FreeBSD 101 : Sysctl - Tweaking the kernel -
Viewing the kernel parameters:
Below is the hierarchy of the "sysctl" parameters:
To be able to view the "tunable" kernel parameters, we could use the below command:
The kernel parameters form a hierarchy called an MIB (Management Information Base).
Each level in the MIB has a sublevel and so on.
The values of the sysctl parameters could have one of the below types:
To get an idea of the role of a specific kernel parameter, we could use the below command:
The values of the sysctl parameters could have one of the below types:
- string: a text.
- integer: a whole number.
- boolean : are either 1 or 0.
- machine-specific: contain code that is only decipherable by another program.
Describing the kernel parameters:
Example:
"net.inet.tcp.cc.algorithm" means Default congestion control algorithm.
Going down the "sysctl" hierarchy:
To be able to see the "net" parameters for example, we use:
We could go a level deeper using the below command:
And one more level down:
Changing a "sysctl" variable:
We could modify a "sysctl" variable using the below:
To be able to make it persist beyond a reboot, we would need to add our parameter to the "/etc/sysctl.conf" file.
Kernel environment:
These variable are loaded at boot-time. They are passed by the bootloader to the kernel.
To view them, we use the below command:
We could modify these environment variables in the"/boot/loader.conf" file.
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