What does OOM killer kill?

While you might think this should not be a problem, the OOM Killer kills processes that the server has deemed non-essential, not the user. For example, the two applications the OOM Killer usually kills first are Apache® and MySQL® because they use a large amount of memory.

What caused OOM killer?

About the OOM Killer In certain situations, the root cause of the issue can be traced to the system running low on memory and killing an important process in order to remain operational. This over-commit model allows the kernel to allocate more memory than it actually has physically available.

How do I know if OOM killer is enabled in Linux?

If you want to enable OOM-Killer runtime, then use sysctl command to enable that. The other way to enable or disable is to write the panic_on_oom variable, you can always check the value in /proc. When you set the value to 0 that means the kernel will not panic when out of memory error occurred.

How can we prevent OOM?

To disable this behaviour:

1. Disable the OOM Killer (Put vm. oom-kill = 0 in /etc/sysctl. conf )
2. Disable memory overcommit (Put vm. overcommit_memory = 2 in /etc/sysctl. conf ) Note that this is a trinary value: 0 = “estimate if we have enough RAM”, 1 = “Always say yes”, 2 = “say no if we don’t have the memory”)

How is OOM calculated?

The score is proportional to the amount of memory used by the process. The score is 10 x percent of memory used by process . So the maximum score is 100% x 10 = 1000. In addition, if a process is running as a privileged user, it gets a slightly lower oom_score as compared to same memory usage by a normal user process.

What’s happening when the Linux kernel is starting the OOM killer How does it choose which process to kill first?

The OOM Killer has to select the best process(es) to kill. Best here refers to that process which will free up the maximum memory upon killing and is also the least important to the system.

What invoked OOM killer?

oom-killer is a process that the kernel will call when a system is over committed on memory. This will only be invoked if your system has exhausted both physical and swap memory. If this happens oom-killer will launch and start to kill processes based on a naughty score which can be found in /proc//oom_score .

What is the boot process in Linux?

In Linux, there are 6 distinct stages in the typical booting process.

1. BIOS. BIOS stands for Basic Input/Output System.
2. MBR. MBR stands for Master Boot Record, and is responsible for loading and executing the GRUB boot loader.
3. GRUB.
4. Kernel.
5. Init.
6. Runlevel programs.

How is oom calculated?

What is memory overcommit in Linux?

Something called overcommit. When a system such a Linux utilizing virtual memory allocates memory to a userspace process (via brk or mmap), there is no fixed correspondence between the virtual memory created in the process’s virtual address space and the physical memory of the machine.

How does OOM killer work in Linux?

The “OOM Killer” or “Out of Memory Killer” is a process that the Linux kernel employs when the system is critically low on memory. This maximises the use of system memory by ensuring that the memory that is allocated to processes is being actively used. Normally, this situation does not cause a problem.

What is OOM score in Linux?

The Linux kernel gives a score to each running process called oom_score which shows how likely it is to be terminated in case of low available memory. The score is proportional to the amount of memory used by the process. The score is 10 x percent of memory used by process . So the maximum score is 100% x 10 = 1000.

How does the OOM killer work in Linux?

This means that the running processes require more memory than is physically available. This situation is critical and must be resolved immediately. The solution that the Linux kernel employs is to invoke the OOM Killer to review all running processes and kill one or more of them in order to free up system memory and keep the system running.

Why does the kernel trigger the OOM killer?

When the kernel fails to retrieve a page from any memory zone in the system, it attempts to obtain free pages by other means, including memory compaction, direct reclaim, and searching again for free pages in case the OOM Killer had terminated a process during the initial search. If no free pages are available, the kernel triggers the OOM Killer.

When does the OOM killer need to be invoked?

The OOM Killer will only get invoked when the system is critically low on memory. Consequently the solution to avoiding it is to either reduce the memory requirements of the server or increase the available memory.

How to diagnose Oom errors on Linux systems?

You can use the OOM Killer logs both to identify which hosts in your system have run out of memory and to get detailed information on how much memory different processes were using at the time of the error. You can find an annotated example in the git commit that added this logging information to the Linux kernel.