People get so touchy about the smallest of things...all the OP wanted to know if there was a way to kill all apps. No, there isn't...you have to manually wiggle and delete the apps one by one.
Agreed!
Now on to the fun...
As for this issue:
Having something running in the background does imply that it's taking up some resources, whether it's RAM, CPU, battery or a combination of those.
Yes, having something RUNNING in the background does mean it's using CPU resources, and therefore reducing battery charge. However, we're talking about SUSPENDED apps. These apps use NO CPU WHATSOEVER. The therefore use no power whatsoever. None, nada.
Right, so it will allocate memory to the app running in the background...doesn't that mean it will need to use the CPU to access that memory? Doesn't that involve having some sort of power (i.e. battery)?
No, that's not correct. The OS doesn't
allocate any more memory to a suspended app - it simply doesn't immediately mark it as available for other uses. The OS simply retains in its memory table a bit of info that says "Suspended application PullMyFinger has been allocated memory blocks xx, yy, etc." When you launch app PullMyFinger, the OS sees that it already has that memory allocated and basically un-suspends the app - that is, it's frozen execution is resumed. (In a particular way - iOS has system calls that tell apps when they've been backgrounded, suspended and resumed. An app doesn't necesarily have to pay attention to those, though.)
The key points to keep in mind are 1) a suspended app uses NO CPU cycles, and 2) memory allocated to a suspended app uses no more electricity than memory marked as free.
A tertiary point, of course, is that an app that utilizes one of Apple's multitasking APIs
will continue using CPU cycles, and will therefore shorten battery life.
Alternatively, if iOS needs additional memory for some other process, it goes through its list of suspended apps and takes their allocated memory from them. This is the same process the OS would go through to allocate free memory, it's just from a different pool, so it is no more battery draining.
With 512 MB of RAM on the phone, I'd say the OS would run more efficiently with having more free RAM than less. Is that conclusion too obvious?
You're forgetting the point that the OS can give itself (and/or other active apps) as much memory as it wants from the suspended apps. So if I have only 16MB RAM free, and 384MB RAM allocated to suspended apps, if the OS needs 128MB of RAM for whatever reason, it'll get that RAM from the suspended apps just as quickly as getting it from an unallocated RAM pool. Free RAM, sitting around doing nothing, is actually quite useless
by definition - it has future potential use, but at the moment that it's free it's serving no use at all.
Might as well use it to keep recently accessed but subsequently suspended apps in memory (but not processing!) so if they're needed in the near term they can start up much more quickly.
