Common groups of information concerning the kernel are grouped into
directories and subdirectories within the /proc/ directory.
5.3.1. Process Directories
Every /proc/ directory contains a number of
directories with numerical names. A listing of them may begins like
dr-xr-xr-x 3 root root 0 Feb 13 01:28 1
dr-xr-xr-x 3 root root 0 Feb 13 01:28 1010
dr-xr-xr-x 3 xfs xfs 0 Feb 13 01:28 1087
dr-xr-xr-x 3 daemon daemon 0 Feb 13 01:28 1123
dr-xr-xr-x 3 root root 0 Feb 13 01:28 11307
dr-xr-xr-x 3 apache apache 0 Feb 13 01:28 13660
dr-xr-xr-x 3 rpc rpc 0 Feb 13 01:28 637
dr-xr-xr-x 3 rpcuser rpcuser 0 Feb 13 01:28 666
These directories are called process
directories, as they are named after a program's process
ID and contain information specific to that process. The owner and
group of each process directory is set to the user running the
process. When the process is terminated, its
/proc/ process directory vanishes.
Each process directory contains the following files:
cmdline — Contains the
command issued when starting the process.
cpu — Provides specific
information about the utilization of each of the system's CPUs.
A process running on a dual CPU system produces output
similar to this:
cpu 11 3
cpu0 0 0
cpu1 11 3
cwd — A symbolic link to the current
working directory for the process.
environ — Gives a list of
the environment variables for the process. The environment
variable is given in all upper-case characters, and the value is in
exe — A symbolic link to the
executable of this process.
fd — A directory containing
all of the file descriptors for a particular process. These are
given in numbered links:
lrwx------ 1 root root 64 May 8 11:31 0 -> /dev/null
lrwx------ 1 root root 64 May 8 11:31 1 -> /dev/null
lrwx------ 1 root root 64 May 8 11:31 2 -> /dev/null
lrwx------ 1 root root 64 May 8 11:31 3 -> /dev/ptmx
lrwx------ 1 root root 64 May 8 11:31 4 -> socket:
lrwx------ 1 root root 64 May 8 11:31 5 -> /dev/ptmx
lrwx------ 1 root root 64 May 8 11:31 6 -> socket:
lrwx------ 1 root root 64 May 8 11:31 7 -> /dev/ptmx
maps — Contains memory maps
to the various executables and library files associated with this
process. This file can be rather long, depending upon the
complexity of the process, but sample output from the
sshd process begins like this:
The information in this output includes the process name and ID,
the state (such as S (sleeping)
or R (running)), user/group ID
running the process, and detailed data regarding memory usage.
The /proc/self/ directory is a link to the
currently running process. This allows a process to look at itself
without having to know its process ID.
Within a shell environment, a listing of the
/proc/self/ directory produces the same contents
as listing the process directory for that process.
This directory contains information specific to the various buses
available on the system. So, for example, on a standard system
containing ISA, PCI, and USB busses, current data on each of these
buses is available in its directory under /proc/bus/.
The contents of the subdirectories and files available varies greatly
on the precise configuration of the system. However, each of the
directories for each of the bus types has at least one directory for
each bus of that type. These individual bus directories, usually
signified with numbers, such as 00,
contain binary files that refer to the various devices available on
So, for example, a system with a USB bus but no USB devices connected
to it has a /proc/bus/usb/ directory containing
dr-xr-xr-x 1 root root 0 May 3 16:25 001
-r--r--r-- 1 root root 0 May 3 16:25 devices
-r--r--r-- 1 root root 0 May 3 16:25 drivers
The /proc/bus/usb/ directory contains files that
track the various devices on any USB buses, as well as the drivers
required to use them. The /proc/bus/usb/001/
directory contains all devices on the first USB bus. By looking at the
contents of the devices file, one can identify
the USB root hub on the motherboard:
This directory contains information for specific drivers in use by the
A common file found here is rtc, which provides
output from the driver for the system's Real Time Clock
(RTC), the device that keeps the time while the system is
switched off. Sample output from /proc/driver/rtc
looks like this:
rtc_time : 01:38:43
rtc_date : 1998-02-13
rtc_epoch : 1900
alarm : 00:00:00
DST_enable : no
BCD : yes
24hr : yes
square_wave : no
alarm_IRQ : no
update_IRQ : no
periodic_IRQ : no
periodic_freq : 1024
batt_status : okay
For more information about the RTC, review
This directory shows which file systems are exported. If running an
NFS server, typing cat /proc/fs/nfs/exports will
display the file systems being shared and the permissions granted for
those file systems. For more on sharing file system with NFS, see
Chapter 9 Network File System (NFS).
This directory holds information about IDE devices on the system. Each
IDE channel is represented as a separate directory, such as
/proc/ide/ide1. In addition, a
drivers file is also available, providing the
version number of the various drivers used on the IDE channels:
ide-cdrom version 4.59
ide-floppy version 0.97
ide-disk version 1.10
Many chipsets also provide an informational file in this directory
that gives additional data concerning the drives connected through the
channels. For example, a generic Intel PIIX4 Ultra 33 chipset produces
a /proc/ide/piix that will tell you whether DMA
or UDMA is enabled for the devices on the IDE channels:
Intel PIIX4 Ultra 33 Chipset.
------------- Primary Channel ---------------- Secondary Channel -------------
------------- drive0 --------- drive1 -------- drive0 ---------- drive1 ------
DMA enabled: yes no yes no
UDMA enabled: yes no no no
UDMA enabled: 2 X X X
Navigating into the directory for an IDE channel, such as
ide0, provides additional
information. The channel file provides the
channel number, while the model tells you the
bus type for the channel (such as
220.127.116.11. The Device Directory
Within each IDE channel directory is a device directory. The name
of the device directory corresponds to the drive letter in the
/dev/ directory. For instance, the first IDE drive on
ide0 would be hda.
There is a symbolic link to each of these device
directories in the /proc/ide/ directory.
Each device directory contains a collection of information and
statistics. The contents of these directories vary according to the
type of device connected. Some of the more useful files common to
many devices include:
cache — The device cache.
capacity — The capacity of the
device, in 512 byte blocks.
driver — The driver and version used
to control the device.
geometry — The physical and logical
geometry of the device.
media — The type of device, such as a
model — The model name or number of
settings — A collection of current
parameters of the device. This file usually contains quite a bit
of useful, technical information. A sample
settings file for a standard IDE hard disk
looks similar to this:
This directory is used to set IRQ to CPU affinity, which allows the
system to connect a particular IRQ to only one CPU. Alternatively, it
can exclude a CPU from handling any IRQs.
Each IRQ has its own directory, allowing for the individual
configuration of each IRQ. The
/proc/irq/prof_cpu_mask file is a bitmask that
contains the default values for the smp_affinity
file in the IRQ directory. The values in
smp_affinity specify which CPUs handle that
For more information about the /proc/irq/
This directory provides a comprehensive look at various networking
parameters and statistics. Each of the files covers a specific range
of information related to networking on the system. Below is a partial
listing of these virtual files:
arp — Contains the kernel's ARP
table. This file is particularly useful for connecting a hardware
address to an IP address on a system.
atm — A directory containing files with
various Asynchronous Transfer Mode (ATM)
settings and statistics. This directory is primarily used with ATM
networking and ADSL cards.
dev — Lists the various network devices
configured on the system, complete with transmit and receive
statistics. This file lists the number of bytes
each interface has sent and received, the number of packets
inbound and outbound, the number of errors seen, the number of
packets dropped, and more.
dev_mcast — Displays the various Layer2
multicast groups each device is listening to.
igmp — Lists the IP multicast
addresses which this system joined.
ip_fwchains — If
ipchains are in use, this virtual file reveals
any current rule.
ip_fwnames — If
ipchains are in use, this virtual file lists all firewall chain
ip_masquerade — Provides a table of
masquerading information under ipchains.
ip_mr_cache — Lists the multicast routing cache.
ip_mr_vif — Lists multicast virtual interfaces.
netstat — Contains a broad yet detailed
collection of networking statistics, including TCP timeouts, SYN
cookies sent and received, and much more.
psched — Lists global packet scheduler parameters.
raw — Lists raw device statistics.
route — Displays the kernel's routing table.
rt_cache — Contains the current routing cache.
snmp — List of Simple Network Management
Protocol (SNMP) data for various networking protocols in use.
sockstat — Provides socket
tcp — Contains detailed TCP socket information.
tr_rif — Lists the token ring RIF routing table.
udp — Contains detailed UDP socket information.
unix — Lists UNIX domain sockets
currently in use.
wireless — Lists wireless interface data.
This directory is analogous to the /proc/ide/
directory, however, it is only for connected SCSI devices.
The primary file in this directory is
/proc/scsi/scsi, which contains a list of every
recognized SCSI device. From this listing, the type of device, as
well as the model name, vendor, SCSI channel and ID data is
For example, if a system contains a SCSI CD-ROM, a tape drive, a hard
drive, and a RAID controller, this file would look similar to this:
Each SCSI driver used by the system has its own directory in
/proc/scsi/, which contains files specific to
each SCSI controller using that driver. So, for the example system
just addressed, aic7xxx and
megaraid directories are present, as those two
drivers are being utilized. The files in each of the directories
typically contain an I/O address range, IRQ information, and
statistics for the particular SCSI controller using that driver. Each
controller can report a different type and amount of information. The
Adaptec AIC-7880 Ultra SCSI host adapter's file in this example system
produces the following output:
Adaptec AIC7xxx driver version: 5.1.20/3.2.4
TCQ Enabled By Default : Disabled
AIC7XXX_PROC_STATS : Enabled
AIC7XXX_RESET_DELAY : 5
SCSI Adapter: Adaptec AIC-7880 Ultra SCSI host adapter
Ultra Narrow Controller
PCI MMAPed I/O Base: 0xfcffe000
Adapter SEEPROM Config: SEEPROM found and used.
Adaptec SCSI BIOS: Enabled
SCBs: Active 0, Max Active 1,
Allocated 15, HW 16, Page 255
BIOS Control Word: 0x18a6
Adapter Control Word: 0x1c5f
Extended Translation: Enabled
Disconnect Enable Flags: 0x00ff
Ultra Enable Flags: 0x0020
Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
Tagged Queue By Device array for aic7xxx host instance 1:
Actual queue depth per device for aic7xxx host instance 1:
Device using Narrow/Sync transfers at 20.0 MByte/sec, offset 15
Transinfo settings: current(12/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 0 (0 reads and 0 writes)
< 2K 2K+ 4K+ 8K+ 16K+ 32K+ 64K+ 128K+
Reads: 0 0 0 0 0 0 0 0
Writes: 0 0 0 0 0 0 0 0
Device using Narrow/Sync transfers at 10.0 MByte/sec, offset 15
Transinfo settings: current(25/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 132 (0 reads and 132 writes)
< 2K 2K+ 4K+ 8K+ 16K+ 32K+ 64K+ 128K+
Reads: 0 0 0 0 0 0 0 0
Writes: 0 0 0 1 131 0 0 0
This output reveals the transfer speed to the various SCSI devices
connected to the controller based on channel ID, as well as detailed
statistics concerning the amount and sizes of files read or written by
that device. For example, this controller is communicating with the
CD-ROM at 20 megabytes per second, while the tape drive is only
communicating at 10 megabytes per second.
The /proc/sys/ directory is different from others
in /proc/ because it not only provides
information about the system but also allows the system administrator
to immediately enable and disable kernel features.
Use caution when changing settings on a production system
using the various files in the /proc/sys/
directory. Changing the wrong setting may render the kernel
unstable, requiring a system reboot.
For this reason, be sure the options are valid for that file before
attempting to change any value in /proc/sys/.
A good way to determine if a particular file can be configured, or if
it is only designed to provide information, is to list it with the
-l option at the shell prompt. If the file is
writable, it may be used to configure the kernel. For example, a
partial listing of /proc/sys/fs looks like this:
-r--r--r-- 1 root root 0 May 10 16:14 dentry-state
-rw-r--r-- 1 root root 0 May 10 16:14 dir-notify-enable
-r--r--r-- 1 root root 0 May 10 16:14 dquot-nr
-rw-r--r-- 1 root root 0 May 10 16:14 file-max
-r--r--r-- 1 root root 0 May 10 16:14 file-nr
In this listing, the files dir-notify-enable and
file-max can be written to and, therefore, can be
used to configure the kernel. The other files only provide feedback on
Changing a value within a /proc/sys/ file is done
by echoing the new value into the file. For example, to enable the
System Request Key on a running kernel, type the command:
echo 1 > /proc/sys/kernel/sysrq
This will change the value for sysrq from
0 (off) to
The purpose of the System Request Key is to allow immediate input to
the kernel through simple key combinations. For example, the System
Request Key can be used to immediately shut down or restart a system,
sync all mounted file systems, or dump important information to your
console. This feature is most useful when using a development kernel
or when experiencing system freezes. However, it is considered a
security risk for an unattended console and is therefore turned off by
default under Red Hat Linux.
for more information on the System Request Key.
A few /proc/sys/ configuration files contain more
than one value. In order to correctly send new values to them, place a
space character between each value passed with the
echo command, such as is done in this example:
echo 4 2 45 > /proc/sys/kernel/acct
Any configuration changes made using the echo
command will disappear when the system is restarted. To make
configuration changes take effect after the system is rebooted, see
Section 5.4 Using the sysctl Command.
The /proc/sys/ directory contains several
subdirectories controlling different aspects of a running kernel.
This directory provides parameters for particular devices on the
system. Most systems have at least two directories,
cdrom and raid. Customized
kernels can have other directories, such as
parport, which provides the ability to share
one parallel port between multiple device drivers.
The cdrom directory contains a file called
info, which reveals a number of important
CD-ROM information, Id: cdrom.c 3.12 2000/10/18
drive name: hdc
drive speed: 32
drive # of slots: 1
Can close tray: 1
Can open tray: 1
Can lock tray: 1
Can change speed: 1
Can select disk: 0
Can read multisession: 1
Can read MCN: 1
Reports media changed: 1
Can play audio: 1
Can write CD-R: 0
Can write CD-RW: 0
Can read DVD: 0
Can write DVD-R: 0
Can write DVD-RAM: 0
This file can be quickly scanned to discover the qualities of an
unknown CD-ROM. If multiple
CD-ROMs are available on a system, each device is given its own
column of information.
Various files in /proc/sys/dev/cdrom, such as
autoclose and checkmedia,
can be used to control the system's CD-ROM. Use the
echo command to enable or disable these features.
If RAID support is compiled into the kernel, a
/proc/sys/dev/raid/ directory will be available
with at least two files in it: speed_limit_min
and speed_limit_max. These settings determine
the acceleration of RAID devices for I/O intensive tasks, such as
resyncing the disks.
This directory contains an array of options and information
concerning various aspects of the file system, including quota, file
handle, inode, and dentry information.
The binfmt_misc directory is used to
provide kernel support for miscellaneous binary formats.
The important files in /proc/sys/fs include:
dentry-state — Provides the status of the
directory cache. The file looks similar to this:
57411 52939 45 0 0 0
The first number reveals the total number of directory cache
entries, while the second number displays the number of unused
entries. The third number tells the number of seconds between
when a directory has been freed and when it can be reclaimed,
and the fourth measures the pages currently requested by the
system. The last two numbers are not used and display only
dquot-nr — Shows the maximum number
of cached disk quota entries.
file-max — Change the
maximum number of file handles that the kernel will
allocate. Raising the value in this file can resolve errors
caused by a lack of available file handles.
file-nr — Displays the number of
allocated file handles, used file handles, and the maximum
number of file handles.
overflowuid — Defines the fixed group
ID and user ID, respectively, for use with file systems that only
support 16-bit group and user IDs.
super-max — Controls the maximum
number of superblocks available.
super-nr — Displays the current
number of superblocks in use.
This directory contains a variety of different configuration files
that directly affect the operation of the kernel. Some of the most important
acct — Controls the suspension of
process accounting based on the percentage of free space
available on the file system containing the log. By default, the
file looks like this:
4 2 30
The second value sets the threshold percentage of free space when
logging will be suspended, while the first value dictates the
percentage of free space required for logging to resume. The
third value sets the interval, in seconds, that the kernel polls the
file system to see if logging should be suspended or resumed.
cap-bound — Controls the
capability bounding settings, which
provides a list of capabilities for any process on the system. If
a capability is not listed here, then no process, no matter how
privileged, can do it. The idea is to make the system
more secure by ensuring that certain things cannot happen, at
least beyond a certain point in the boot process.
For a valid list of values for this virtual file, consult
More information on capability bounding is available online at
the following URL: http://lwn.net/1999/1202/kernel.php3.
ctrl-alt-del — Controls whether
will gracefully restart the computer using
0) or force an immediate reboot
without syncing the dirty buffers to disk (value
domainname — Configures
the system domain name, such as example.com.
hostname — Configures the system
hostname, such as
hotplug — Configures the utility
to be used when a configuration change is detected by the
system. This is primarily used with USB and Cardbus PCI. The
default value of /sbin/hotplug
should not be changed unless you are testing a new program to
fulfill this role.
modprobe — Sets the location of the
program to be used to load kernel modules when necessary. The
default value of /sbin/modprobe
signifies that kmod will call it to actually
load the module when a kernel thread calls
msgmax — Sets the maximum size of any
message sent from one process to another and is set to
8192 bytes by default. You
should be careful about raising this value, as queued messages
between processes are stored in non-swappable kernel memory.
Any increase in msgmax would increase RAM
requirements for the system.
msgmnb — Sets the maximum number
of bytes in a single message queue. The default is
msgmni — Sets the maximum number
of message queue identifiers. The default is
osrelease — Lists the Linux kernel
release number. This file can only be altered by changing the
kernel source and recompiling.
ostype — Displays the type of
operating system. By default, this file is set to
Linux, and this value can only
be changed by changing the kernel source and recompiling.
overflowuid — Defines the fixed group
ID and user ID, respectively, for use with system calls on
architectures that only support 16-bit group and user IDs.
panic — Defines the number of seconds
the kernel will postpone rebooting when the system experiences a kernel
panic. By default, the value is set to
0, which disables automatic
rebooting after a panic.
printk — This file controls a variety
of settings related to printing or logging error messages. Each
error message reported by the kernel has a
loglevel associated with it that defines
the importance of the message. The loglevel values break down in
0 — Kernel
emergency. The system is unusable.
1 — Kernel
alert. Action must be taken immediately.
2 — Condition of the
kernel is considered critical.
3 — General kernel
4 — General kernel
5 — Kernel
notice of a normal but significant condition.
6 — Kernel
7 — Kernel
Four values are found in the printk file:
6 4 1 7
Each of these values defines a different rule for dealing with
error messages. The first value, called the console
loglevel, defines the lowest priority of messages
that will be printed to the console. (Note that, the lower the
priority, the higher the loglevel number.) The second value sets
the default loglevel for messages without an explicit loglevel
attached to them. The third value sets the lowest possible
loglevel configuration for the console loglevel. The last value
sets the default value for the console loglevel.
rtsig-max — Configures the maximum
number of POSIX realtime signals that the system may have queued
at any one time. The default value is
rtsig-nr — The current number of
POSIX realtime signals queued by the kernel.
sem — Configures semaphore
settings within the kernel. A semaphore
is a System V IPC object that is used to control utilization of
a particular process.
shmall — Sets the total amount of
shared memory that can be used at one time on the system, in
bytes. By default, this value is
shmmax — Sets the largest shared
memory segment size allowed by the kernel, in bytes. By default,
this value is
33554432. However, the kernel
supports much larger values than this.
shmmni — Sets the maximum number of
shared memory segments for the whole system, in bytes. By default, this
value is 4096
sysrq — Activates the System
Request Key, if this value is set to anything other than the
default of 0. See Section 5.3.9 /proc/sys/ for details about the System Request
threads-max — Sets the maximum number
of threads to be used by the kernel, with a default value of
version — Displays the date and time
the kernel was last compiled. The first field in this file, such
as #3, relates to the number of
times a kernel was built from the source base.
The random directory stores a number of values
related to generating random numbers for the kernel.
This directory contains subdirectories concerning various networking
topics. Various configurations at the time of kernel compilation
make different directories available here, such as
ipv4, ipx, and
ipv6. Within these directories, system
administrators are able to adjust the network configuration on a
Given the wide variety of possible networking options available with
Linux, only the most common /proc/sys/net/
directories will be discussed.
The /proc/sys/net/core/ directory contains a variety of
settings that control the interaction between the kernel and
networking layers. The most important of these files are:
message_burst — The amount of
time in tenths of a second required to write a new warning
message. This is used to prevent Denial of
Service (DoS) attacks. The
default setting is 50.
message_cost — Also used to prevent
DoS attacks by placing a cost on every warning message. The
higher the value of this file (default of
5), the more likely the warning
message will be ignored.
The idea of a DoS attack is to bombard the targeted system with
requests that generate errors and fill up disk partitions with
log files or require all of the system's resources to handle
the error logging. The settings in
message_cost are designed to be modified
based on the system's acceptable risk versus the need for
netdev_max_backlog — Sets the maximum
number of packets allowed to queue when a particular interface
receives packets faster than the kernel can process them. The
default value for this file is 300.
optmem_max — Configures the maximum
ancillary buffer size allowed per socket.
rmem_default — Sets the receive
socket buffer default size in bytes.
rmem_max — Sets the receive socket
buffer maximum size in bytes.
wmem_default — Sets the send socket
buffer default size in bytes.
wmem_max — Sets the send socket
buffer maximum size in bytes.
The /proc/sys/net/ipv4/ directory contains additional
networking settings. Many of these settings, used in conjunction
with one another, are very useful in preventing attacks on the
system or using the system to act as a router.
An erroneous change to these files may affect remote connectivity
to the system.
Here are some of the most important files in the
icmp_timeexeed_rate — Set the maximum
ICMP send packet rate, in 1/100 of a second, to hosts under
certain conditions. A setting of
0 removes any delay and is not
a good idea.
icmp_echo_ignore_broadcasts — Allows
the kernel to ignore ICMP ECHO packets from every host or only
those originating from broadcast and multicast addresses,
respectively. A value of 0
allows the kernel to respond, while a value of
1 ignores the packets.
ip_default_ttl — Sets the default
Time To Live (TTL), which limits the
number of hops a packet may make before reaching its
destination. Increasing this value can diminish system
ip_forward — Permits interfaces
on the system to forward packets to one other. By default, this
file is set to 0. Setting this
file to 1 enables network
ip_local_port_range — Specifies
the range of ports to be used by TCP or UDP when a local port is
needed. The first number is the lowest port to be used and the
second number specifies the highest port. Any systems that
expect to require more ports than the default 1024 to 4999
should use the 32768 to 61000 range in this file.
tcp_syn_retries — Provides a limit on
the number of times the system will re-transmit a SYN packet
when attempting to make a connection.
tcp_retries1 — Sets the number of
permitted re-transmissions attempting to answer an incoming
connection. Default of 3.
tcp_retries2 — Sets the number of
permitted re-transmissions of TCP packets. Default of
file contains a complete list of files and options available in the
A number of other directories exist within the
/proc/sys/net/ipv4/ directory and each covers
specific topics. The /proc/sys/net/ipv4/conf/
directory allows each system interface to be configured in different
ways, including the use of default settings for unconfigured devices
(in the /proc/sys/net/ipv4/conf/default/
subdirectory) and settings that override all special configurations
(in the /proc/sys/net/ipv4/conf/all/
The /proc/sys/net/ipv4/neigh/ directory
contains settings for communicating with a host directly connected
to the system (called a network neighbor) and also contains
different settings for systems more than one hop away.
Routing over IPV4 also has its own directory,
conf/ and neigh/, the
/proc/sys/net/ipv4/route/ directory contains
specifications that apply to routing with any interfaces on the
system. Many of these settings, such as
max_size, max_delay, and
min_delay, relate to controlling the size of
the routing cache. To clear the routing cache, write any value to
the flush file.
Additional information about these directories and the possible
values for their configuration files can be found in
This directory facilitates the configuration of the Linux kernel's
virtual memory (VM) subsystem. The kernel makes extensive and
intelligent use of virtual memory, which is commonly called swap space.
The following files are commonly found in the
bdflush — Sets various values related
to the bdflush kernel daemon.
buffermem — Allows you to control the
percentage amount of total system memory to be used for buffer
memory. Typical output for this file looks like this:
2 10 60
The first and last values set the minimum and maximum percentage
of memory to be used as buffer memory, respectively. The middle
value sets the percentage of system memory dedicated to buffer
memory where the memory management subsystem will begin to clear
buffer cache more than other kinds of memory to compensate for a
general lack of free memory.
kswapd — Sets various values
concerned with the kernel swap-out daemon,
kswapd. This file has three values:
512 32 8
The first value sets the maximum number of pages that
kswapd will attempt to free in a single
attempt. The larger this number, the more aggressively the
kernel can move to free pages. The second value sets the minimum
number of times that kswapd attempts to free
a page. The third value sets the number of pages
kswapd attempts to write in a single
attempt. Proper tuning of this final value can improve
performance on a system using a lot of swap space by telling the
kernel to write pages in large chunks, minimizing the number of
max_map_count — Configures the
maximum number of memory map areas a process may have. In most
cases, the default value of
65536 is appropriate.
overcommit_memory — When set to
the default value of 0 the kernel
estimates the amount of memory available and fails requests that
are blatantly invalid. Unfortunately, since memory is allocated
using a heuristic rather than a precise algorithm, it can
sometimes overload the system.
If overcommit_memory is set to
1, then the potential for system
overload is increased, but so is the performance for memory intensive
tasks, such as those used by some scientific software.
For those who desire less risk of over memory commitment,
the following two options have been added. Setting
2 fails if a memory request adds up
to more that half of the physical RAM, plus swap. Setting it to
3 fails if a memory request adds
up to more than swap alone can hold.
pagecache — Controls the amount of
memory used by the page cache. The values in
pagecache are percentages, and they work in
a similar way as buffermem to enforce
minimums and maximums of available page cache memory.
page-cluster — Sets the number of
pages read in a single attempt. The default value of
4, which actually relates to 16
pages, is appropriate for most systems.
pagetable_cache — Controls the number
of page tables that are cached on a per-processor basis. The
first and second values relate to the minimum and maximum number
of page tables to set aside, respectively.
contains additional information on these various files.
This directory contains information about System V IPC resources. The files
in this directory relate to System V IPC calls for messages
(msg), semaphores (sem), and
shared memory (shm).
This directory contains information about the available and currently
used tty devices on the system. Originally
called teletype devices, any character-based
data terminals are called tty devices.
In Linux, there are three different kinds of tty
devices. Serial devices are used with serial
connections, such as over a modem or using a serial
cable. Virtual terminals create the common
console connection, such as the virtual consoles available when
pressing [Alt]-[<F-key>] at the system console. Pseudo
terminals create a two-way communication that is used by
some higher level applications, such as
XFree86. The drivers file
is a list of the current tty devices in use:
The /proc/tty/driver/serial file lists the usage
statistics and status of each of the serial tty lines.
In order that tty devices can be used in a similar way as network
devices, the Linux kernel will enforce line
discipline on the device. This allows the driver to place
a specific type of header with every block of data transmitted over
the device, making it possible for the remote end of the connection to
see a block of data as just one in a stream of data blocks. SLIP and
PPP are common line disciplines, and each are commonly used to connect
systems to one other over a serial link.
Registered line disciplines are stored in the
ldiscs file, with detailed information available
in the ldisc directory.