Releasing allocations in response to low memory warning, but app still crashes

I'm building an app for viewing photos I pull down from an API. Each photo is ~1MB in size. I've set up a "slideshow" to show a photo, then move onto the next one, like a user would actually use the app. I'm testing on an iPad 1 in Instruments.

When my app receives a low memory warning, I'm dumping all photos that are currently not being displayed to the user, as well as all cached model data returned from the API. I'm seeing a significant drop in my allocations in Instruments, and a similar drop in the virtual memory use. Even with this drop in consumed memory, my app is still being killed by the OS.

The application responds to 2-3 memory warnings without crashing before being terminated.

I've recently switched to ARC, so maybe there's something I'm not understanding? I assume setting my references to nil is sufficient. Here's my code for the in-memory models dumping their image data:

[[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidReceiveMemoryWarningNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
    NSLog(@"Received memory warning; clear image for photo named \"%@\"", _name);
        _image = nil;
        _imageThumbnail = nil;
}];

Which is getting called. I also have an NSMutableDictionary which I'm calling removeAllObjects on when I received the low memory warning. I'm getting the following in the device console:

Oct  5 19:43:46 unknown configd[25] <Notice>: jetsam: kernel termination snapshot being created
Oct  5 19:43:46 unknown com.apple.launchd[1] <Notice>: (com.apple.accessoryd) Exited: Killed: 9
Oct  5 19:43:46 unknown com.apple.launchd[1] <Notice>: (com.apple.locationd) Exited: Killed: 9
Oct  5 19:43:46 unknown com.apple.launchd[1] <Notice>: (com.apple.mediaserverd) Exited: Killed: 9
Oct  5 19:43:46 unknown com.apple.launchd[1] <Notice>: (UIKitApplication:com.500px[0xd492]) Exited: Killed: 9
Oct  5 19:43:47 unknown kernel[0] <Debug>: launchd[1996] Builtin profile: accessoryd (sandbox)
Oct  5 19:43:47 unknown ReportCrash[1999] <Error>: libMobileGestalt loadBasebandMobileEquipmentInfo: CommCenter error: 1:45
Oct  5 19:43:47 unknown ReportCrash[1999] <Error>: libMobileGestalt copyInternationalMobileEquipmentIdentity: Could not get mobile equipment info dictionary
Oct  5 19:43:47 unknown ReportCrash[1999] <Error>: Saved crashreport to /Library/Logs/CrashReporter/LowMemory-2011-10-05-194347.plist using uid: 0 gid: 0, synthetic_euid: 0 egid: 0
Oct  5 19:43:47 unknown DTMobileIS[1655] <Warning>: _memoryNotification : <NSThread: 0x1cd31410>{name = (null), num = 1}
Oct  5 19:43:47 unknown DTMobileIS[1655] <Warning>: _memoryNotification : {
        OSMemoryNotificationLevel = 0;
        timestamp = "2011-10-05 23:43:47 +0000";
    }
Oct  5 19:43:47 unknown DTMobileIS[1655] <Warning>: _memoryNotification : <NSThread: 0x1cd31410>{name = (null), num = 1}
Oct  5 19:43:47 unknown DTMobileIS[1655] <Warning>: _memoryNotification : {
        OSMemoryNotificationLevel = 0;
        timestamp = "2011-10-05 23:43:47 +0000";
    }
Oct  5 19:43:48 unknown com.apple.locationd[1997] <Notice>: locationd was started after an unclean shut开发者_JAVA技巧down
Oct  5 19:43:49 unknown SpringBoard[29] <Warning>: Application '500px' exited abnormally with signal 9: Killed: 9

Does anyone have any idea why my app is being killed even though it's freeing memory?

    _image = nil;
    _imageThumbnail = nil;

This is just setting the pointers to nil, not releasing the actual objects. Release the objects, then they'll get deallocated (if their retain count hits 0).

Since you're using ARC, just set the properties to nil.

Turns out I was hanging onto references to the model classes somewhere else - they weren't getting dealloc'd, even if they released their image data during memory warnings. Eventually there were too many of them and the app crashed.