Thursday, 21 July 2011

Linux Tuning Parameters


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


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


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


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_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

Choose the Right File System
Use 'ext3' file system in Linux. 
- It is enhanced version of ext2
- With journaling capability - high level of data integrity (in event of unclean shutdown)
- It does not need to check disks on unclean shutdown and reboot (time consuming)
- Faster write - ext3 journaling optimizes hard drive head motion

# mke2fs -j -b 2048 -i 4096 /dev/sda
mke2fs 1.32 (09-Nov-2002)
/dev/sda is entire device, not just one partition!
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=2048 (log=1)
Fragment size=2048 (log=1)
13107200 inodes, 26214400 blocks
1310720 blocks (5.00%) reserved for the super user
First data block=0
1600 block groups
16384 blocks per group, 16384 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
16384, 49152, 81920, 114688, 147456, 409600, 442368, 802816, 1327104,
2048000, 3981312, 5619712, 10240000, 11943936

Writing inode tables: done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 28 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.

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.

Disable Unnecessary Daemons (They Take up Memory and CPU)
There are daemons (background services) running on every server that are probably not needed. Disabling these daemons frees memory, decreases startup time, and decreases the number of processes that the CPU has to handle. A side benefit to this is increased security of the server because fewer daemons mean fewer exploitable processes.

Some example Linux daemons running by default (and should be disabled). Use command:
#/sbin/chkconfig --levels 2345 sendmail off 
#/sbin/chkconfig sendmail off
Advanced power management daemon
Automatically mounts file systems on demand (i.e.: mounts a CD-ROM automatically)
Common UNIX� Printing System
HP OfficeJet support
ISDN modem support
Used in support of exporting NFS shares
Used for file locking with NFS
PCMCIA support on a server
Red Hat Network update service for checking for updates and security errata
Mail Transport Agent
Font server for X Windows
Shutdown GUI

Normally, there is no need for a GUI on a Linux server. All administration tasks can be achieved by the command line, redirecting the X display or through a Web browser interface.  Modify the 'inittab' file to set boot level as 3:
To set the initial runlevel (3 instead of 5) of a machine at boot, 
modify the /etc/inittab file as shown:

Linux recommended parameter values
Parameter nameDescription
Recommended value
SHMMNIThe maximum number of segments
256 * <size of RAM in GB>
SHMMAXThe maximum size of a shared memory segment (bytes)
<size of RAM in bytes>
SHMMAX - Set this value to automatically start the server on Red Hat Enterprise Linux 6The maximum size of a shared memory segment (bytes)268435456
SHMALLThe maximum allocation of shared memory pages (bytes)
2 * <size of RAM in bytes> (setting is in 4K pages)
SEMMNIThe maximum number of arrays
256 * <size of RAM in GB>
SEMMSLThe maximum semaphores per array
SEMMNSThe maximum semaphores per system
SEMOPMThe maximum operations per semaphore call
MSGMNIThe maximum number of system-wide message queues
1024 * <size of RAM in GB>
MSGMAXThe maximum size of messages (bytes)
MSGMNBThe default maximum size of queue (bytes)

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