Oracle UNIX/Linux Kernel Parameters Significance
Oracle’s OS specific installation instructions provide guidelines for the OS configuration, but the settings for the OS parameters can make an enormous difference in Oracle performance.
Semaphores
Semaphores act as flags for shared memory. Semaphores are either set on or off. When an Oracle process accesses the SGA in shared memory, it checks for a semaphore for that portion of memory. If it finds a semaphore set on for that portion of memory, indicating another process is already using that portion, the process will sleep and check again later. If there is no semaphore set on for that portion of memory, it sets one on and proceeds with its operation. When it is done, it switches that semaphore back to off.
Oracle specifies semaphore values for semmsl, semmns, semopm and semmni as 250, 3200, 100 and 128, respectively. These can be found in the output of the sysctl command in this same order.
# /sbin/sysctl -a | grep sem
kernel.sem = 250 32000 32 128
The values for the semaphores represent the following:
semmsl: The number of semaphores per set
semmns: The total number of semaphores available
semopm: The number of operations which can be made per semaphore call
semmni: The maximum number of shared memory segments available in the system
The Oracle recommended values is a good starting point for these parameters, but when running multiple Oracle databases on a system, semmsl and semmns may need to be increased to accommodate the additional instances.
To change this setting, edit the /etc/sysctl.conf file. If there is already a line for kernel.sem, edit the values given; otherwise, add a line in the same format as the output above. The line should look like this:
kernel.sem = 250 32000 100 128
This line can be added anywhere in the file, but it is best to keep all the changes in one place within the file. Comments can be added by starting a line with a # character.
Shared Memory Settings
The parameters shmall, shmmax, and shmmni determine how much shared memory is available for Oracle to use. These parameters are set in memory pages, not in bytes, so the usable sizes are the value multiplied by the page size, typically 4096 bytes. To confirm the page size, use the command getconf -a | grep PAGE_SIZE.
shmall: The total amount of shared memory (in pages) which can be allocated on the system
shmmax: The maximum size of a shared memory segment (in pages)
shmmni: The maximum number of shared memory segments available on the system
Given that the SGA for a database must be kept in shared memory, the value of shmmax needs to be as big as the largest SGA. The value for shmall needs to be bigger than the sum of all your databases. The value for shmmni needs to be at least as high as the number of databases that are intended to be put on the system, but in practice is generally much higher (Oracle recommends 4096.)
The quick install guide includes directions on checking these parameters. If they need to be modified, they can be set in the /etc/sysctl.conf file with entries like the following:
kernel.shmall = 2097152
kernel.shmmax = 2147483648
kernel.shmmni = 4096
Keep in mind that shmall and shmmax are set in 4 KB pages, not in bytes.
Oracle’s OS specific installation instructions provide guidelines for the OS configuration, but the settings for the OS parameters can make an enormous difference in Oracle performance.
Semaphores
Semaphores act as flags for shared memory. Semaphores are either set on or off. When an Oracle process accesses the SGA in shared memory, it checks for a semaphore for that portion of memory. If it finds a semaphore set on for that portion of memory, indicating another process is already using that portion, the process will sleep and check again later. If there is no semaphore set on for that portion of memory, it sets one on and proceeds with its operation. When it is done, it switches that semaphore back to off.
Oracle specifies semaphore values for semmsl, semmns, semopm and semmni as 250, 3200, 100 and 128, respectively. These can be found in the output of the sysctl command in this same order.
# /sbin/sysctl -a | grep sem
kernel.sem = 250 32000 32 128
The values for the semaphores represent the following:
semmsl: The number of semaphores per set
semmns: The total number of semaphores available
semopm: The number of operations which can be made per semaphore call
semmni: The maximum number of shared memory segments available in the system
The Oracle recommended values is a good starting point for these parameters, but when running multiple Oracle databases on a system, semmsl and semmns may need to be increased to accommodate the additional instances.
To change this setting, edit the /etc/sysctl.conf file. If there is already a line for kernel.sem, edit the values given; otherwise, add a line in the same format as the output above. The line should look like this:
kernel.sem = 250 32000 100 128
This line can be added anywhere in the file, but it is best to keep all the changes in one place within the file. Comments can be added by starting a line with a # character.
Shared Memory Settings
The parameters shmall, shmmax, and shmmni determine how much shared memory is available for Oracle to use. These parameters are set in memory pages, not in bytes, so the usable sizes are the value multiplied by the page size, typically 4096 bytes. To confirm the page size, use the command getconf -a | grep PAGE_SIZE.
shmall: The total amount of shared memory (in pages) which can be allocated on the system
shmmax: The maximum size of a shared memory segment (in pages)
shmmni: The maximum number of shared memory segments available on the system
Given that the SGA for a database must be kept in shared memory, the value of shmmax needs to be as big as the largest SGA. The value for shmall needs to be bigger than the sum of all your databases. The value for shmmni needs to be at least as high as the number of databases that are intended to be put on the system, but in practice is generally much higher (Oracle recommends 4096.)
The quick install guide includes directions on checking these parameters. If they need to be modified, they can be set in the /etc/sysctl.conf file with entries like the following:
kernel.shmall = 2097152
kernel.shmmax = 2147483648
kernel.shmmni = 4096
Keep in mind that shmall and shmmax are set in 4 KB pages, not in bytes.
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