Kernel Tuning

General Information

Kernel tuning using sysctl.conf

Checklist

• Any distro, but many examples are done via Red Hat based systems

Problem: Unable to access a VM's console, and it shows out of memory errors on boot.

Solution: Reduce the value of “vm.nr_hugepages” to less than 1/2 of total memory in MBs.

/etc/sysctl.conf

vm.nr_hugepages = 1024

1 hugepage = 2 MBs, so the above is equal to 2GB

Explanation: Hugepages that is configured to be greater than or equal to total installed memory causes memory starvation during boot due to no memory available for normal use.

Alternative config location: Hugepages could also be configured at /boot/grub/grub.conf

Swappiness controls how likely the kernel is to move processes out of memory and onto swap disk space.

The setting is from 0 to 100.

• 0 = Avoid swapping for as long as possible
• 60 = Linux default
• 100 = Aggressively swap from memory to disk

Check the setting:

cat /proc/sys/vm/swappiness
60

To change the setting: edit sysctl.conf, add a line, save, and re-read the config:

vim /etc/sysctl.conf
vm.swappiness=1
:wq
sysctl -p

Viewing and setting the system wide file descriptors.

View system wide file descriptor information

sysctl fs.file-nr
fs.file-nr = 6144	0	809542

The three numbers returned:

• 6144 = used file descriptors
• 0 = allocated but not used
• 809542 = system max

Change System FD Limits

Edit /etc/sysctl.conf

fs.file-max = 810542

Example above increases system wide max by 1,000.

More details, including per user settings: https://owlbearconsulting.com/doku.php?id=linux_wiki:file_descriptors