of the possible system interrupts. The first column is the total of all interrupts serviced;
each subsequent column is the total for a particular interrupt.
disk_io: (2,0):(31,30,5764,1,2) (3,0):...
(major,disk_idx):(noinfo, read_io_ops, blks_read, write_io_ops, blks_written)
(Linux 2.4 only)
ctxt 115315
The number of context switches that the system underwent.
btime 769041601
boot time, in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
processes 86031
Number of forks since boot.
procs_running 6
Number of processes in runnable state. (Linux 2.5.45 onwards.)
procs_blocked 2
Number of processes blocked waiting for I/O to complete. (Linux 2.5.45 onwards.)
Swap areas in use. See also swapon(8).
This directory (present since 1.3.57) contains a number of files and subdirectories corresponding to kernel variables. These variables can be read and sometimes modified using the /proc file system, and the (deprecated) sysctl(2) system call.
/proc/sys/abi (since Linux 2.4.10)
This directory may contain files with application binary information. See the kernel source file Documentation/sysctl/abi.txt for more information.
This directory may be empty.
This directory contains device-specific information (e.g., dev/cdrom/info). On some systems, it may be empty.
This directory contains the files and subdirectories for kernel variables related to file systems.
Documentation for files in this directory can be found in the kernel sources in Documentation/binfmt_misc.txt.
/proc/sys/fs/dentry-state (since Linux 2.2)
This file contains information about the status of the directory cache (dcache). The file contains six numbers, nr_dentry, nr_unused, age_limit (age in seconds), want_pages (pages requested by system) and two dummy values.
* nr_dentry is the number of allocated dentries (dcache
entries). This field is unused in Linux 2.2.
* nr_unused is the number of unused dentries.
* age_limit is the age in seconds after which dcache entries can
be reclaimed when memory is short.
* want_pages is nonzero when the kernel has called
shrink_dcache_pages() and the dcache isn't pruned yet.
This file can be used to disable or enable the dnotify interface described in fcntl(2) on a system-wide basis. A value of 0 in this file disables the interface, and a value of 1 enables it.
This file shows the maximum number of cached disk quota entries. On some (2.4) systems, it is not present. If the number of free cached disk quota entries is very low and you have some awesome number of simultaneous system users, you might want to raise the limit.
This file shows the number of allocated disk quota entries and the number of free disk quota entries.
/proc/sys/fs/epoll (since Linux 2.6.28)
This directory contains the file max_user_watches, which can be used to limit the amount of kernel memory consumed by the epoll interface. For further details, see epoll(7).
This file defines a system-wide limit on the number of open files for all processes. (See also setrlimit(2), which can be used by a process to set the per-process limit, RLIMIT_NOFILE, on the number of files it may open.) If you get lots of error messages about running out of file handles, try increasing this value:
echo 100000 > /proc/sys/fs/file-max
The kernel constant NR_OPEN imposes an upper limit on the value that may be placed in file-max.
If you increase /proc/sys/fs/file-max, be sure to increase /proc/sys/fs/inode-max to 3-4 times the new value of /proc/sys/fs/file-max, or you will run out of inodes.
This (read-only) file gives the number of files presently opened. It contains three numbers: the number of allocated file handles; the number of free file handles; and the maximum number of file handles. The kernel allocates file handles dynamically, but it doesn't free them again. If the number of allocated files is close to the maximum, you should consider increasing the maximum. When the number of free file handles is large, you've encountered a peak in your usage of file handles and you probably don't need to increase the maximum.
This file contains the maximum number of in-memory inodes. On some (2.4) systems, it may not be present. This value should be 3-4 times larger than the value in file-max, since stdin, stdout and network sockets also need an inode to handle them. When you regularly run out of inodes, you need to increase this value.
This file contains the first two values from inode-state.
This file contains seven numbers: nr_inodes, nr_free_inodes, preshrink, and four dummy values. nr_inodes is the number of inodes the system has allocated. This can be slightly more than inode-max because Linux allocates them one page full at a time. nr_free_inodes represents the number of free inodes. preshrink is nonzero when the nr_inodes > inode-max and the system needs to prune the inode list instead of allocating more.
/proc/sys/fs/inotify (since Linux 2.6.13)
This directory contains files max_queued_events, max_user_instances, and max_user_watches, that can be used to limit the amount of kernel memory consumed by the inotify interface. For further details, see inotify(7).
This file specifies the grace period that the kernel grants to a process holding a file lease (fcntl(2)) after it has sent a signal to that process notifying it that another process is waiting to open the file. If the lease holder does not remove or downgrade the lease within this grace period, the kernel forcibly breaks the lease.
This file can be used to enable or disable file leases (fcntl(2)) on a system-wide basis. If this file contains the value 0, leases are disabled. A nonzero value enables leases.
/proc/sys/fs/mqueue (since Linux 2.6.6)
This directory contains files msg_max, msgsize_max, and queues_max, controlling the resources used by POSIX message queues. See mq_overview(7) for details.
/proc/sys/fs/overflowgid and /proc/sys/fs/overflowuid
These files allow you to change the value of the fixed UID and GID. The default is 65534. Some file systems only support 16-bit UIDs and GIDs, although in Linux UIDs and GIDs are 32 bits. When one of these file systems is mounted with writes enabled, any UID or GID that would exceed 65535 is translated to the overflow value before being written to disk.
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
The value in this file defines an upper limit for raising the capacity of a pipe using the fcntl(2) F_SETPIPE_SZ operation. This limit applies only to unprivileged processes. The default value for this file is 1,048,576. The value assigned to this file may be rounded upward, to reflect the value actually employed for a convenient implementation. To determine the rounded-up value, display the contents of this file after assigning a value to it. The minimum value that can be assigned to this file is the system page size.
/proc/sys/fs/suid_dumpable (since Linux 2.6.13)
The value in this file determines whether core dump files are produced for set-user-ID or otherwise protected/tainted binaries. Three different integer values can be specified:
0 (default) This provides the traditional (pre-Linux 2.6.13) behavior. A core dump will not be produced for a process which has changed credentials (by calling seteuid(2), setgid(2), or similar, or by executing a set-user-ID or set-group-ID program) or whose binary does not have read permission enabled.
1 ("debug") All processes dump core when possible. The core dump is owned by the file system user ID of the dumping process and no security is applied. This is intended for system debugging situations only. Ptrace is unchecked.
2 ("suidsafe") Any binary which normally would not be dumped (see "0" above) is dumped readable by root only. This allows the user to remove the core dump file but not to read it. For security reasons core dumps in this mode will not overwrite one another or other files. This mode is appropriate when administrators are attempting to debug problems in a normal environment.
This file controls the maximum number of superblocks, and thus the maximum number of mounted file systems the kernel can have. You only need to increase super-max if you need to mount more file systems than the current value in super-max allows you to.
This file contains the number of file systems currently mounted.
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 file system 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/cap-bound (from Linux 2.2 to 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_uses_pid See core(5).
/proc/sys/kernel/ctrl-alt-delThis 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(8) 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/hotplug This file contains the path for the hotplug policy agent. The default value in this file is /sbin/hotplug.
/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.
/proc/sys/kernel/htab-reclaim(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.
(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 path for the kernel module loader. The default value is /sbin/modprobe. The file is only present if the kernel is built with the CONFIG_KMOD option enabled. It is described by the kernel source file Documentation/kmod.txt (only present in kernel 2.4 and earlier).
This file defines a system-wide limit specifying the maximum number of bytes in a single message written on a System V message queue.
This file defines the system-wide limit on the number of message queue identifiers. (This file is only present in Linux 2.4 onwards.)
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/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). 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/printkThe four values in this file are console_loglevel, default_message_loglevel, minimum_console_level, and default_console_loglevel. These values influence printk() behavior when printing or logging error messages. See syslog(2) for more info on the different loglevels. Messages with a higher priority than console_loglevel will be printed to the console. Messages without an explicit priority will be printed with priority default_message_level. minimum_console_loglevel is the minimum (highest) value to which console_loglevel can be set. default_console_loglevel is the default value for console_loglevel.
/proc/sys/kernel/pty (since Linux 2.6.4) This directory contains two files relating to the number of Unix 98 pseudo-terminals (see pts(4)) on the system.
/proc/sys/kernel/pty/max This file defines the maximum number of pseudo-terminals.
/proc/sys/kernel/pty/nrThis read-only file indicates how many pseudo-terminals are currently in use.
/proc/sys/kernel/randomThis directory contains various parameters controlling the operation of the file /dev/random. See random(4) for further information.
/proc/sys/kernel/real-root-devThis file is documented in the kernel source file Documentation/initrd.txt.
/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 (Only in kernels up to and including 2.6.7; see setrlimit(2)) This file can be used to tune the maximum number of POSIX realtime (queued) signals that can be outstanding in the system.
/proc/sys/kernel/rtsig-nr (Only in kernels up to and including 2.6.7.) This file shows the number POSIX real-time signals currently queued.
/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.
This file contains the system-wide limit on the total number of pages of System V shared memory.
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 1GB are now supported in the kernel. This value defaults to SHMMAX.
(available in Linux 2.4 and onwards) This file specifies the system-wide maximum number of System V shared memory segments that can be created.
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 - bitmask 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 signalling of processes (term, kill, oom
128 - allow reboot/poweroff
256 - allow nicing of all real-time tasks
This file is only present if the CONFIG_MAGIC_SYSRQ kernel configuration option is enabled. For further details see the kernel source file Documentation/sysrq.txt.
/proc/sys/kernel/version This file contains a string like:

#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 behind 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.
/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.
This directory contains networking stuff. Explanations for some of the files under this directory can be found in tcp(7) and ip(7).
/proc/sys/net/core/somaxconn This file defines a ceiling value for the backlog argument of listen(2); see the listen(2) manual page for details.
This directory may be empty.
This directory supports Sun remote procedure call for network file system (NFS). On some systems, it is not present.
/proc/sys/vmThis directory contains files for memory management tuning, buffer and cache management.
/proc/sys/vm/drop_caches (since Linux 2.6.16) Writing to this file causes the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
To free pagecache, use echo 1 > /proc/sys/vm/drop_caches; to free dentries and inodes, use echo 2 > /proc/sys/vm/drop_caches;
to free pagecache, dentries and inodes, use echo 3 > /proc/sys/vm/drop_caches.
Because this is a nondestructive operation and dirty objects are not freeable, the user should run sync(8) first.
/proc/sys/vm/legacy_va_layout (since Linux 2.6.9) If nonzero, this disables the new 32-bit memory-mapping layout;
the kernel will use the legacy (2.4) layout for all processes.
/proc/sys/vm/memory_failure_early_kill (since Linux 2.6.32) Control how to kill processes when an uncorrected memory error (typically a 2-bit error in a memory module) that cannot be handled by the kernel is detected in the background by hardware. In some cases (like the page still having a valid copy on disk), the kernel will handle the failure transparently without affecting any applications. But if there is no other up-to-date copy of the data, it will kill processes to prevent any data corruptions from propagating.
The file has one of the following values:
1: Kill all processes that have the corrupted-and-not-reload able page mapped as soon as the corruption is detected. Note this is not supported for a few types of pages, like kernel internally allocated data or the swap cache, but works for the majority of user pages.
0: Only unmap the corrupted page from all processes and only kill a process who tries to access it.
The kill is performed using a SIGBUS signal with si_code set to BUS_MCEERR_AO. Processes can handle this if they want to; see sigaction(2) for more details.
This feature is only active on architectures/platforms with advanced machine check handling and depends on the hardware capabilities.
Applications can override the memory_failure_early_kill setting individually with the prctl(2) PR_MCE_KILL operation.
Only present if the kernel was configured with CONFIG_MEMORY_FAILURE.
/proc/sys/vm/memory_failure_recovery (since Linux 2.6.32) Enable memory failure recovery (when supported by the platform)
1: Attempt recovery.
0: Always panic on a memory failure.
Only present if the kernel was configured with CONFIG_MEMORY_FAILURE.
/proc/sys/vm/oom_dump_tasks (since Linux 2.6.25) Enables a system-wide task dump (excluding kernel threads) to be produced when the kernel performs an OOM-killing. The dump includes the following information for each task (thread, process): thread ID, real user ID, thread group ID (process ID), virtual memory size, resident set size, the CPU that the task is scheduled on, oom_adj score (see the description of /proc/[pid]/oom_adj), and command name. This is helpful to determine why the OOM-killer was invoked and to identify the rogue task that caused it.
If this contains the value zero, this information is suppressed. On very large systems with thousands of tasks, it may not be feasible to dump the memory state information for each one. Such systems should not be forced to incur a performance penalty in OOM situations when the information may not be desired.
If this is set to nonzero, this information is shown whenever the OOM-killer actually kills a memory-hogging task.
The default value is 0.
/proc/sys/vm/oom_kill_allocating_task (since Linux 2.6.24)This enables or disables killing the OOM-triggering task in outof-memory situations.
If this is set to zero, the OOM-killer will scan through the entire tasklist and select a task based on heuristics to kill. This normally selects a rogue memory-hogging task that frees up a large amount of memory when killed.
If this is set to nonzero, the OOM-killer simply kills the task that triggered the out-of-memory condition. This avoids a possibly expensive tasklist scan.
If /proc/sys/vm/panic_on_oom is nonzero, it takes precedence over whatever value is used in /proc/sys/vm/oom_kill_allocating_task.
The default value is 0.
/proc/sys/vm/overcommit_memory This file contains the kernel virtual memory accounting mode. Values are:

0: heuristic overcommit (this is the default)
1: always overcommit, never check
2: always check, never overcommit
In mode 0, calls of mmap(2) with MAP_NORESERVE are not checked, and the default check is very weak, leading to the risk of getting a process "OOM-killed". Under Linux 2.4 any nonzero value implies mode 1. In mode 2 (available since Linux 2.6), the total virtual address space on the system is limited to (SS + RAM*(r/100)), where SS is the size of the swap space, and RAM is the size of the physical memory, and r is the contents of the file /proc/sys/vm/overcommit_ratio.
/proc/sys/vm/overcommit_ratio See the description of /proc/sys/vm/overcommit_memory.
/proc/sys/vm/panic_on_oom (since Linux 2.6.18)This enables or disables a kernel panic in an out-of-memory situation.
If this file is set to the value 0, the kernel's OOM-killer will kill some rogue process. Usually, the OOM-killer is able to kill a rogue process and the system will survive.
If this file is set to the value 1, then the kernel normally panics when out-of-memory happens. However, if a process limits allocations to certain nodes using memory policies (mbind(2) MPOL_BIND) or cpusets (cpuset(7)) and those nodes reach memory exhaustion status, one process may be killed by the OOM-killer. No panic occurs in this case: because other nodes' memory may be free, this means the system as a whole may not have reached an out-of-memory situation yet.
If this file is set to the value 2, the kernel always panics when an out-of-memory condition occurs.
The default value is 0. 1 and 2 are for failover of clustering. Select either according to your policy of failover.
The value in this file controls how aggressively the kernel will swap memory pages. Higher values increase agressiveness, lower values decrease aggressiveness. The default value is 60.
/proc/sysrq-trigger (since Linux 2.4.21)Writing a character to this file triggers the same SysRq function as typing ALT-SysRq-<character> (see the description of /proc/sys/kernel/sysrq). This file is normally only writable by root. For further details see the kernel source file Documentation/sysrq.txt.
Subdirectory containing the pseudo-files msg, sem and shm. These files list the System V Interprocess Communication (IPC) objects (respectively: message queues, semaphores, and shared memory) that currently exist on the system, providing similar information to that available via ipcs(1). These files have headers and are formatted (one IPC object per line) for easy understanding. svipc(7) provides further background on the information shown by these files.
Subdirectory containing the pseudo-files and subdirectories for tty drivers and line disciplines.
/proc/uptimeThis file contains two numbers: the uptime of the system (seconds), and the amount of time spent in idle process (seconds).
/proc/versionThis string identifies the kernel version that is currently running. It includes the contents of /proc/sys/kernel/ostype, /proc/sys/kernel/osrelease and /proc/sys/kernel/version. For example:
Linux version 1.0.9 (quinlan@phaze) #1 Sat May 14 01:51:54 EDT 1994
/proc/vmstat (since Linux 2.6)
This file displays various virtual memory statistics.
/proc/zoneinfo (since Linux 2.6.13)This file display information about memory zones. This is useful for analyzing virtual memory behavior.


Many strings (i.e., the environment and command line) are in the internal format, with subfields terminated by null bytes ('\0'), so you may find that things are more readable if you use od -c or tr "\000" "\n" to read them. Alternatively, echo `cat <file>` works well.

This manual page is incomplete, possibly inaccurate, and is the kind of thing that needs to be updated very often.


cat(1), dmesg(1), find(1), free(1), ps(1), tr(1), uptime(1), chroot(2), mmap(2), readlink(2), syslog(2), slabinfo(5), hier(7), time(7), arp(8), hdparm(8), ifconfig(8), init(8), lsmod(8), lspci(8), mount(8), netstat(8), procinfo(8), route(8)
The kernel source files: Documentation/filesystems/proc.txt, Documentation/sysctl/vm.txt


This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at
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