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--- linux_wiki:kernel_tuning [2015/04/28 20:39]
billdozor
+++ linux_wiki:kernel_tuning [2019/05/25 23:50]
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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
-<code bash>
-vm.nr_hugepages = 1024
-</code>
-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
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-===== 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 = 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:
-<code bash>
-cat /proc/sys/vm/swappiness
-</code>
-To change the setting: edit sysctl.conf, add a line, save, and re-read the config:
-<code bash>
-vim /etc/sysctl.conf
-vm.swappiness=10
-:wq
-sysctl -p
-</code>
-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.
-Background (async) pagecache flushing:
-<code bash>
-vm.dirty_background_ratio = 3
-</code>
-Explanation: Start flushing pages to disk asynchronously when pagecache is equal to 3% of memory.
-Foreground (sync) pagecache flushing:
-<code bash>
-vm.dirty_ratio = 15
-</code>
-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.
-----
-===== File Descriptors =====
-Viewing and setting the system wide file descriptors.
-**View system wide file descriptor information**
-<code bash>
-sysctl fs.file-nr
-fs.file-nr = 6144	0	809542
-</code>
-The three numbers returned:
-  * 6144 = used file descriptors
-  * 0 = allocated but not used
-  * 809542 = system max
-**Change System FD Limits**
-Edit /etc/sysctl.conf
-<code bash>
-fs.file-max = 810542
-</code>
-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
-----