Search code examples
iosmemorymemory-managementinstrumentsheapsort

Unreasonable Heap Growth

Hi I have truble with allocated memory, because I noticed in Instruments a lot of Heap Growth, so I designed a test app.

Test app contained two ViewControllers and each have one button. First ViewController was linked thru Segue Modal to SecondViewController (and it has NO code at all - beside auto-generated).

Second ViewController has only function 

 -(IBAction)back:(id)sender{
      [self dismissModalViewControllerAnimated:YES];
}

so I could flip throw views.

When I test it whit Instrument I noticed heap growth after I go to second view and back.

enter image description here enter image description here

How is that posible? What am I missing?

Solution

  • The size of the heap is not the app memory usage.

    When your app is alive, the kernel will have to allocate memory for you.
    Modern systems uses virtual memory. Basically, they map physical addresses to virtual addresses, that your process will access.

    This mapping is handled by the kernel, and it needs memory for it.

    If you request 1MB of memory, it will have to allocate memory to keep track of the physical pages allocated, by increasing the size of your adress space.

    If you free all memory, the kernel will usually keep the memory used for the mapping, and re-use it for the next allocations, avoiding the need to reallocate space for it.

    This is why the heap size doesn't change. But it does not indicate your application's memory usage at all.

    If using Instruments, look at the VM Tracker tool for this.