Optimizing Linux

Linux distributions are adding more and more features and not all of them are useful for every desktop or server application. This document helps by suggesting ways to make your Linux distribution optimized for its intended role. Many distributions have features enabled which will never be used and should be disabled or removed from the system. Some of the methods in this document use values that can be tailored to suit individual roles of the machine being optimized. The value given is a generalized value and should work for most systems.
I will not address individual distributions in this document as it will cover all distributions in a general manner.

Disable IPV6

This is a next generation protocol and if you do not use it disable it.
Performance of IPV4 DNS queries is much better without IPV6 enabled.
Most distributions enable IPV6 with a module which you can disable in the file /etc/modprobe.conf or /etc/modules.conf by adding the line:

alias net-pf-10 off

In some cases you may need to also add the line:

alias ipv6 off

And edit /etc/sysconfig/network where you change NETWORKING_IPV6=yes to NETWORKING_IPV6=no
Also make sure that there is no iptables firewall starting up by disabling any ip6tables rules the server may have, and check the /etc/hosts file where you can delete or comment out the entry

::1 localhost.localdomain localhost

A reboot is required to disable IPV6.

Disable services which are not required

Many services are enabled by default for convenience.
You can save memory, cpu cycles, and have added security if you disable all services that are not being used.
Check what services are enabled and then decide which services are required for the machines role.
Disable extra console gettys by editing /etc/inittab then comment out all but one or two gettys.

# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
#2:2345:respawn:/sbin/mingetty tty2
#3:2345:respawn:/sbin/mingetty tty3
#4:2345:respawn:/sbin/mingetty tty4
#5:2345:respawn:/sbin/mingetty tty5
#6:2345:respawn:/sbin/mingetty tty6

It's important to know what runlevel the machine starts off in, to verify this:
Type

grep ":initdefault" /etc/inittab cut -d":" -f2 or runlevel

This example will cover runlevel 3 and the examples will work for runlevel 5 if you substitute 3 for 5.
Type

ls /etc/rc3.d grep S

These are the services that automatically start when the machine boots up and the numbers next to the S are to signify which order the services start in. Some systems may have different orders so the numbers are only for reference.
The best way to disable the services is to uninstall the services package. You can also delete the symlink in the runlevel directory. Here is an example of deleting S06cpuspeed from runlevel 3

rm -f /etc/rc3.d/S06cpuspeed

I'd suggest the following services need to be removed from most systems:
S05kudzu is useful when you add or remove hardware but should be deleted
S06cpuspeed this could slow the cpu down and make the system less responsive
S08ip6tables you should already have IPV6 disabled so you do not need this
S09isdn unless you live in the dark ages and require ISDN then remove this also
S13irqbalance this feature uses a lot of cpu cycles and should be disabled, a great performance gain
S13mcstrans this is for selinux and you may have already or will disable that in the next section
S13named if this is not a name server delete this
S13 portmap if you are not using NFS, NIS, or rpc connections delete this one
S14 nfslock delete unless you use nfs
S15mdmonitor unless you are using software raid there is no reason to have this running
S18rpcidmapd if you don't use nfs delete this
S19rpcgssd again if you are not doing nfs delete this
S25netfs if you don't have any remote filesystems that are locally mounted delete this
S25bluetooth sure it could be useful in a laptop, otherwise remove this also
S25pcscd if you have smart cards or crypto tokens leave this, otherwise delete
S26apmd sure it's nice to have the machine use less power but then you can't use it, delete this
S26hidd you do not need Bluetooth keyboard or mouse so delete this also
S28autofs you can start and stop this if you need it but enabled it's a security risk, wastes cpu and mem
S44acpid unless you use a laptop delete this
S80sendmail if you are running a sendmail mail server then leave this otherwise delete
S89rdisc you can delete this one also
90xfs if you use graphical login then leave this, otherwise delete
S95anacron I don't think I know anyone who ever used this so delete it
S95atd here is another one you need to delete
S97yum-updatesd automatic updates in linux are lame delete this
S98avahi-daemon is for netzeroconf which should also be disabled and will be covered later
S98haldaemon you can delete this one also
To have the changes take place you can stop all the services manually, reboot, or in some cases if you type:

init 3; init q

That is if you are in runlevel 3 of course.

Disable netzeroconf or zero conf

This is for service discovery and should be disabled unless you want your computer to scan the network
Most distributions have it enabled in /etc/sysconfig/network
Add anything to NOZEROCONF= because the scripts only check to see if there is something there so no or yes disable it.
Other distros you can remove the package avahi-autoip

Turn off filesystem counters

This updates the times of all files and directories that are accessed within any file system and to disable them you need to edit /etc/fstab to add the options: noatime,nodiratime

/dev/hda1 /boot ext3 defaults,noatime,nodiratime 1 2

Then remount the file systems that you alter in /etc/fstab

mount -o remount /boot

You can type mount to show the file systems to verify the change

/dev/hda1 on /boot type ext3 (rw,noatime,nodiratime)

Optimize the TCP/IP stack

Some of these modifications will increase throughput and decrease cpu cycles.
Put the following commands in /etc/rc.d/rc.local or wherever rc.local is

#lets not leave sockets in fin wait for too long just close them in 30
echo 30 >/proc/sys/net/ipv4/tcp_fin_timeout
#lets not keep the connection alive longer than it would if it were live
echo 1800 >/proc/sys/net/ipv4/tcp_keepalive_time
#if you expect windows larger than 64k leave this set at 1
echo 0 >/proc/sys/net/ipv4/tcp_window_scaling
#selective acknowledgement is good only when receiving out of order packets
#and is bad for the cpu.
echo 0 >/proc/sys/net/ipv4/tcp_sack
#forward acknowledgement is also good to disable
echo 0 >/proc/sys/net/ipv4/tcp_fack
#calculation of RTT is cpu overhead
echo 0 >/proc/sys/net/ipv4/tcp_timestamps
#lowers the syn retries
echo 0 >/proc/sys/net/ipv4/tcp_syn_retries
You could also modify /etc/sysctl.cfg adding the settings then run sysctl -p
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_keepalive_time = 1800
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_sack = 0
net.ipv4.tcp_fack = 0
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_syn_retries = 2

Reduce kernel swapping

The kernel swaps to disk when it thinks it needs to which may be too often on a server that's required for performance so we reduce the swappiness.
Put the following command in /etc/rc.d/rc.local or wherever rc.local is

echo 10 > /proc/sys/vm/swappiness

You could also modify /etc/sysctl.cfg adding the settings then run

sysctl -p
vm.swappiness=10

This can be a lower value if you still notice swapping.