Linux Kernel Architecture

Chapter 10: Filesystems without Persistent Storage

transmission quality of the network because it also includes incorrectly transmitted and rejected

packages as well as collision data.

Some network card drivers (e.g., for the popular IntelPRO/100 chipset) create additional subdirectories

in/proc/netwith more detailed hardware-specific information.

System ControlParameters

The system control parameters used to check and modify the behavior of the kernel dynamically make

up the lion’s share of entries in theprocfilesystem. However, this interface is not the only way of manip-

ulating data — this can also be done using thesysctlsystem call. This requires more effort because it

is first necessary to write a program to support communication with the kernel via the system call inter-

face. As a result, the numeric sysctl mechanism was tagged as being obsolete during development of 2.5

(the kernel outputs a warning message to this effect each timesysctlis invoked) and was planned to be

dropped at some point. Removing the system call has, however, created a controversial discussion, and

up to 2.6.25, the call is still in the kernel — although a message warns the user that it is deprecated.

Thesysctlsystem call is not really needed because the/procinterface is a kernel data manipulation

option of unrivaled simplicity. The sysctl parameters are managed in a separate subdirectory named

/proc/sys, which is split into further subdirectories in line with the various kernel subsystems:

wolfgang@meitner>ls -l /proc/sys

total 0

dr-xr-xr-x 0 root root 0 2008-02-15 04:29 abi

dr-xr-xr-x 0 root root 0 2008-02-15 04:29 debug

dr-xr-xr-x 0 root root 0 2008-02-14 22:26 dev

dr-xr-xr-x 0 root root 0 2008-02-14 22:22 fs

dr-xr-xr-x 0 root root 0 2008-02-14 22:22 kernel

dr-xr-xr-x 0 root root 0 2008-02-14 22:22 net

dr-xr-xr-x 0 root root 0 2008-02-14 22:26 vm

The subdirectories contain a series of files that reflect the characteristic data of the associated kernel

subsystems. For example,/proc/sys/vmincludes the following entries:

wolfgang@meitner>ls -l /proc/sys/vm

total 0

-rw-r--r-- 1 root root 0 2008-02-17 01:32 block_dump

-rw-r--r-- 1 root root 0 2008-02-16 20:55 dirty_background_ratio

-rw-r--r-- 1 root root 0 2008-02-16 20:55 dirty_expire_centisecs

-rw-r--r-- 1 root root 0 2008-02-16 20:55 dirty_ratio

-rw-r--r-- 1 root root 0 2008-02-16 20:55 dirty_writeback_centisecs

...

-rw-r--r-- 1 root root 0 2008-02-17 01:32 swappiness

-rw-r--r-- 1 root root 0 2008-02-17 01:32 vfs_cache_pressure

-rw-r--r-- 1 root root 0 2008-02-17 01:32 zone_reclaim_mode

Unlike the files discussed earlier, the contents of the files in these directories can not only be read, but also

supplied with new values by means of normal file operations. For instance, thevmsubdirectory includes

aswappinessfile to indicate how ‘‘aggressively‘‘ the swapping algorithm goes about its job of swapping

out pages. The default value is 60, as shown when the file contents are output usingcat:

wolfgang@meitner>cat /proc/sys/vm/swappiness

60