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--- linux_wiki:kernel_tuning [2015/04/28 20:06]
billdozor created
+++ linux_wiki:kernel_tuning [2019/05/25 23:50] (current)
@@ Line 6: / Line 6: @@
 **Checklist**
-  * Any distro, but many examples are done via Red Hat based systems
+  * Distro(s): Any
 ----
-===== Hugepages =====
+====== Hugepages ======
 Problem: Unable to access a VM's console, and it shows out of memory errors on boot.
@@ Line 28: / Line 28: @@
 ----
-===== Swappiness =====
+==== HugePage Bug ====
-TODO: VM Swappiness content will go here.
+On some kernels, khugepaged can start running at 100% CPU utilization. This is typically seen on systems that have process/memory intensive processes.
+Other symptoms include:
+  * Normal commands hang (w, uptime, ps)
+  * Processes showing above 100% CPU (ie sometimes up to 1,300%)
+Details
+  * Known Affected kernels
+    * CentOS 6: 2.6.32-431.el6.x86_64
+  * Reference bug report: https://bugzilla.redhat.com/show_bug.cgi?id=879801
+=== Workaround Fix ===
+Disable hugepage defragmenting:
+<code bash>
+echo never > /sys/kernel/mm/redhat_transparent_hugepage/defrag
+</code>
+Add new cron entry
+<code bash>
+vim /etc/cron.d/hugepage_defrag
+# Disable kernel huge page defrag due to bugzilla bug: 879801
+@reboot root /bin/echo never > /sys/kernel/mm/transparent_hugepage/defrag
+</code>
+=== Permanent Fix ===
+Ultimately, the permanent fix is to update the kernel to a newer version.
+TODO: Will add confirmed kernel versions that this bug is fixed in.
+----
+====== 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:
+<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.
+Check current settings:
+<code bash>
+sysctl -a | grep dirty
+</code>
+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.
+  * Default value = 10
+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.
+  * Default value = 20
+----
+====== 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
+----
+====== Kernel Panic ======
+Reboot a system after a 10 seconds of kernel panic
+<code bash>
+kernel.panic = 10
+</code>
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