“A more severe memory latency is a major page fault. These can occur when the system has to synchronize memory buffers with the disk, swap memory pages belonging to other processes, or undertake any other Input/Output activity to free memory. This occurs when the processor references a virtual memory address that has not had a physical page allocated to it. The reference to an empty page causes the processor to execute a fault, and instructs the kernel code to allocate a page and return, all of which increases latency dramatically.” Chapter 2. Memory allocation
As stated in the previous definition a high number of Major Page Fault can cause a serious degradation in server performance due to the added disk latency to the interrupted program execution. This can occur in case of high memory utilization or when the parameter swappiness is set to a high value.
https://en.wikipedia.org/wiki/Swappiness
vm.swappiness = 0 |
The kernel will swap only to avoid an out of memory condition, when free memory will be below vm.min_free_kbytes limit. See the “VM Sysctl documentation”. |
vm.swappiness = 1 |
Kernel version 3.5 and over, as well as kernel version 2.6.32-303 and over: Minimum amount of swapping without disabling it entirely. |
vm.swappiness = 10 |
This value is sometimes recommended to improve performance when sufficient memory exists in a system. |
vm.swappiness = 60 |
The default value. |
vm.swappiness = 100 |
The kernel will swap aggressively. |
So how to assess the impact of a Major Page Fault on an Oracle session ?