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Kernel Tuning
General Information
Kernel tuning using sysctl.conf
Checklist
- Any distro, but many examples are done via Red Hat based systems
Hugepages
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
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 = Very aggressively avoid swapping for as long as possible
- High risk of OOM killing from memory and I/O pressure
- 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
Page Cache
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
File Descriptors
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