release does not free up memory in low-memory condidtion

Question

I am trying to follow the Apple's recommendation to handle low-memory warnings (found in Session 416 of WWDC 2009 videos) by freeing up resources used by freeing up my dataController object (referenced in my app delegate) that contains a large number of strings for read from a plist:

- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application {
   [_dataController release];
   _dataController = nil;
   NSLog([NSString stringWithFormat:@"applicationDidReceiveMemoryWarning bottom...      retain count:%i", [_dataController retainCount]]);
}      

But when I run ObjectAlloc within Instruments and simulate a Low-Memory Condition, I don't see a decrease in the memory used by my app even though I see the NSLog statements written out and the retain count is zero for the object. I do pass references to the app delegate around to some of the view controllers. But the code above releases the reference to the _dataController object (containing the plist data) so I would expect the memory to be freed.

Any help would be appreciated.

Answer 1

Are you sure that app delegate is the only owner of _dataController? -release only decreases the ref count, it won't deallocate the object unless the ref count drops to zero.

If _dataController is owned by other objects too, send a message to them to release it.

Answer 2

I am double-checking that. Thanks for the input! I did reread the memory management docs from Apple and I did put an NSLog statement in the dealloc method of my DataController and it is being called. I also put wrote out the retain count before the release and setting to nil of _dataController. The retain count is 1.

So this drives me back to why I am not seeing a significant decrease in memory usage. I think I need to understand the ObjectAlloc display in Instruments better and where the largest allocations of memory are taking place in my app. After searching for help in this area, I am frustrated with trying to determine from Instruments where this occurs. I can see that there is 3.54 MB for "All Allocations" and 608 MB for Malloc 32.00 KB. If I drill down on Malloc, I only see the Responsible Caller as being framework calls like png_malloc and inflateEnd. I am looking for calls within my code that is responsible for the Malloc but I don't see that. All this to say that I wonder if I am releasing the object or objects that will really make a significant difference in the amount of memory used for the low-memory condition. I think I need an in-depth tutorial for Instruments. The Apple help docs are okay but an example with code would be more helpful.