30 - Kernel Tuning

30.1 What is Kernel Tunables?

Imagine kernel as the engine of your computer.

- Because the Linux OS might be installed on a different medium (laptop, database server, network router etc.).
- Each medium needs different configurations.
- These adjustable configurations are Kernel Tunables.

30.2 Where to edit Kernel Tunables?

In Linux, everything is treated as a file.

User can edit kernel tunables through:

/proc/sys/ for temporary changes.
usually is /etc/sysctl.conf for permanent changes.

30.3 Swappiness

Imagine physical RAM as your working desk. Your desk holds two distinct categories of materials:
- File Caches (Page Cache): Reference papers or books pulled from a bookshelf. If your desk gets messy, you can put these books back on the shelf without losing any written work. If you need them again, you simply reach back to the shelf.
- Application Data (Anonymous Memory): Notes you are actively writing. You cannot just discard them. To free up space on your desk, you must carefully pack them into a box, carry them to a storage room (the swap space on your disk), and retrieve them later when needed.
When RAM starts running low, the operating system must free up desk space.
- When the swappiness value is higher, the kernel is more willing to move Application Data (Anonymous Memory) out of RAM and into swap space.

30.4 Tutorial

Change Swappiness Temporary

Check current Swappiness number
- cat /proc/sys/vm/swappiness
Edit Swappiness number temporary
- sudo bash -c 'echo [Swappiness number] > /proc/sys/vm/swappiness'
- Example: sudo bash -c 'echo 40 > /proc/sys/vm/swappiness'

Change Swappiness Permanently

Direct to sysctl.d and list all configuration files
- cd /etc/sysctl.d/
Decide naming of new configuration file
- [Index]-[File Name].conf. Example: 10-swappiness.conf
- Linux reads configuration files in alphabetical order. Numbers are read before letters.
  1. 10-swappiness.conf (read first)
  2. 99-sysctl.conf (read last, overriding anything set before it, default there's no configuration inside.)
Check current Swappiness number via sysctl
- sysctl vm.swappiness
Create new configuration file in /etc/sysctl.d/
- echo "vm.swappiness=[Swappiness number]" | sudo tee -a /etc/sysctl.d/[Index]-[File Name].conf
- Example: echo "vm.swappiness=40" | sudo tee -a /etc/sysctl.d/10-swappiness.conf
Verify configuration
- cat -b [Index]-[File Name].conf
- Example: cat -b 10-swappiness.conf
Apply configuration
- sudo sysctl --system

Cleanup

Delete configuration file
- sudo rm /etc/sysctl.d/[Index]-[File Name].conf
- Example: sudo rm /etc/sysctl.d/10-swappiness.conf
Apply configuration
- sudo sysctl --system

Result

Change Swappiness Temporary

student@localhost:/etc/sysctl.d$ cat /proc/sys/vm/swappiness

student@localhost:/etc/sysctl.d$ sudo bash -c 'echo 40 > /proc/sys/vm/swappiness'

[sudo] password for student:

Change Swappiness Permanently

student@localhost:/etc/sysctl.d$ cd /etc/sysctl.d/

student@localhost:/etc/sysctl.d$ sysctl vm.swappiness

vm.swappiness = 40

student@localhost:/etc/sysctl.d$ echo "vm.swappiness=40" | sudo tee -a /etc/sysctl.d/10-swappiness.conf

vm.swappiness=40

student@localhost:/etc/sysctl.d$ cat -b 10-swappiness.conf

1 vm.swappiness=40

student@localhost:/etc/sysctl.d$ sudo sysctl --system

* Applying /usr/lib/sysctl.d/10-default-yama-scope.conf ...

* Applying /usr/lib/sysctl.d/10-map-count.conf ...

* Applying /etc/sysctl.d/10-swappiness.conf ...

* Applying /usr/lib/sysctl.d/50-coredump.conf ...

* Applying /usr/lib/sysctl.d/50-default.conf ...

* Applying /usr/lib/sysctl.d/50-libkcapi-optmem_max.conf ...

* Applying /usr/lib/sysctl.d/50-pid-max.conf ...

* Applying /usr/lib/sysctl.d/50-redhat.conf ...

* Applying /etc/sysctl.d/99-sysctl.conf ...

* Applying /etc/sysctl.conf ...

kernel.yama.ptrace_scope = 0

vm.max_map_count = 1048576

vm.swappiness = 40

kernel.core_pattern = |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %d %F

kernel.core_pipe_limit = 16

fs.suid_dumpable = 2

kernel.sysrq = 16

kernel.core_uses_pid = 1

net.ipv4.conf.default.rp_filter = 2

net.ipv4.conf.enp0s3.rp_filter = 2

net.ipv4.conf.lo.rp_filter = 2

net.ipv4.conf.default.accept_source_route = 0

net.ipv4.conf.enp0s3.accept_source_route = 0

net.ipv4.conf.lo.accept_source_route = 0

net.ipv4.conf.default.promote_secondaries = 1

net.ipv4.conf.enp0s3.promote_secondaries = 1

net.ipv4.conf.lo.promote_secondaries = 1

net.ipv4.ping_group_range = 0 2147483647

net.core.default_qdisc = fq_codel

fs.protected_hardlinks = 1

fs.protected_symlinks = 1

fs.protected_regular = 1

fs.protected_fifos = 1

net.core.optmem_max = 81920

kernel.pid_max = 4194304

kernel.kptr_restrict = 1

net.ipv4.conf.default.rp_filter = 1

net.ipv4.conf.enp0s3.rp_filter = 1

net.ipv4.conf.lo.rp_filter = 1