Kernel space: Avoiding the OOM killer with mem_notify

Use too much memory, and Linux's "Dreaded OOM Killer" will, without warning, kill one of the processes on your system. A new kernel feature would allow processes to back off on memory usage and escape the OOM Killer's wrath.

By Jake Edge, LinuxWorld.com, 02/05/08

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Having applications that use up all the available memory can be a fairly painful experience. For Linux systems, it generally means a visit from the out-of-memory (OOM) killer, which will try to find processes to kill. As one would guess, coming up with rules governing which process to kill is challenging—someone, somewhere, will always be unhappy with a choice the OOM killer makes. Avoiding it altogether is the goal of the mem_notify patch.

When memory gets tight, it is quite possible that applications have memory allocated—often caches for better performance—that they could free. After all, it is generally better to lose some performance than to face the consequences of being chosen by the OOM killer. But, currently, there is no way for a process to know that the kernel is feeling memory pressure. The patch provides a way for interested programs to monitor the /dev/mem_notify file to be notified if memory starts to run low.

/dev/mem_notify is a character device that signals memory pressure by becoming readable. Interested programs can open the file and then use poll() or select() to monitor the file descriptor. Alternatively, signal-driven I/O can be enabled via the FASYNC flag and the system will deliver a SIGIO signal to the process when the device becomes readable. If it becomes readable, the process should free any memory that it can afford to give up. If enough memory is freed this way, the kernel will have no need to call in the OOM killer.

The crux of the patch is how to decide that memory pressure is occurring. mem_notify modifies shrink_active_list() to look for movement of an anonymous page to the inactive list, which is an indication that some will likely be swapped out soon. When that occurs, memory_pressure_notify() (with the pressure flag set to 1) will be called for that zone. When the number of free pages for the zone increase above a threshold—based on pages_high and lowmem_reserve for the zone—memory_pressure_notify() is called again, but with the pressure flag set to 0, effectively ending the memory pressure event for that zone.

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