Thanks for correcting me guys. The iPad is a good example. However, I have had both the first iPad and now the iPad 2 and I still have yet to see any real differences in speed. I understand the benefits of dual core but in most real world cases, the benefits are not easily detected by the user.
Ah - ok.
Android is based on Linux, so that uses what we call pre-emptive multitasking (just like Mac OS X, actually).
iOS as used on the iPhone/iPad uses what we call cooperative multitasking.
iOS apps ARE monolithic, unlike Android apps, and everything they do is basically self-contained - that's why an iOS app is typically bigger than its counterpart on Android.
In that iOS system, because the app does its own heavy lifting, each individual app may need to be re-done to really get the advantage of the iPad2 dual-core - the Apple stuff, including the app management, on the iPad2 is likely taking a lot of advantage of it right off, but you're not necessarily seeing it on many apps yet.
All of this increase on the Android side is predicated on the phone maker providing the system updates under the hood to make the Linux/GNU parts capable of using the dual-core stuff. From what I'm reading, some phone makers are doing a better job of that than others.
Because iOS apps are monolithic, a given app maker can actually optimize like crazy and develop absolute-performance, no-compromise dual-core apps - do not expect this in Android.
Because Android apps share common resources, all sharing rules are in effect.
Because there isn't sharing in iOS, perfectly ok for an iOS app to hog the heck out resources, it won't affect its sibling apps. (FWIW - it's this kind of app structuring that leads to Apple's products having different memory structuring, and requirements for more memory than Android for an overall capability equivalency.)
If this helps:
iOS multitasking is basically like that used in Win95.
Android multitasking is basically like that used in Win7.
The trade-offs are: Android, more even playing field for all apps to exploit dual core; iOS, more opportunity to have more advanced specialty apps than Android.
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The type of dual-core expansion that we call SMP (symmetric multi-processing) is really not terribly advanced or the most efficient way in the world to use multiple processors - but it is highly effective as a simple architecture. I wrote about half of an SMP operating system back in the day (in assembly!) that was used for defense avionics stuff, so I know.