10. Improving DB2 Version 7.1 performance on Linux

This variable, defined in /usr/src/linux/include/asm/shmparam.h, determines the number of shared memory segment identifiers available to Linux. The default value for _SHM_ID_BITS in the kernel source is 7, which allows for a total of 2^7, or 128, shared memory segment identifiers.

On a single-processor machine, DB2 itself uses a set number (~15) of shared memory segment identifiers. On a machine with multiple processors, DB2 also requires one shared memory segment identifier per agent to take advantage of the Fast Communication Manager (FCM) feature of DB2. Because each DB2 connection requires one agent per processor, on a quad-processor machine the default value of _SHM_ID_BITS allows less than 32 simultaneous connections to a DB2 instance.

If you recompile the kernel yourself, you should be able to safely increase this value to 9.

This variable determines the maximum number of message queue identifiers. For DB2 Version 7.1 to function correctly, the minimum value is 128, but for heavier use consider setting this value to 1024 or higher.

In the 2.2 kernel source, this variable is defined in /usr/src/linux/include/linux/msg.h. The default value of 128 is acceptable for small-scale use of DB2 Version 7.1.

In the 2.4 kernel source, this variable is defined in /proc/sys/kernel/msgmni. The default value of 16 must be increased to enable DB2 Version 7.1 to function correctly. The good news is that you can change this value without recompiling the kernel or rebooting your machine. For more information, see Section 12.

This variable, defined in /usr/src/linux/include/linux/tasks.h, determines the number of simultaneous processes that Linux supports. A second variable, MAX_TASKS_PER_USER, is defined as NR_TASKS/2. Since DB2 instances are treated as users by Linux, and each connection uses a single process, the maximum number of connections per instance is capped at the value of NR_TASKS/2.

The default value for NR_TASKS in the kernel source is 512, allowing a maximum of 256 simultaneous connections to a single DB2 instance. DB2 itself requires a few connections for overhead processes. If you recompile the kernel yourself, you should increase this value to something like 2048. The stock kernels shipped with the Red Hat, SuSE, and TurboLinux distributions increase the value of NR_TASKS to 2560 or above. However, Caldera OpenLinux eDesktop 2.4 ships with a kernel in which NR_TASKS is set to the default value of 512. Caldera users should increase the value of this variable and recompile the kernel.

This variable, defined in /usr/src/linux/include/linux/sem.h, determines the number of semaphore identifiers that Linux supports. This variable is particularly important on symmetric multi-processing (SMP) machines. A unique semaphor identifier is required for each processor per agent (or connection); therefore, on a quad-processor machine, four semaphore identifiers are required per connection.

The default value for SEMMNI in the kernel source is 128, which, on a quad-processor machine, will only allow 32 simultaneous connections to a DB2 instance. If you recompile the kernel yourself, increase this value to something like 1024.