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

Basically the warnings mean that the device is running low on memory, and that, "If you could please free some memory you aren't actively using that'd be swell!". If your memory management is tight, and you have no objects that could practically be discarded, just pass the message along and ignore it.

Answer 2

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.

---

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.