Added sysctl notes to the knowledge base

knowledge base/sysctl.md

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+# Sysctl
+
+Default configuration files locations:
+
+1. `/run/sysctl.d/*.conf`
+1. `/etc/sysctl.d/*.conf`
+1. `/usr/local/lib/sysctl.d/*.conf`
+1. `/usr/lib/sysctl.d/*.conf`
+1. `/lib/sysctl.d/*.conf`
+1. `/etc/sysctl.conf`
+
+## TL;DR
+
+```shell
+# see the value of a single setting
+sysctl kernel.ostype
+
+# see the values of all settings
+sysctl -a
+
+# change the current value of a setting
+sudo sysctl vm.swappiness=10
+sudo sysctl -w net.ipv4.ip_forward=1
+
+# reload settings from a single configuration file
+sudo sysctl -p
+sudo sysctl -p /etc/sysctl.d/99-swappiness.conf
+
+# reload settings from the default configuration files locations
+sudo sysctl --system
+
+# persistent configuration
+echo 'vm.swappiness=10'  | sudo tee -a /etc/sysctl.conf
+echo 'vm.swappiness = 5' | sudo tee -a /etc/sysctl.d/99-swappiness.conf
+```
+
+## Further readings
+
+- [How to reload sysctl.conf variables on Linux]
+
+[how to reload sysctl.conf variables on linux]: https://www.cyberciti.biz/faq/reload-sysctl-conf-on-linux-using-sysctl/