Linux kernel可设置参数说明(sysctl.conf)

kernel 3.8.13 

目录：

/usr/share/doc/kernel-doc-3.8.13/Documentation/sysctl/

abi.txt

---

Documentation for /proc/sys/abi/* kernel version 2.6.0.test2
     (c) 2003,  Fabian Frederick

For general info : README.

==============================================================

This path is binary emulation relevant aka personality types aka abi.
When a process is executed, it's linked to an exec_domain whose
personality is defined using values available from /proc/sys/abi.
You can find further details about abi in include/linux/personality.h.

Here are the files featuring in 2.6 kernel :

- defhandler_coff
- defhandler_elf
- defhandler_lcall7
- defhandler_libcso
- fake_utsname
- trace

===========================================================
defhandler_coff:
defined value :
PER_SCOSVR3
0x0003 | STICKY_TIMEOUTS | WHOLE_SECONDS | SHORT_INODE

===========================================================
defhandler_elf:
defined value :
PER_LINUX
0

===========================================================
defhandler_lcall7:
defined value :
PER_SVR4
0x0001 | STICKY_TIMEOUTS | MMAP_PAGE_ZERO,

===========================================================
defhandler_libsco:
defined value:
PER_SVR4
0x0001 | STICKY_TIMEOUTS | MMAP_PAGE_ZERO,

===========================================================
fake_utsname:
Unused

===========================================================
trace:
Unused

===========================================================

---

fs.txt

---

Documentation for /proc/sys/fs/*     kernel version 2.2.10
     (c) 1998, 1999,  Rik van Riel
     (c) 2009,        Shen Feng

For general info and legal blurb, please look in README.

==============================================================

This file contains documentation for the sysctl files in
/proc/sys/fs/ and is valid for Linux kernel version 2.2.

The files in this directory can be used to tune and monitor
miscellaneous and general things in the operation of the Linux
kernel. Since some of the files _can_ be used to screw up your
system, it is advisable to read both documentation and source
before actually making adjustments.

1. /proc/sys/fs
----------------------------------------------------------

Currently, these files are in /proc/sys/fs:
- aio-max-nr
- aio-nr
- dentry-state
- dquot-max
- dquot-nr
- file-max
- file-nr
- inode-max
- inode-nr
- inode-state
- nr_open
- overflowuid
- overflowgid
- protected_hardlinks
- protected_symlinks
- suid_dumpable
- super-max
- super-nr

==============================================================

aio-nr & aio-max-nr:

aio-nr is the running total of the number of events specified on the
io_setup system call for all currently active aio contexts.  If aio-nr
reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
raising aio-max-nr does not result in the pre-allocation or re-sizing
of any kernel data structures.

==============================================================

dentry-state:

From linux/fs/dentry.c:
--------------------------------------------------------------
struct {
        int nr_dentry;
        int nr_unused;
        int age_limit;         /* age in seconds */
        int want_pages;        /* pages requested by system */
        int dummy[2];
} dentry_stat = {0, 0, 45, 0,};
--------------------------------------------------------------

Dentries are dynamically allocated and deallocated, and
nr_dentry seems to be 0 all the time. Hence it's safe to
assume that only nr_unused, age_limit and want_pages are
used. Nr_unused seems to be exactly what its name says.
Age_limit is the age in seconds after which dcache entries
can be reclaimed when memory is short and want_pages is
nonzero when shrink_dcache_pages() has been called and the
dcache isn't pruned yet.

==============================================================

dquot-max & dquot-nr:

The file dquot-max shows the maximum number of cached disk
quota entries.

The file dquot-nr shows the number of allocated disk quota
entries and the number of free disk quota entries.

If the number of free cached disk quotas is very low and
you have some awesome number of simultaneous system users,
you might want to raise the limit.

==============================================================

file-max & file-nr:

The value in file-max denotes the maximum number of file-
handles that the Linux kernel will allocate. When you get lots
of error messages about running out of file handles, you might
want to increase this limit.

Historically,the kernel was able to allocate file handles
dynamically, but not to free them again. The three values in
file-nr denote the number of allocated file handles, the number
of allocated but unused file handles, and the maximum number of
file handles. Linux 2.6 always reports 0 as the number of free
file handles -- this is not an error, it just means that the
number of allocated file handles exactly matches the number of
used file handles.

Attempts to allocate more file descriptors than file-max are
reported with printk, look for "VFS: file-max limit
reached".
==============================================================

nr_open:

This denotes the maximum number of file-handles a process can
allocate. Default value is 1024*1024 (1048576) which should be
enough for most machines. Actual limit depends on RLIMIT_NOFILE
resource limit.

==============================================================

inode-max, inode-nr & inode-state:

As with file handles, the kernel allocates the inode structures
dynamically, but can't free them yet.

The value in inode-max denotes the maximum number of inode
handlers. This value should be 3-4 times larger than the value
in file-max, since stdin, stdout and network sockets also
need an inode struct to handle them. When you regularly run
out of inodes, you need to increase this value.

The file inode-nr contains the first two items from
inode-state, so we'll skip to that file...

Inode-state contains three actual numbers and four dummies.
The actual numbers are, in order of appearance, nr_inodes,
nr_free_inodes and preshrink.

Nr_inodes stands for the number of inodes the system has
allocated, this can be slightly more than inode-max because
Linux allocates them one pageful at a time.

Nr_free_inodes represents the number of free inodes (?) and
preshrink is nonzero when the nr_inodes > inode-max and the
system needs to prune the inode list instead of allocating
more.

==============================================================

overflowgid & overflowuid:

Some filesystems only support 16-bit UIDs and GIDs, although in Linux
UIDs and GIDs are 32 bits. When one of these filesystems is mounted
with writes enabled, any UID or GID that would exceed 65535 is translated
to a fixed value before being written to disk.

These sysctls allow you to change the value of the fixed UID and GID.
The default is 65534.

==============================================================

protected_hardlinks:

A long-standing class of security issues is the hardlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given hardlink (i.e. a
root process follows a hardlink created by another user). Additionally,
on systems without separated partitions, this stops unauthorized users
from "pinning" vulnerable setuid/setgid files against being upgraded by
the administrator, or linking to special files.

When set to "0", hardlink creation behavior is unrestricted.

When set to "1" hardlinks cannot be created by users if they do not
already own the source file, or do not have read/write access to it.

This protection is based on the restrictions in Openwall and grsecurity.

==============================================================

protected_symlinks:

A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user). For a likely
incomplete list of hundreds of examples across the years, please see:
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

When set to "0", symlink following behavior is unrestricted.

When set to "1" symlinks are permitted to be followed only when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.

This protection is based on the restrictions in Openwall and grsecurity.

==============================================================

suid_dumpable:

This value can be used to query and set the core dump mode for setuid
or otherwise protected/tainted binaries. The modes are

0 - (default) - traditional behaviour. Any process which has changed
     privilege levels or is execute only will not be dumped.
1 - (debug) - all processes dump core when possible. The core dump is
     owned by the current user and no security is applied. This is
     intended for system debugging situations only. Ptrace is unchecked.
     This is insecure as it allows regular users to examine the memory
     contents of privileged processes.
2 - (suidsafe) - any binary which normally would not be dumped is dumped
     anyway, but only if the "core_pattern" kernel sysctl is set to
     either a pipe handler or a fully qualified path. (For more details
     on this limitation, see CVE-2006-2451.) This mode is appropriate
     when administrators are attempting to debug problems in a normal
     environment, and either have a core dump pipe handler that knows
     to treat privileged core dumps with care, or specific directory
     defined for catching core dumps. If a core dump happens without
     a pipe handler or fully qualifid path, a message will be emitted
     to syslog warning about the lack of a correct setting.

==============================================================

super-max & super-nr:

These numbers control the maximum number of superblocks, and
thus the maximum number of mounted filesystems the kernel
can have. You only need to increase super-max if you need to
mount more filesystems than the current value in super-max
allows you to.

==============================================================

aio-nr & aio-max-nr:

aio-nr shows the current system-wide number of asynchronous io
requests.  aio-max-nr allows you to change the maximum value
aio-nr can grow to.

==============================================================


2. /proc/sys/fs/binfmt_misc
----------------------------------------------------------

Documentation for the files in /proc/sys/fs/binfmt_misc is
in Documentation/binfmt_misc.txt.


3. /proc/sys/fs/mqueue - POSIX message queues filesystem
----------------------------------------------------------

The "mqueue"  filesystem provides  the necessary kernel features to enable the
creation of a  user space  library that  implements  the  POSIX message queues
API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
Interfaces specification.)

The "mqueue" filesystem contains values for determining/setting  the amount of
resources used by the file system.

/proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
maximum number of message queues allowed on the system.

/proc/sys/fs/mqueue/msg_max  is  a  read/write file  for  setting/getting  the
maximum number of messages in a queue value.  In fact it is the limiting value
for another (user) limit which is set in mq_open invocation. This attribute of
a queue must be less or equal then msg_max.

/proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
maximum  message size value (it is every  message queue's attribute set during
its creation).

/proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
default number of messages in a queue value if attr parameter of mq_open(2) is
NULL. If it exceed msg_max, the default value is initialized msg_max.

/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
the default message size value if attr parameter of mq_open(2) is NULL. If it
exceed msgsize_max, the default value is initialized msgsize_max.

4. /proc/sys/fs/epoll - Configuration options for the epoll interface
--------------------------------------------------------

This directory contains configuration options for the epoll(7) interface.

max_user_watches
----------------

Every epoll file descriptor can store a number of files to be monitored
for event readiness. Each one of these monitored files constitutes a "watch".
This configuration option sets the maximum number of "watches" that are
allowed for each user.
Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
on a 64bit one.
The current default value for  max_user_watches  is the 1/32 of the available
low memory, divided for the "watch" cost in bytes.

---

kernel.txt

---

Documentation for /proc/sys/kernel/*     kernel version 2.2.10
     (c) 1998, 1999,  Rik van Riel
     (c) 2009,        Shen Feng

For general info and legal blurb, please look in README.

==============================================================

This file contains documentation for the sysctl files in
/proc/sys/kernel/ and is valid for Linux kernel version 2.2.

The files in this directory can be used to tune and monitor
miscellaneous and general things in the operation of the Linux
kernel. Since some of the files _can_ be used to screw up your
system, it is advisable to read both documentation and source
before actually making adjustments.

Currently, these files might (depending on your configuration)
show up in /proc/sys/kernel:

- acct
- acpi_video_flags
- auto_msgmni
- bootloader_type          [ X86 only ]
- bootloader_version          [ X86 only ]
- callhome               [ S390 only ]
- cap_last_cap
- core_pattern
- core_pipe_limit
- core_uses_pid
- ctrl-alt-del
- dmesg_restrict
- domainname
- hostname
- hotplug
- kptr_restrict
- kstack_depth_to_print       [ X86 only ]
- l2cr                        [ PPC only ]
- modprobe                    ==> Documentation/debugging-modules.txt
- modules_disabled
- msg_next_id                [ sysv ipc ]
- msgmax
- msgmnb
- msgmni
- nmi_watchdog
- osrelease
- ostype
- overflowgid
- overflowuid
- panic
- panic_on_oops
- panic_on_unrecovered_nmi
- panic_on_stackoverflow
- pid_max
- powersave-nap               [ PPC only ]
- printk
- printk_delay
- printk_ratelimit
- printk_ratelimit_burst
- randomize_va_space
- real-root-dev               ==> Documentation/initrd.txt
- reboot-cmd                  [ SPARC only ]
- rtsig-max
- rtsig-nr
- sem
- sem_next_id                [ sysv ipc ]
- sg-big-buff                 [ generic SCSI device (sg) ]
- shm_next_id                [ sysv ipc ]
- shm_rmid_forced
- shmall
- shmmax                      [ sysv ipc ]
- shmmni
- softlockup_thresh
- stop-a                      [ SPARC only ]
- sysrq                       ==> Documentation/sysrq.txt
- tainted
- threads-max
- unknown_nmi_panic
- version

==============================================================

acct:

highwater lowwater frequency

If BSD-style process accounting is enabled these values control
its behaviour. If free space on filesystem where the log lives
goes below % accounting suspends. If free space gets
above % accounting resumes. determines
how often do we check the amount of free space (value is in
seconds). Default:
4 2 30
That is, suspend accounting if there left <= 2% free; resume it
if we got >=4%; consider information about amount of free space
valid for 30 seconds.

==============================================================

acpi_video_flags:

flags

See Doc*/kernel/power/video.txt, it allows mode of video boot to be
set during run time.

==============================================================

auto_msgmni:

Enables/Disables automatic recomputing of msgmni upon memory add/remove
or upon ipc namespace creation/removal (see the msgmni description
above). Echoing "1" into this file enables msgmni automatic recomputing.
Echoing "0" turns it off. auto_msgmni default value is 1.


==============================================================

bootloader_type:

x86 bootloader identification

This gives the bootloader type number as indicated by the bootloader,
shifted left by 4, and OR'd with the low four bits of the bootloader
version.  The reason for this encoding is that this used to match the
type_of_loader field in the kernel header; the encoding is kept for
backwards compatibility.  That is, if the full bootloader type number
is 0x15 and the full version number is 0x234, this file will contain
the value 340 = 0x154.

See the type_of_loader and ext_loader_type fields in
Documentation/x86/boot.txt for additional information.

==============================================================

bootloader_version:

x86 bootloader version

The complete bootloader version number.  In the example above, this
file will contain the value 564 = 0x234.

See the type_of_loader and ext_loader_ver fields in
Documentation/x86/boot.txt for additional information.

==============================================================

callhome:

Controls the kernel's callhome behavior in case of a kernel panic.

The s390 hardware allows an operating system to send a notification
to a service organization (callhome) in case of an operating system panic.

When the value in this file is 0 (which is the default behavior)
nothing happens in case of a kernel panic. If this value is set to "1"
the complete kernel oops message is send to the IBM customer service
organization in case the mainframe the Linux operating system is running
on has a service contract with IBM.

==============================================================

cap_last_cap

Highest valid capability of the running kernel.  Exports
CAP_LAST_CAP from the kernel.

==============================================================

core_pattern:

core_pattern is used to specify a core dumpfile pattern name.
. max length 128 characters; default value is "core"
. core_pattern is used as a pattern template for the output filename;
  certain string patterns (beginning with '%') are substituted with
  their actual values.
. backward compatibility with core_uses_pid:
     If core_pattern does not include "%p" (default does not)
     and core_uses_pid is set, then .PID will be appended to
     the filename.
. corename format specifiers:
     %     '%' is dropped
     %%     output one '%'
     %p     pid
     %u     uid
     %g     gid
     %d     dump mode, matches PR_SET_DUMPABLE and
          /proc/sys/fs/suid_dumpable
     %s     signal number
     %t     UNIX time of dump
     %h     hostname
     %e     executable filename (may be shortened)
     %E     executable path
     % both are dropped
. If the first character of the pattern is a '|', the kernel will treat
  the rest of the pattern as a command to run.  The core dump will be
  written to the standard input of that program instead of to a file.

==============================================================

core_pipe_limit:

This sysctl is only applicable when core_pattern is configured to pipe
core files to a user space helper (when the first character of
core_pattern is a '|', see above).  When collecting cores via a pipe
to an application, it is occasionally useful for the collecting
application to gather data about the crashing process from its
/proc/pid directory.  In order to do this safely, the kernel must wait
for the collecting process to exit, so as not to remove the crashing
processes proc files prematurely.  This in turn creates the
possibility that a misbehaving userspace collecting process can block
the reaping of a crashed process simply by never exiting.  This sysctl
defends against that.  It defines how many concurrent crashing
processes may be piped to user space applications in parallel.  If
this value is exceeded, then those crashing processes above that value
are noted via the kernel log and their cores are skipped.  0 is a
special value, indicating that unlimited processes may be captured in
parallel, but that no waiting will take place (i.e. the collecting
process is not guaranteed access to /proc//).  This
value defaults to 0.

==============================================================

core_uses_pid:

The default coredump filename is "core".  By setting
core_uses_pid to 1, the coredump filename becomes core.PID.
If core_pattern does not include "%p" (default does not)
and core_uses_pid is set, then .PID will be appended to
the filename.

==============================================================

ctrl-alt-del:

When the value in this file is 0, ctrl-alt-del is trapped and
sent to the init(1) program to handle a graceful restart.
When, however, the value is > 0, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.

Note: when a program (like dosemu) has the keyboard in 'raw'
mode, the ctrl-alt-del is intercepted by the program before it
ever reaches the kernel tty layer, and it's up to the program
to decide what to do with it.

==============================================================

dmesg_restrict:

This toggle indicates whether unprivileged users are prevented
from using dmesg(8) to view messages from the kernel's log buffer.
When dmesg_restrict is set to (0) there are no restrictions. When
dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
dmesg(8).

The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
default value of dmesg_restrict.

==============================================================

domainname & hostname:

These files can be used to set the NIS/YP domainname and the
hostname of your box in exactly the same way as the commands
domainname and hostname, i.e.:
# echo "darkstar" > /proc/sys/kernel/hostname
# echo "mydomain" > /proc/sys/kernel/domainname
has the same effect as
# hostname "darkstar"
# domainname "mydomain"

Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS (Network
Information Service) or YP (Yellow Pages) domainname. These two
domain names are in general different. For a detailed discussion
see the hostname(1) man page.

==============================================================

hotplug:

Path for the hotplug policy agent.
Default value is "/sbin/hotplug".

==============================================================

kptr_restrict:

This toggle indicates whether restrictions are placed on
exposing kernel addresses via /proc and other interfaces.  When
kptr_restrict is set to (0), there are no restrictions.  When
kptr_restrict is set to (1), the default, kernel pointers
printed using the %pK format specifier will be replaced with 0's
unless the user has CAP_SYSLOG.  When kptr_restrict is set to
(2), kernel pointers printed using %pK will be replaced with 0's
regardless of privileges.

==============================================================

kstack_depth_to_print: (X86 only)

Controls the number of words to print when dumping the raw
kernel stack.

==============================================================

l2cr: (PPC only)

This flag controls the L2 cache of G3 processor boards. If
0, the cache is disabled. Enabled if nonzero.

==============================================================

modules_disabled:

A toggle value indicating if modules are allowed to be loaded
in an otherwise modular kernel.  This toggle defaults to off
(0), but can be set true (1).  Once true, modules can be
neither loaded nor unloaded, and the toggle cannot be set back
to false.

==============================================================

msg_next_id, sem_next_id, and shm_next_id:

These three toggles allows to specify desired id for next allocated IPC
object: message, semaphore or shared memory respectively.

By default they are equal to -1, which means generic allocation logic.
Possible values to set are in range {0..INT_MAX}.

Notes:
1) kernel doesn't guarantee, that new object will have desired id. So,
it's up to userspace, how to handle an object with "wrong" id.
2) Toggle with non-default value will be set back to -1 by kernel after
successful IPC object allocation.

==============================================================

nmi_watchdog:

Enables/Disables the NMI watchdog on x86 systems. When the value is
non-zero the NMI watchdog is enabled and will continuously test all
online cpus to determine whether or not they are still functioning
properly. Currently, passing "nmi_watchdog=" parameter at boot time is
required for this function to work.

If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
parameter), the NMI watchdog shares registers with oprofile. By
disabling the NMI watchdog, oprofile may have more registers to
utilize.

==============================================================

osrelease, ostype & version:

# cat osrelease
2.1.88
# cat ostype
Linux
# cat version
#5 Wed Feb 25 21:49:24 MET 1998

The files osrelease and ostype should be clear enough. Version
needs a little more clarification however. The '#5' means that
this is the fifth kernel built from this source base and the
date behind it indicates the time the kernel was built.
The only way to tune these values is to rebuild the kernel :-)

==============================================================

overflowgid & overflowuid:

if your architecture did not always support 32-bit UIDs (i.e. arm,
i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
applications that use the old 16-bit UID/GID system calls, if the
actual UID or GID would exceed 65535.

These sysctls allow you to change the value of the fixed UID and GID.
The default is 65534.

==============================================================

panic:

The value in this file represents the number of seconds the kernel
waits before rebooting on a panic. When you use the software watchdog,
the recommended setting is 60.

==============================================================

panic_on_unrecovered_nmi:

The default Linux behaviour on an NMI of either memory or unknown is
to continue operation. For many environments such as scientific
computing it is preferable that the box is taken out and the error
dealt with than an uncorrected parity/ECC error get propagated.

A small number of systems do generate NMI's for bizarre random reasons
such as power management so the default is off. That sysctl works like
the existing panic controls already in that directory.

==============================================================

panic_on_oops:

Controls the kernel's behaviour when an oops or BUG is encountered.

0: try to continue operation

1: panic immediately.  If the `panic' sysctl is also non-zero then the
   machine will be rebooted.

==============================================================

panic_on_stackoverflow:

Controls the kernel's behavior when detecting the overflows of
kernel, IRQ and exception stacks except a user stack.
This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.

0: try to continue operation.

1: panic immediately.

==============================================================


pid_max:

PID allocation wrap value.  When the kernel's next PID value
reaches this value, it wraps back to a minimum PID value.
PIDs of value pid_max or larger are not allocated.

==============================================================

ns_last_pid:

The last pid allocated in the current (the one task using this sysctl
lives in) pid namespace. When selecting a pid for a next task on fork
kernel tries to allocate a number starting from this one.

==============================================================

powersave-nap: (PPC only)

If set, Linux-PPC will use the 'nap' mode of powersaving,
otherwise the 'doze' mode will be used.

==============================================================

printk:

The four values in printk denote: console_loglevel,
default_message_loglevel, minimum_console_loglevel and
default_console_loglevel respectively.

These values influence printk() behavior when printing or
logging error messages. See 'man 2 syslog' for more info on
the different loglevels.

- console_loglevel: messages with a higher priority than
  this will be printed to the console
- default_message_loglevel: messages without an explicit priority
  will be printed with this priority
- minimum_console_loglevel: minimum (highest) value to which
  console_loglevel can be set
- default_console_loglevel: default value for console_loglevel

==============================================================

printk_delay:

Delay each printk message in printk_delay milliseconds

Value from 0 - 10000 is allowed.

==============================================================

printk_ratelimit:

Some warning messages are rate limited. printk_ratelimit specifies
the minimum length of time between these messages (in jiffies), by
default we allow one every 5 seconds.

A value of 0 will disable rate limiting.

==============================================================

printk_ratelimit_burst:

While long term we enforce one message per printk_ratelimit
seconds, we do allow a burst of messages to pass through.
printk_ratelimit_burst specifies the number of messages we can
send before ratelimiting kicks in.

==============================================================

randomize_va_space:

This option can be used to select the type of process address
space randomization that is used in the system, for architectures
that support this feature.

0 - Turn the process address space randomization off.  This is the
    default for architectures that do not support this feature anyways,
    and kernels that are booted with the "norandmaps" parameter.

1 - Make the addresses of mmap base, stack and VDSO page randomized.
    This, among other things, implies that shared libraries will be
    loaded to random addresses.  Also for PIE-linked binaries, the
    location of code start is randomized.  This is the default if the
    CONFIG_COMPAT_BRK option is enabled.

2 - Additionally enable heap randomization.  This is the default if
    CONFIG_COMPAT_BRK is disabled.

    There are a few legacy applications out there (such as some ancient
    versions of libc.so.5 from 1996) that assume that brk area starts
    just after the end of the code+bss.  These applications break when
    start of the brk area is randomized.  There are however no known
    non-legacy applications that would be broken this way, so for most
    systems it is safe to choose full randomization.

    Systems with ancient and/or broken binaries should be configured
    with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
    address space randomization.

==============================================================

reboot-cmd: (Sparc only)

??? This 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. ???

==============================================================

rtsig-max & rtsig-nr:

The file rtsig-max can be used to tune the maximum number
of POSIX realtime (queued) signals that can be outstanding
in the system.

rtsig-nr shows the number of RT signals currently queued.

==============================================================

sg-big-buff:

This file shows the size of the generic SCSI (sg) buffer.
You can't tune it just yet, but you could change it on
compile time by editing include/scsi/sg.h and changing
the value of SG_BIG_BUFF.

There shouldn't be any reason to change this value. If
you can come up with one, you probably know what you
are doing anyway :)

==============================================================

shmall:

This parameter sets the total amount of shared memory pages that
can be used system wide. Hence, SHMALL should always be at least
ceil(shmmax/PAGE_SIZE).

If you are not sure what the default PAGE_SIZE is on your Linux
system, you can run the following command:

# getconf PAGE_SIZE

==============================================================

shmmax:

This value can be used to query and set the run time limit
on the maximum 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.

==============================================================

shm_rmid_forced:

Linux lets you set resource limits, including how much memory one
process can consume, via setrlimit(2).  Unfortunately, shared memory
segments are allowed to exist without association with any process, and
thus might not be counted against any resource limits.  If enabled,
shared memory segments are automatically destroyed when their attach
count becomes zero after a detach or a process termination.  It will
also destroy segments that were created, but never attached to, on exit
from the process.  The only use left for IPC_RMID is to immediately
destroy an unattached segment.  Of course, this breaks the way things are
defined, so some applications might stop working.  Note that this
feature will do you no good unless you also configure your resource
limits (in particular, RLIMIT_AS and RLIMIT_NPROC).  Most systems don't
need this.

Note that if you change this from 0 to 1, already created segments
without users and with a dead originative process will be destroyed.

==============================================================

softlockup_thresh:

This value can be used to lower the softlockup tolerance threshold.  The
default threshold is 60 seconds.  If a cpu is locked up for 60 seconds,
the kernel complains.  Valid values are 1-60 seconds.  Setting this
tunable to zero will disable the softlockup detection altogether.

==============================================================

tainted:

Non-zero if the kernel has been tainted.  Numeric values, which
can be ORed together:

   1 - A module with a non-GPL license has been loaded, this
       includes modules with no license.
       Set by modutils >= 2.4.9 and module-init-tools.
   2 - A module was force loaded by insmod -f.
       Set by modutils >= 2.4.9 and module-init-tools.
   4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
   8 - A module was forcibly unloaded from the system by rmmod -f.
  16 - A hardware machine check error occurred on the system.
  32 - A bad page was discovered on the system.
  64 - The user has asked that the system be marked "tainted".  This
       could be because they are running software that directly modifies
       the hardware, or for other reasons.
128 - The system has died.
256 - The ACPI DSDT has been overridden with one supplied by the user
        instead of using the one provided by the hardware.
512 - A kernel warning has occurred.
1024 - A module from drivers/staging was loaded.
2048 - The system is working around a severe firmware bug.
4096 - An out-of-tree module has been loaded.

==============================================================

unknown_nmi_panic:

The value in this file affects behavior of handling NMI. When the
value is non-zero, unknown NMI is trapped and then panic occurs. At
that time, kernel debugging information is displayed on console.

NMI switch that most IA32 servers have fires unknown NMI up, for

example.  If a system hangs up, try pressing the NMI switch.

---

net.txt

---

Documentation for /proc/sys/net/*     kernel version 2.4.0-test11-pre4
     (c) 1999          Terrehon Bowden
                    Bodo Bauer
     (c) 2000          Jorge Nerin
     (c) 2009          Shen Feng

For general info and legal blurb, please look in README.

==============================================================

This file contains the documentation for the sysctl files in
/proc/sys/net and is valid for Linux kernel version 2.4.0-test11-pre4.

The interface  to  the  networking  parts  of  the  kernel  is  located  in
/proc/sys/net. The following table shows all possible subdirectories.You may
see only some of them, depending on your kernel's configuration.


Table : Subdirectories in /proc/sys/net
..............................................................................
Directory Content             Directory  Content
core      General parameter   appletalk  Appletalk protocol
unix      Unix domain sockets netrom     NET/ROM
802       E802 protocol       ax25       AX25
ethernet  Ethernet protocol   rose       X.25 PLP layer
ipv4      IP version 4        x25        X.25 protocol
ipx       IPX                 token-ring IBM token ring
bridge    Bridging            decnet     DEC net
ipv6      IP version 6
..............................................................................

1. /proc/sys/net/core - Network core options
-------------------------------------------------------

bpf_jit_enable
--------------

This enables Berkeley Packet Filter Just in Time compiler.
Currently supported on x86_64 architecture, bpf_jit provides a framework
to speed packet filtering, the one used by tcpdump/libpcap for example.
Values :
     0 - disable the JIT (default value)
     1 - enable the JIT
     2 - enable the JIT and ask the compiler to emit traces on kernel log.

dev_weight
--------------

The maximum number of packets that kernel can handle on a NAPI interrupt,
it's a Per-CPU variable.
Default: 64

rmem_default
------------

The default setting of the socket receive buffer in bytes.

rmem_max
--------

The maximum receive socket buffer size in bytes.

wmem_default
------------

The default setting (in bytes) of the socket send buffer.

wmem_max
--------

The maximum send socket buffer size in bytes.

message_burst and message_cost
------------------------------

These parameters  are used to limit the warning messages written to the kernel
log from  the  networking  code.  They  enforce  a  rate  limit  to  make  a
denial-of-service attack  impossible. A higher message_cost factor, results in
fewer messages that will be written. Message_burst controls when messages will
be dropped.  The  default  settings  limit  warning messages to one every five
seconds.

warnings
--------

This controls console messages from the networking stack that can occur because
of problems on the network like duplicate address or bad checksums. Normally,
this should be enabled, but if the problem persists the messages can be
disabled.

netdev_budget
-------------

Maximum number of packets taken from all interfaces in one polling cycle (NAPI
poll). In one polling cycle interfaces which are registered to polling are
probed in a round-robin manner. The limit of packets in one such probe can be
set per-device via sysfs class/net//weight .

netdev_max_backlog
------------------

Maximum number  of  packets,  queued  on  the  INPUT  side, when the interface
receives packets faster than kernel can process them.

netdev_tstamp_prequeue
----------------------

If set to 0, RX packet timestamps can be sampled after RPS processing, when
the target CPU processes packets. It might give some delay on timestamps, but
permit to distribute the load on several cpus.

If set to 1 (default), timestamps are sampled as soon as possible, before
queueing.

optmem_max
----------

Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
of struct cmsghdr structures with appended data.

2. /proc/sys/net/unix - Parameters for Unix domain sockets
-------------------------------------------------------

There is only one file in this directory.
unix_dgram_qlen limits the max number of datagrams queued in Unix domain
socket's buffer. It will not take effect unless PF_UNIX flag is specified.


3. /proc/sys/net/ipv4 - IPV4 settings
-------------------------------------------------------
Please see: Documentation/networking/ip-sysctl.txt and ipvs-sysctl.txt for
descriptions of these entries.


4. Appletalk
-------------------------------------------------------

The /proc/sys/net/appletalk  directory  holds the Appletalk configuration data
when Appletalk is loaded. The configurable parameters are:

aarp-expiry-time
----------------

The amount  of  time  we keep an ARP entry before expiring it. Used to age out
old hosts.

aarp-resolve-time
-----------------

The amount of time we will spend trying to resolve an Appletalk address.

aarp-retransmit-limit
---------------------

The number of times we will retransmit a query before giving up.

aarp-tick-time
--------------

Controls the rate at which expires are checked.

The directory  /proc/net/appletalk  holds the list of active Appletalk sockets
on a machine.

The fields  indicate  the DDP type, the local address (in network:node format)
the remote  address,  the  size of the transmit pending queue, the size of the
received queue  (bytes waiting for applications to read) the state and the uid
owning the socket.

/proc/net/atalk_iface lists  all  the  interfaces  configured for appletalk.It
shows the  name  of the interface, its Appletalk address, the network range on
that address  (or  network number for phase 1 networks), and the status of the
interface.

/proc/net/atalk_route lists  each  known  network  route.  It lists the target
(network) that the route leads to, the router (may be directly connected), the
route flags, and the device the route is using.


5. IPX
-------------------------------------------------------

The IPX protocol has no tunable values in proc/sys/net.

The IPX  protocol  does,  however,  provide  proc/net/ipx. This lists each IPX
socket giving  the  local  and  remote  addresses  in  Novell  format (that is
network:node:port). In  accordance  with  the  strange  Novell  tradition,
everything but the port is in hex. Not_Connected is displayed for sockets that
are not  tied to a specific remote address. The Tx and Rx queue sizes indicate
the number  of  bytes  pending  for  transmission  and  reception.  The  state
indicates the  state  the  socket  is  in and the uid is the owning uid of the
socket.

The /proc/net/ipx_interface  file lists all IPX interfaces. For each interface
it gives  the network number, the node number, and indicates if the network is
the primary  network.  It  also  indicates  which  device  it  is bound to (or
Internal for  internal  networks)  and  the  Frame  Type if appropriate. Linux
supports 802.3,  802.2,  802.2  SNAP  and DIX (Blue Book) ethernet framing for
IPX.

The /proc/net/ipx_route  table  holds  a list of IPX routes. For each route it
gives the  destination  network, the router node (or Directly) and the network

address of the router (or Connected) for internal networks.

sunrpc.txt

Documentation for /proc/sys/sunrpc/*     kernel version 2.2.10
     (c) 1998, 1999,  Rik van Riel

For general info and legal blurb, please look in README.

==============================================================

This file contains the documentation for the sysctl files in
/proc/sys/sunrpc and is valid for Linux kernel version 2.2.

The files in this directory can be used to (re)set the debug
flags of the SUN Remote Procedure Call (RPC) subsystem in
the Linux kernel. This stuff is used for NFS, KNFSD and
maybe a few other things as well.

The files in there are used to control the debugging flags:
rpc_debug, nfs_debug, nfsd_debug and nlm_debug.

These flags are for kernel hackers only. You should read the

source code in net/sunrpc/ for more information.

---

vm.txt

---

Documentation for /proc/sys/vm/*     kernel version 2.6.29
     (c) 1998, 1999,  Rik van Riel
     (c) 2008         Peter W. Morreale

For general info and legal blurb, please look in README.

==============================================================

This file contains the documentation for the sysctl files in
/proc/sys/vm and is valid for Linux kernel version 2.6.29.

The files in this directory can be used to tune the operation
of the virtual memory (VM) subsystem of the Linux kernel and
the writeout of dirty data to disk.

Default values and initialization routines for most of these
files can be found in mm/swap.c.

Currently, these files are in /proc/sys/vm:

- block_dump
- compact_memory
- dirty_background_bytes
- dirty_background_ratio
- dirty_bytes
- dirty_expire_centisecs
- dirty_ratio
- dirty_writeback_centisecs
- drop_caches
- extfrag_threshold
- hugepages_treat_as_movable
- hugetlb_shm_group
- laptop_mode
- legacy_va_layout
- lowmem_reserve_ratio
- max_map_count
- memory_failure_early_kill
- memory_failure_recovery
- min_free_kbytes
- min_slab_ratio
- min_unmapped_ratio
- mmap_min_addr
- nr_hugepages
- nr_overcommit_hugepages
- nr_trim_pages         (only if CONFIG_MMU=n)
- numa_zonelist_order
- oom_dump_tasks
- oom_kill_allocating_task
- overcommit_memory
- overcommit_ratio
- page-cluster
- panic_on_oom
- percpu_pagelist_fraction
- stat_interval
- swappiness
- vfs_cache_pressure
- zone_reclaim_mode

==============================================================

block_dump

block_dump enables block I/O debugging when set to a nonzero value. More
information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.

==============================================================

compact_memory

Available only when CONFIG_COMPACTION is set. When 1 is written to the file,
all zones are compacted such that free memory is available in contiguous
blocks where possible. This can be important for example in the allocation of
huge pages although processes will also directly compact memory as required.

==============================================================

dirty_background_bytes

Contains the amount of dirty memory at which the background kernel
flusher threads will start writeback.

Note: dirty_background_bytes is the counterpart of dirty_background_ratio. Only
one of them may be specified at a time. When one sysctl is written it is
immediately taken into account to evaluate the dirty memory limits and the
other appears as 0 when read.

==============================================================

dirty_background_ratio

Contains, as a percentage of total system memory, the number of pages at which
the background kernel flusher threads will start writing out dirty data.

==============================================================

dirty_bytes

Contains the amount of dirty memory at which a process generating disk writes
will itself start writeback.

Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be
specified at a time. When one sysctl is written it is immediately taken into
account to evaluate the dirty memory limits and the other appears as 0 when
read.

Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
value lower than this limit will be ignored and the old configuration will be
retained.

==============================================================

dirty_expire_centisecs

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the kernel flusher threads.  It is expressed in 100'ths
of a second.  Data which has been dirty in-memory for longer than this
interval will be written out next time a flusher thread wakes up.

==============================================================

dirty_ratio

Contains, as a percentage of total system memory, the number of pages at which
a process which is generating disk writes will itself start writing out dirty
data.

==============================================================

dirty_writeback_centisecs

The kernel flusher threads will periodically wake up and write `old' data
out to disk.  This tunable expresses the interval between those wakeups, in
100'ths of a second.

Setting this to zero disables periodic writeback altogether.

==============================================================

drop_caches

Writing to this will cause the kernel to drop clean caches, dentries and
inodes from memory, causing that memory to become free.

To free pagecache:
     echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
     echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
     echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.

==============================================================

extfrag_threshold

This parameter affects whether the kernel will compact memory or direct
reclaim to satisfy a high-order allocation. /proc/extfrag_index shows what
the fragmentation index for each order is in each zone in the system. Values
tending towards 0 imply allocations would fail due to lack of memory,
values towards 1000 imply failures are due to fragmentation and -1 implies
that the allocation will succeed as long as watermarks are met.

The kernel will not compact memory in a zone if the
fragmentation index is <= extfrag_threshold. The default value is 500.

==============================================================

hugepages_treat_as_movable

This parameter is only useful when kernelcore= is specified at boot time to
create ZONE_MOVABLE for pages that may be reclaimed or migrated. Huge pages
are not movable so are not normally allocated from ZONE_MOVABLE. A non-zero
value written to hugepages_treat_as_movable allows huge pages to be allocated
from ZONE_MOVABLE.

Once enabled, the ZONE_MOVABLE is treated as an area of memory the huge
pages pool can easily grow or shrink within. Assuming that applications are
not running that mlock() a lot of memory, it is likely the huge pages pool
can grow to the size of ZONE_MOVABLE by repeatedly entering the desired value
into nr_hugepages and triggering page reclaim.

==============================================================

hugetlb_shm_group

hugetlb_shm_group contains group id that is allowed to create SysV
shared memory segment using hugetlb page.

==============================================================

laptop_mode

laptop_mode is a knob that controls "laptop mode". All the things that are
controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.

==============================================================

legacy_va_layout

If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel
will use the legacy (2.4) layout for all processes.

==============================================================

lowmem_reserve_ratio

For some specialised workloads on highmem machines it is dangerous for
the kernel to allow process memory to be allocated from the "lowmem"
zone.  This is because that memory could then be pinned via the mlock()
system call, or by unavailability of swapspace.

And on large highmem machines this lack of reclaimable lowmem memory
can be fatal.

So the Linux page allocator has a mechanism which prevents allocations
which _could_ use highmem from using too much lowmem.  This means that
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.

(The same argument applies to the old 16 megabyte ISA DMA region.  This
mechanism will also defend that region from allocations which could use
highmem or lowmem).

The `lowmem_reserve_ratio' tunable determines how aggressive the kernel is
in defending these lower zones.

If you have a machine which uses highmem or ISA DMA and your
applications are using mlock(), or if you are running with no swap then
you probably should change the lowmem_reserve_ratio setting.

The lowmem_reserve_ratio is an array. You can see them by reading this file.
-
% cat /proc/sys/vm/lowmem_reserve_ratio
256     256     32
-
Note: # of this elements is one fewer than number of zones. Because the highest
      zone's value is not necessary for following calculation.

But, these values are not used directly. The kernel calculates # of protection
pages for each zones from them. These are shown as array of protection pages
in /proc/zoneinfo like followings. (This is an example of x86-64 box).
Each zone has an array of protection pages like this.

-
Node 0, zone      DMA
  pages free     1355
        min      3
        low      3
        high     4
     :
     :
    numa_other   0
        protection: (0, 2004, 2004, 2004)
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  pagesets
    cpu: 0 pcp: 0
        :
-
These protections are added to score to judge whether this zone should be used
for page allocation or should be reclaimed.

In this example, if normal pages (index=2) are required to this DMA zone and
watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should
not be used because pages_free(1355) is smaller than watermark + protection[2]
(4 + 2004 = 2008). If this protection value is 0, this zone would be used for
normal page requirement. If requirement is DMA zone(index=0), protection[0]
(=0) is used.

zone[i]'s protection[j] is calculated by following expression.

(i < j):
  zone[i]->protection[j]
  = (total sums of present_pages from zone[i+1] to zone[j] on the node)
    / lowmem_reserve_ratio[i];
(i = j):
   (should not be protected. = 0;
(i > j):
   (not necessary, but looks 0)

The default values of lowmem_reserve_ratio[i] are
    256 (if zone[i] means DMA or DMA32 zone)
    32  (others).
As above expression, they are reciprocal number of ratio.
256 means 1/256. # of protection pages becomes about "0.39%" of total present
pages of higher zones on the node.

If you would like to protect more pages, smaller values are effective.
The minimum value is 1 (1/1 -> 100%).

==============================================================

max_map_count:

This file contains the maximum number of memory map areas a process
may have. Memory map areas are used as a side-effect of calling
malloc, directly by mmap and mprotect, and also when loading shared
libraries.

While most applications need less than a thousand maps, certain
programs, particularly malloc debuggers, may consume lots of them,
e.g., up to one or two maps per allocation.

The default value is 65536.

=============================================================

memory_failure_early_kill:

Control how to kill processes when uncorrected memory error (typically
a 2bit error in a memory module) is detected in the background by hardware
that cannot be handled by the kernel. 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 uptodate copy of the data it will kill to prevent any data
corruptions from propagating.

1: Kill all processes that have the corrupted and not reloadable 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 done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can
handle this if they want to.

This is only active on architectures/platforms with advanced machine
check handling and depends on the hardware capabilities.

Applications can override this setting individually with the PR_MCE_KILL prctl

==============================================================

memory_failure_recovery

Enable memory failure recovery (when supported by the platform)

1: Attempt recovery.

0: Always panic on a memory failure.

==============================================================

min_free_kbytes:

This is used to force the Linux VM to keep a minimum number
of kilobytes free.  The VM uses this number to compute a
watermark[WMARK_MIN] value for each lowmem zone in the system.
Each lowmem zone gets a number of reserved free pages based
proportionally on its size.

Some minimal amount of memory is needed to satisfy PF_MEMALLOC
allocations; if you set this to lower than 1024KB, your system will
become subtly broken, and prone to deadlock under high loads.

Setting this too high will OOM your machine instantly.

=============================================================

min_slab_ratio:

This is available only on NUMA kernels.

A percentage of the total pages in each zone.  On Zone reclaim
(fallback from the local zone occurs) slabs will be reclaimed if more
than this percentage of pages in a zone are reclaimable slab pages.
This insures that the slab growth stays under control even in NUMA
systems that rarely perform global reclaim.

The default is 5 percent.

Note that slab reclaim is triggered in a per zone / node fashion.
The process of reclaiming slab memory is currently not node specific
and may not be fast.

=============================================================

min_unmapped_ratio:

This is available only on NUMA kernels.

This is a percentage of the total pages in each zone. Zone reclaim will
only occur if more than this percentage of pages are in a state that
zone_reclaim_mode allows to be reclaimed.

If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared
against all file-backed unmapped pages including swapcache pages and tmpfs
files. Otherwise, only unmapped pages backed by normal files but not tmpfs
files and similar are considered.

The default is 1 percent.

==============================================================

mmap_min_addr

This file indicates the amount of address space  which a user process will
be restricted from mmapping.  Since kernel null dereference bugs could
accidentally operate based on the information in the first couple of pages
of memory userspace processes should not be allowed to write to them.  By
default this value is set to 0 and no protections will be enforced by the
security module.  Setting this value to something like 64k will allow the
vast majority of applications to work correctly and provide defense in depth
against future potential kernel bugs.

==============================================================

nr_hugepages

Change the minimum size of the hugepage pool.

See Documentation/vm/hugetlbpage.txt

==============================================================

nr_overcommit_hugepages

Change the maximum size of the hugepage pool. The maximum is
nr_hugepages + nr_overcommit_hugepages.

See Documentation/vm/hugetlbpage.txt

==============================================================

nr_trim_pages

This is available only on NOMMU kernels.

This value adjusts the excess page trimming behaviour of power-of-2 aligned
NOMMU mmap allocations.

A value of 0 disables trimming of allocations entirely, while a value of 1
trims excess pages aggressively. Any value >= 1 acts as the watermark where
trimming of allocations is initiated.

The default value is 1.

See Documentation/nommu-mmap.txt for more information.

==============================================================

numa_zonelist_order

This sysctl is only for NUMA.
'where the memory is allocated from' is controlled by zonelists.
(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
you may be able to read ZONE_DMA as ZONE_DMA32...)

In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
ZONE_NORMAL -> ZONE_DMA
This means that a memory allocation request for GFP_KERNEL will
get memory from ZONE_DMA only when ZONE_NORMAL is not available.

In NUMA case, you can think of following 2 types of order.
Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL

(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.

Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
will be used before ZONE_NORMAL exhaustion. This increases possibility of
out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.

Type(B) cannot offer the best locality but is more robust against OOM of
the DMA zone.

Type(A) is called as "Node" order. Type (B) is "Zone" order.

"Node order" orders the zonelists by node, then by zone within each node.
Specify "[Nn]ode" for node order

"Zone Order" orders the zonelists by zone type, then by node within each
zone.  Specify "[Zz]one" for zone order.

Specify "[Dd]efault" to request automatic configuration.  Autoconfiguration
will select "node" order in following case.
(1) if the DMA zone does not exist or
(2) if the DMA zone comprises greater than 50% of the available memory or
(3) if any node's DMA zone comprises greater than 60% of its local memory and
    the amount of local memory is big enough.

Otherwise, "zone" order will be selected. Default order is recommended unless
this is causing problems for your system/application.

==============================================================

oom_dump_tasks

Enables a system-wide task dump (excluding kernel threads) to be
produced when the kernel performs an OOM-killing and includes such
information as pid, uid, tgid, vm size, rss, nr_ptes, swapents,
oom_score_adj score, and name.  This is helpful to determine why the
OOM killer was invoked, to identify the rogue task that caused it,
and to determine why the OOM killer chose the task it did to kill.

If this is set to 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 conditions when the
information may not be desired.

If this is set to non-zero, this information is shown whenever the
OOM killer actually kills a memory-hogging task.

The default value is 1 (enabled).

==============================================================

oom_kill_allocating_task

This enables or disables killing the OOM-triggering task in
out-of-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 non-zero, the OOM killer simply kills the task that
triggered the out-of-memory condition.  This avoids the expensive
tasklist scan.

If panic_on_oom is selected, it takes precedence over whatever value
is used in oom_kill_allocating_task.

The default value is 0.

==============================================================

overcommit_memory:

This value contains a flag that enables memory overcommitment.

When this flag is 0, the kernel attempts to estimate the amount
of free memory left when userspace requests more memory.

When this flag is 1, the kernel pretends there is always enough
memory until it actually runs out.

When this flag is 2, the kernel uses a "never overcommit"
policy that attempts to prevent any overcommit of memory.

This feature can be very useful because there are a lot of
programs that malloc() huge amounts of memory "just-in-case"
and don't use much of it.

The default value is 0.

See Documentation/vm/overcommit-accounting and
security/commoncap.c::cap_vm_enough_memory() for more information.

==============================================================

overcommit_ratio:

When overcommit_memory is set to 2, the committed address
space is not permitted to exceed swap plus this percentage
of physical RAM.  See above.

==============================================================

page-cluster

page-cluster controls the number of pages up to which consecutive pages
are read in from swap in a single attempt. This is the swap counterpart
to page cache readahead.
The mentioned consecutivity is not in terms of virtual/physical addresses,
but consecutive on swap space - that means they were swapped out together.

It is a logarithmic value - setting it to zero means "1 page", setting
it to 1 means "2 pages", setting it to 2 means "4 pages", etc.
Zero disables swap readahead completely.

The default value is three (eight pages at a time).  There may be some
small benefits in tuning this to a different value if your workload is
swap-intensive.

Lower values mean lower latencies for initial faults, but at the same time
extra faults and I/O delays for following faults if they would have been part of
that consecutive pages readahead would have brought in.

=============================================================

panic_on_oom

This enables or disables panic on out-of-memory feature.

If this is set to 0, the kernel will kill some rogue process,
called oom_killer.  Usually, oom_killer can kill rogue processes and
system will survive.

If this is set to 1, the kernel panics when out-of-memory happens.
However, if a process limits using nodes by mempolicy/cpusets,
and those nodes become memory exhaustion status, one process
may be killed by oom-killer. No panic occurs in this case.
Because other nodes' memory may be free. This means system total status
may be not fatal yet.

If this is set to 2, the kernel panics compulsorily even on the
above-mentioned. Even oom happens under memory cgroup, the whole
system panics.

The default value is 0.
1 and 2 are for failover of clustering. Please select either
according to your policy of failover.
panic_on_oom=2+kdump gives you very strong tool to investigate
why oom happens. You can get snapshot.

=============================================================

percpu_pagelist_fraction

This is the fraction of pages at most (high mark pcp->high) in each zone that
are allocated for each per cpu page list.  The min value for this is 8.  It
means that we don't allow more than 1/8th of pages in each zone to be
allocated in any single per_cpu_pagelist.  This entry only changes the value
of hot per cpu pagelists.  User can specify a number like 100 to allocate
1/100th of each zone to each per cpu page list.

The batch value of each per cpu pagelist is also updated as a result.  It is
set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)

The initial value is zero.  Kernel does not use this value at boot time to set
the high water marks for each per cpu page list.

==============================================================

stat_interval

The time interval between which vm statistics are updated.  The default
is 1 second.

==============================================================

swappiness

This control is used to define how aggressive the kernel will swap
memory pages.  Higher values will increase agressiveness, lower values
decrease the amount of swap.

The default value is 60.

==============================================================

vfs_cache_pressure
------------------

Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
never reclaim dentries and inodes due to memory pressure and this can easily
lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.

==============================================================

zone_reclaim_mode:

Zone_reclaim_mode allows someone to set more or less aggressive approaches to
reclaim memory when a zone runs out of memory. If it is set to zero then no
zone reclaim occurs. Allocations will be satisfied from other zones / nodes
in the system.

This is value ORed together of

1     = Zone reclaim on
2     = Zone reclaim writes dirty pages out
4     = Zone reclaim swaps pages

zone_reclaim_mode is set during bootup to 1 if it is determined that pages
from remote zones will cause a measurable performance reduction. The
page allocator will then reclaim easily reusable pages (those page
cache pages that are currently not used) before allocating off node pages.

It may be beneficial to switch off zone reclaim if the system is
used for a file server and all of memory should be used for caching files
from disk. In that case the caching effect is more important than
data locality.

Allowing zone reclaim to write out pages stops processes that are
writing large amounts of data from dirtying pages on other nodes. Zone
reclaim will write out dirty pages if a zone fills up and so effectively
throttle the process. This may decrease the performance of a single process
since it cannot use all of system memory to buffer the outgoing writes
anymore but it preserve the memory on other nodes so that the performance
of other processes running on other nodes will not be affected.

Allowing regular swap effectively restricts allocations to the local
node unless explicitly overridden by memory policies or cpuset
configurations.

============ End of Document =================================