Tuning Kernel Parameters

Many of the tunable performance items can be configured directly by the kernel. The command sysctl is used to view current kernel settings and adjust them. For example, to display all available parameters (in a sorted list), use:

sudo sysctl -a | sort | more

Note: There are a few tunable parameters that can only be accessed by root. Without sudo, you can still view most of the kernel parameters.

Each of the kernel parameters are in a field = value format. For example, the parameter kernel.threads-max = 16379 sets the maximum number of concurrent processes to 16,379. This is smaller than the maximum number of unique PIDs (65,536). Lowering the number of PIDs can improve performance on systems with slow CPUs or little RAM since it reduces the number of simultaneous tasks. On high-performance computers with dual processors, this value can be large. As an example, my 350 MHz iMac is set to 2,048, my dual-processor 200 MHz PC is set to 1024, and my 2.8 GHz dual processor PC is set to 16,379.

Tip: The kernel configures the default number of threads based on the available resources. Installing the same Ubuntu version on different hardware may set a different value. If you need an identical system (for testing, critical deployment, or sensitive compatibility), be sure to explicitly set this value.

There are two ways to adjust the kernel parameters. First, you can do it on the command line. For example, sudo sysctl -w kernel.threads-max=16000. This change takes effect immediately but is not permanent; if you reboot, this change will be lost. The other way to make a kernel change is to add the parameter to the /etc/sysctl.conf file. Adding the line kernel.threads-max=16000 will make the change take effect on the next reboot. Usually when tuning, you first use sysctl –w. If you like the change, then you can add it to /etc/sysctl.conf. Using sysctl –w first allows you to test modifications. In the event that everything breaks, you can always reboot to recover before committing the changes to /etc/sysctl.conf