What's exactly behind / inside iPhone memory management?

Question

This question is NOT about retain/release things in iphone memory management. I understand the routine quite well and there is no memory leak things in my app.

I pop up the question shown in the title, when I use Activity Instruments to monitor the overall memory activity of my app.

The instrument always shows that the amount of "real memory", which my app is using, keeps being between 21 MB and 30MB, never higher. I think this amount is relatively not big. However, sometimes, my app will give level 1 or 2 memory warning (never crash and I don't do anything for this warning in my code).

so I am wondering what's really behind iphone memory thing. I mean, does real memory the only things that triggers warnings? or there is anything else (such as virtual memory, as shown in the Instruments) inside the whole memory I should take care of?

Although my app never crash due to memory issues, this warning thing (especially level 2 warning) really annoys me and makes me fear of crashing once I release it to public in the future.

Any help?

Thanks

Answer 1

Memory warnings exist to tell your app you're nearing your limit. They are not necessarily a 'bad' thing - plenty of applications simply ignore them.

The actual implementation details about when a memory warning is triggered are not important, and in fact will vary considerably from device to device. An iPhone 4 might have 512MB of RAM to play with, but a 3GS will have half that.

That said, there are some things worth knowing about memory warnings:

• A memory warning is triggered when the overall amount of available free memory reaches a certain level
• These levels are undocumented . So you don't know what the difference is between a level 1 warning and a level 2 warning, other than the fact 2 is worse (more urgent) than 1
• Memory warnings are not application specific. A memory warning is delivered to all applications currently running and not suspended. So you may not be directly responsible for triggering one.
• When memory warnings are received the system will try and free up memory on your behalf

Again, the exact implementation details are undocumented, and you shouldn't need to care about them. A memory warning is an opportunity for you to help the system by freeing up any objects you don't need.