Modifying kernel parameters (Linux)
For root installations, the database manager uses a formula to automatically adjust kernel parameter settings and eliminate the need for manual updates to these settings.
You must have root authority to modify kernel parameters.
Before you begin
To update kernel parameters on Red Hat and SUSE Linux®:
- Run the ipcs -l command to list the current kernel parameter settings.
Analyze the command output to determine whether
you have to change kernel settings or not by comparing the current
values with the enforced minimum settings in
the following table.
IPC kernel parameter Enforced minimum setting kernel.shmmni (SHMMNI) 256 * <size of RAM in GB> kernel.shmmax (SHMMAX) <size of RAM in bytes>1 kernel.shmall (SHMALL) 2 * <size of RAM in the default system page size>2 kernel.sem (SEMMNI) 256 * <size of RAM in GB> kernel.sem (SEMMSL) 250 kernel.sem (SEMMNS) 256 000 kernel.sem (SEMOPM) 32 kernel.msgmni (MSGMNI) 1 024 * <size of RAM in GB> kernel.msgmax (MSGMAX) 65 536 kernel.msgmnb (MSGMNB) 65 536 3
The following text is an example of the ipcs command output with comments added after
- On 32-bit Linux operating systems, the enforced minimum setting for SHMMAX is limited to 4 294 967 295 bytes.
- SHMALL limits the total amount of virtual shared memory that can be allocated on a system. Each Db2® data server efficiently manages the amount of system memory it consumes, also know as committed memory. The Db2 data server allocates more virtual memory than it commits to support memory preallocation and dynamic memory management. Memory preallocation benefits performance. Dynamic memory management is the process of growing and shrinking real memory usage within separate virtual shared memory areas. To support memory preallocation and dynamic memory management effectively, data servers frequently have to allocate more virtual shared memory on a system than the amount of physical RAM. The kernel requires this value as a number of pages.
- Load performance might benefit from a larger message queue size limit, which is specified in
bytes by MSGMNB. You can view message queue usage by running the
ipcs -qcommand. If the message queues are at capacity, or reaching capacity, during load operations, consider increasing the number of bytes the message queue size limit.
//to show what the parameter names are:
# ipcs -l ------ Shared Memory Limits -------- max number of segments = 4096 // SHMMNI max seg size (kbytes) = 32768 // SHMMAX max total shared memory (kbytes) = 8388608 // SHMALL min seg size (bytes) = 1 ------ Semaphore Limits -------- max number of arrays = 1024 // SEMMNI max semaphores per array = 250 // SEMMSL max semaphores system wide = 256000 // SEMMNS max ops per semop call = 32 // SEMOPM semaphore max value = 32767 ------ Messages: Limits -------- max queues system wide = 1024 // MSGMNI max size of message (bytes) = 65536 // MSGMAX default max size of queue (bytes) = 65536 // MSGMNB
- Beginning with the first section on Shared Memory Limits, the SHMMAX limit
is the maximum size of a shared memory segment on a Linux system. The SHMALL
limit is the maximum allocation of shared memory pages on a system.
- It is recommended to set the SHMMAX value to be equal to the amount of physical memory on your system. However, the minimum that is required on x86 systems is 268435456 (256 MB) and for 64-bit systems, it is 1073741824 (1 GB).
- The next section covers the amount of semaphores available to the operating system. The kernel parameter sem consists of four tokens, SEMMSL, SEMMNS, SEMOPM, and SEMMNI. SEMMNS is the result of SEMMSL multiplied by SEMMNI. The database manager requires that the number of arrays (SEMMNI) be increased as necessary. Typically, SEMMNI must be twice the maximum number of agents expected on the system multiplied by the number of logical partitions on the database server computer plus the number of local application connections on the database server computer.
- The third section covers messages on the system.
- The MSGMNI parameter affects the number of agents that can be started. The MSGMAX parameter affects the size of the message that can be sent in a queue, and the MSGMNB parameter affects the size of the queue.
- The MSGMAX parameter must be changed to 64 KB (that is, 65536 bytes), and the MSGMNB parameter must be increased to 65536.
Modify the kernel parameters that you have to adjust by editing the
/etc/sysctl.conf file. If this file does not exist, create it.
The following lines are examples of what must be placed into the file:
#Example for a computer with 16GB of RAM: kernel.shmmni=4096 kernel.shmmax=17179869184 kernel.shmall=8388608 #kernel.sem=<SEMMSL> <SEMMNS> <SEMOPM> <SEMMNI> kernel.sem=250 1024000 32 4096 kernel.msgmni=16384 kernel.msgmax=65536 kernel.msgmnb=65536
- Run sysctl with -p parameter
to load in sysctl settings from the default file /etc/sysctl.conf:
- Optional: Have the changes persist after every
- (SUSE Linux) Make boot.sysctl active.
- (Red Hat) The rc.sysinit initialization script reads the /etc/sysctl.conf file automatically.