iPhone OS Memory Warnings. What Do The Different Levels Mean?

Question

Regarding the black art of managing memory on iPhone OS devices: what do the different levels of memory warning mean. Level 1? Level 2? Does the dial go to 11?

Context: After an extensive memory stress testing period - including running my iPad app with the iPod music player app playing, I am inclined to ignore the random yet infrequent memory warnings I am receiving. My app never crashes. Ever. My app is leak free. And, well, the mems warnings just don't seem to matter.

Thanks,
Doug

Answer 1

Memory level warnings are logged by SpringBoard. As an app developer you don't need to care about it. Just responding to -{application}didReceiveMemoryWarning is enough.

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There are 4 levels of warnings (0 to 3). These are set from the kernel memory watcher, and can be obtained by the not-so-public function OSMemoryNotificationCurrentLevel().

typedef enum {
    OSMemoryNotificationLevelAny      = -1,
    OSMemoryNotificationLevelNormal   =  0,
    OSMemoryNotificationLevelWarning  =  1,
    OSMemoryNotificationLevelUrgent   =  2,
    OSMemoryNotificationLevelCritical =  3
} OSMemoryNotificationLevel;

How the levels are triggered is not documented. SpringBoard is configured to do the following in each memory level:

1. Warning (not-normal) — Relaunch, or delay auto relaunch of nonessential background apps e.g. Mail.
2. Urgent — Quit all background apps, e.g. Safari and iPod.
3. Critical and beyond — The kernel will take over, probably killing SpringBoard or even reboot.

Killing the active app (jetsam) is not handled by SpringBoard, but launchd.