IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 8. Open system servers - Linux for xSeries 273

Using the sysctl commands

The sysctl commands use the names of files in the /proc/sys directory tree as parameters.

For example, to modify the shmmax kernel parameter, you can display (using cat) and

change (using echo) the file /proc/sys/kernel/shmmax:

#cat /proc/sys/kernel/shmmax

33554432

#echo 33554430 > /proc/sys/kernel/shmmax

#cat /proc/sys/kernel/shmmax

33554430

However, using these commands can easily introduce errors, so we recommend that you use

the sysctl command because it checks the consistency of the data before it makes any

change. For example:

#sysctl kernel.shmmax

kernel.shmmax = 33554432

#sysctl -w kernel.shmmax=33554430

kernel.shmmax = 33554430

#sysctl kernel.shmmax

kernel.shmmax = 33554430

This change to the kernel will stay in effect only until the next reboot. If you want to make the

change permanent, you can edit the /etc/sysctl.conf or /etc/sysconfig/sysctl file and add the

appropriate command. In our example:

kernel.shmmax = 33554439

The next time you reboot, the parameter file will be read. You can do the same thing without

rebooting by issuing the following command:

#sysctl -p

8.2.6 Kernel parameters

The Linux kernel has many parameters that can improve performance for your installation.

Table 8-2 lists the SUSE Linux V2.4 kernel parameters that are most relevant to performance.

Table 8-2 List of the SUSE LINUX V2.4 kernel parameters that are most relevant

/proc/sys/fs/* Used to increase the number of open files the OS allows and to handle quota.

/proc/sys/kernel/* For tuning purposes, you can enable hotplug, manipulate shared memory, and specify

the maximum number of pid files and level of debug in syslog.

/proc/sys/net/* Tuning of network in general, IPV4 and IPV6.

/proc/sys/vm/* Management of cache memory and buffer.

File/directory Purpose

Parameter Description/example of use

kernel.shm-bigpages-per-file Normally used for tuning database servers. The default is 32768. To

calculate a suitable value, take the amount of System Global Area

(SGA) memory in GB and multiply by 1024. For example:

sysctl -w kernel.shm-bigpages-per-file=16384

274 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

kernel.sched_yield_scale Enables the dynamic resizing of time slices given to processes. When

enabled, the kernel reserves more time slices for busy processes and

fewer for idle processes. The parameters kernel.min-timeslice and

kernel.max-timeslice are used to specify the range of time slices that

the kernel can supply as needed. If disabled, the time slices given to

each process are the same.

The default is 0 (disabled). Applications such as ERP and Java™ can

benefit from this being enabled. For real-time applications such as

streaming audio and video, leave it disabled. For example:

sysctl -w kernel.sched_yield_scale=1

kernel.shm-use-bigpages Enables the use of bigpages (typically for databases). Default is 0

(disabled). For example:

sysctl -w kernel.shm-use-bigpages=1

net.ipv4.conf.all.hidden All interface addresses are hidden from Address Resolution Protocol

(ARP) broadcasts and will be included in the ARP response of other

addresses. Default is 0 (disabled). For example:

sysctl -w net.ipv4.conf.all.hidden=1

net.ipv4.conf.default.hidden Enables all interfaces as hidden by default. Default is 0 (disabled).

sysctl -w net.ipv4.conf.default.hidden=1

net.ipv4.conf.eth0.hidden Enables only interface eth0 as hidden. Uses the ID of your network

card. Default is 0 (disabled).

sysctl -w net.ipv4.conf.eth0.hidden=1

net.ipv4.ip_conntrack_max This setting is the number of separate connections that can be tracked.

Default is 65536.

sysctl -w net.ipv4.ip_conntrack_max=32768

net.ipv6.conf.all.mtu Default maximum for transfer unit on IPV6. Default is 1280.

sysctl -w net.ipv6.conf.all.mtu=9000

net.ipv6.conf.all.router_solicitation_delay Determines whether to wait after interface opens before sending router

solicitations. Default is 1 (the kernel should wait). For example:

sysctl -w net.ipv6.conf.all.router_solicitation_delay=0

net.ipv6.conf.all.router_solicitation_interval

Number of seconds to wait between router solicitations. Default is 4

seconds. For example:

sysctl -w net.ipv6.conf.all.router_solicitation_interval=3

net.ipv6.conf.all.router_solicitations

Number of router solicitations to send until assuming no routers are

present. Default is 3.

sysctl -w net.ipv6.conf.all.router_solicitations=2

net.ipv6.conf.all.temp_prefered_lft

Lifetime preferred in seconds for temporary addresses. Default is

86400 (1 day).

sysctl -w net.ipv6.conf.all.temp_prefered_lft=259200

Parameter Description/example of use