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
• 10 = Red Hat recommended for Oracle databases
• 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=10
:wq
sysctl -p

More details about OOM: https://owlbearconsulting.com/doku.php?id=linux_wiki:oom_killer

Tuning how often cached pages in memory are flushed to disk.

Check current settings:

sysctl -a | grep dirty

Background (async) pagecache flushing:

vm.dirty_background_ratio = 3

• Explanation: Start flushing pages to disk asynchronously when pagecache is equal to 3% of memory.
• Default value = 10

Foreground (sync) pagecache flushing:

vm.dirty_ratio = 15

• Explanation: Sync flush when pagecache is 15% of total memory. This blocks ANY process from using I/O when this happens. If this process takes longer than 120 seconds, kernel panics can happen.
• Default value = 20

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