KernelTrap - embedded

Signaling When Out of Memory

Jeremy — Fri, 19 Oct 2007 13:29:00 +0000

The previous 2.4 Linux kernel maintainer, Marcelo Tossati, resurrected a discussion on adding support for out of memory notifications to the Linux kernel. He explained, "AIX contains the SIGDANGER signal to notify applications to free up some unused cached memory," then noting, "there have been a few discussions on implementing such an idea on Linux, but nothing concrete has been achieved." In a request for discussion, Marcelo added, "on the kernel side Rik suggested two notification points: 'about to swap' (for desktop scenarios) and 'about to OOM' (for embedded-like scenarios)." Rik van Riel explained:

"The first threshold - 'we are about to swap' - means the application frees memory that it can. Eg. free()d memory that glibc has not yet given back to the kernel, or JVM running the garbage collector, or ...

"The second threshold - 'we are out of memory' - means that the first approach has failed and the system needs to do something else. On an embedded system, I would expect some application to exit or maybe restart itself."

Out of Memory Notification

Jeremy — Fri, 28 Sep 2007 14:29:58 +0000

"Applications with dynamic input and dynamic memory usage have some issues with the current overcommitting kernel," Daniel Spång explained looking for ideas on how to best manage out of memory (OOM) situations on embedded systems with little memory and without swap. He noted, "some kind of notification to the application that the available memory is scarce and let the application free up some memory (e.g., by flushing caches), could be used to improve the situation and avoid the OOM killer." Daniel then briefly described four possible solutions, looking for other ideas:

"1) Turn off overcommit. Results in a waste of memory. 2) Nokia uses a lowmem security module to signal on predetermined thresholds. Currently available in the -omap tree. But this requires manual tuning of the thresholds. 3) Using madvise() with MADV_FREE to get the kernel to free mmaped memory, typically application caches, when the kernel needs the memory. 4) A OOM handler that the application registers with the kernel, and that the kernel executes before the OOM-killer steps in."