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Linux Tuning Parameters

Using all the resources available to you? Many default settings in Linux suck. Check out these tunings that can give you lots of computing juice…

Kernel

To successfully run enterprise applications, such as a database server, on your Linux distribution, you may be required to update some of the default kernel parameter settings. For example, the 2.4.x series kernel message queue parameter msgmni has a default value (for example, shared memory, or shmmax is only 33,554,432 bytes on Red Hat Linux by default) that allows only a limited number of simultaneous connections to a database. Here are some recommended values (by the IBM DB2 Support Web site) for database servers to run optimally:

kernel.shmmax=268435456 for 32-bit
kernel.shmmax=1073741824 for 64-bit
kernel.msgmni=1024
fs.file-max=8192
kernel.sem="250 32000 32 1024"

Shared Memory
To view current settings, run command:

# more /proc/sys/kernel/shmmax

To set it to a new value for this running session, which takes effect immediately, run command:

# echo 268435456 > /proc/sys/kernel/shmmax

To set it to a new value permanently (so it survives reboots), modify the sysctl.conf file:

...
kernel.shmmax = 268435456
...

Semaphores
To view current settings, run command:

# more /proc/sys/kernel/sem
250 32000 32 1024

To set it to a new value for this running session, which takes effect immediately, run command:

# echo 500 512000 64 2048 > /proc/sys/kernel/sem

Parameters meaning:
SEMMSL – semaphores per ID
SEMMNS – (SEMMNI*SEMMSL) max semaphores in system
SEMOPM – max operations per semop call
SEMMNI – max semaphore identifiers

ulimits
To view current settings, run command:

# ulimit -a

To set it to a new value for this running session, which takes effect immediately, run command:

# ulimit -n 8800
# ulimit -n -1 #for unlimited; recommended if server isn't shared

Alternatively, if you want the changes to survive reboot, do the following:
– Exit all shell sessions for the user you want to change limits on.
– As root, edit the file /etc/security/limits.conf and add these two lines toward the end:

	user1        soft    nofile          16000
	user1        hard    nofile          20000

** the two lines above changes the max number of file handles – nofile – to new settings.
– Save the file.
– Login as the user1 again. The new changes will be in effect.

Message queues
To view current settings, run command:

# more /proc/sys/kernel/msgmni
# more /proc/sys/kernel/msgmax

To set it to a new value for this running session, which takes effect immediately, run command:

# echo 2048 > /proc/sys/kernel/msgmni
# echo 64000 > /proc/sys/kernel/msgmax

Network

Gigabit-based network interfaces have many performance-related parameters inside of their device driver such as CPU affinity. Also, the TCP protocol can be tuned to increase network throughput for connection-hungry applications.

Tune TCP
To view current TCP settings, run command:

# sysctl net.ipv4.tcp_keepalive_time
net.ipv4.tcp_keepalive_time = 7200 # 2 hours

where net.ipv4.tcp_keepalive_time is a TCP tuning parameter.

To set a TCP parameter to a value, run command:

# sysctl -w net.ipv4.tcp_keepalive_time=1800
A list of recommended TCP parameters, values, and their meanings:
Tuning Parameter 	Tuning Value 			Description of impact 
------------------------------------------------------------------------------
net.ipv4.tcp_tw_reuse 
net.ipv4.tcp_tw_recycle 	1 			Reuse sockets in the time-wait state 
---
net.core.wmem_max 		8388608 		Increase the maximum write buffer queue size 
---
net.core.rmem_max 		8388608 		Increase the maximum read buffer queue size 
---
net.ipv4.tcp_rmem 		4096 87380 8388608 	Set the minimum, initial, and maximum sizes for the 
							read buffer. Note that this maximum should be less 
							than or equal to the value set in net.core.rmem_max. 
---
net.ipv4.tcp_wmem 		4096 87380 8388608 	Set the minimum, initial, and maximum sizes for the 
							write buffer. Note that this maximum should be less 
							than or equal to the value set in net.core.wmem_max. 
---
timeout_timewait 		echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout Determines the time that must elapse before 
							TCP/IP can release a closed connection and reuse its resources. 
							This interval between closure and release is known as the TIME_WAIT 
							state or twice the maximum segment lifetime (2MSL) state. 
							During this time, reopening the connection to the client and 
							server cost less than establishing a new connection. By reducing the 
							value of this entry, TCP/IP can release closed connections faster, providing 
							more resources for new connections. Adjust this parameter if the running application 
							requires rapid release, the creation of new connections, and a low throughput 
							due to many connections sitting in the TIME_WAIT state.

Disk I/O

Use ‘noatime’ File System Mount Option

Use ‘noatime’ option in the file system boot-up configuration file ‘fstab’. Edit the fstab file under /etc. This option works the best if external storage is used, for example, SAN:

# more /etc/fstab
LABEL=/                 /                       ext3    defaults        1 1
none                    /dev/pts                devpts  gid=5,mode=620  0 0
none                    /proc                   proc    defaults        0 0
none                    /dev/shm                tmpfs   defaults        0 0
/dev/sdc2               swap                    swap    defaults        0 0
/dev/cdrom              /mnt/cdrom              udf,iso9660 noauto,owner,kudzu,ro 0 0
/dev/fd0                /mnt/floppy             auto    noauto,owner,kudzu 0 0
/dev/sda                /database               ext3    defaults,noatime 1 2
/dev/sdb                /logs                   ext3    defaults,noatime 1 2
/dev/sdc                /multimediafiles        ext3    defaults,noatime 1 2

Tune the Elevator Algorithm in Linux Kernel for Disk I/O

After choosing the file system, there are several kernel and mounting options that can affect it. One such kernel setting is the elevator algorithm. Tuning the elevator algorithm helps the system balance the need for low latency with the need to collect enough data to efficiently organize batches of read and write requests to the disk. The elevator algorithm can be adjusted with the following command:

# elvtune -r 1024 -w 2048 /dev/sda
/dev/sda elevator ID 2 
read_latency: 1024 
write_latency: 2048 
max_bomb_segments: 6

The parameters are: read latency (-r), write latency (-w) and the device affected.
Red Hat recommends using a read latency half the size of the write latency (as shown).
As usual, to make this setting permanent, add the ‘elvtune’ command to the /etc/rc.d/rc.local script.

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