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.

To update kernel parameters on Red Hat and SUSE Linux:

1. Run the ipcs -l command to list the current kernel parameter settings.
2. 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
   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, specified in bytes by MSGMNB. You can view message queue usage can by running the ipcs -q command. If the message queues are at capacity, or reaching capacity, during load operations, consider increasing the number of bytes the message queue size limit.

   The following text is an example of the ipcs command output with comments added after // 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 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 should 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 should be changed to 64 KB (that is, 65536 bytes), and the MSGMNB parameter should be increased to 65536.
3. 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 should 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

4. Run sysctl with -p parameter to load in sysctl settings from the default file /etc/sysctl.conf:

      sysctl -p

5. Optional: Have the changes persist after every reboot:
   • (SUSE Linux) Make boot.sysctl active.
   • (Red Hat) The rc.sysinit initialization script reads the /etc/sysctl.conf file automatically.