iPhone OS 4 and Multitasking

iPhone OS 4 and Multitasking

Apple is adding a set of service APIs that allow background processes access to a subset of the device’s functionality. Such services have access to, at least, notifications, geolocation, deferred computation, VoIP, and media playback. Having a service API limits the ability of background applications to run errant and destroy battery, but that is not a primary concern. As I stated in my previous post, the real concern with multitasking on an embedded, swapless device is memory consumption. Battery life is a straw man. So how do services solve the memory consumption problem? Alone, as described in the event, they don’t. But iPhone OS will continue to kill applications that leave the foreground. Thus, applications will need to be refactored to provide a background component that only uses the new Service APIs. Without the risk of unbounded multitasking, memory pressure is greatly relieved.

One difference between Android and iPad & iPhone is that Android does not kill applications on task switch. The iPad & iPhone will continue to do so. Thus, in some sense, Android has a third solution to application multitasking: We allow apps to actually multitask until the system experiences memory pressure, at which point our OOM killer is able to kill applications in least-recently-used order.

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