iPhone iOS6 how to return Image IO type "resident dirty memory" to the OS? - objective-c

Question

I"m looking at the WWDC 2010 video which deals with advanced memory analysis( session 311):
At around 45:00 into the video, the performance engineer discusses what to do with "Resident Dirty memory" that your app has loaded in RAM. The engineer suggests that in response to memory warnings, your app should clear this. The engineer pastes in his custom class "flush" method into didReceiveMemoryWarning and everything is fine, but the code does not really offer any examples of HOW the memory is to be freed.
The question that I have is - how to do I flush large chunks of dirty memory used by "Image IO"? :
Here's around 74 mb of memory just sitting around dirty ( for close to 6 minutes now), waiting for someone to return it to iOS6. Nothing is happening to it. Since it does not go away on its own, I need to know how to return it to iOS.
These blocks appear to originate from code like this and (maybe other image related operations).
UIImage *screenshot = nil;
#autoreleasepool {
if ([[UIScreen mainScreen] respondsToSelector:#selector(scale)])
UIGraphicsBeginImageContextWithOptions(iPhoneRetinaIconSize, NO, [UIScreen mainScreen].scale);
else
UIGraphicsBeginImageContext(iPhoneRetinaIconSize);
[self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
screenshot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
The issue is that there's a lot of memory sitting around, loaded in RAM, unable to be returned to the operating system until the app crashes.
For webview-related dirty memory, I found that this may work:
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
[[NSURLCache sharedURLCache] removeAllCachedResponses];
[[NSURLCache sharedURLCache] setDiskCapacity:0];
[[NSURLCache sharedURLCache] setMemoryCapacity:0];
// Dispose of any resources that can be recreated.
}
Is there an equivalent for UIImage, CALayer or UIGraphics ?

Answer 1

I am far from an expert in these issues, but based on the tests I conducted with the code you provided, I'd say you just have to release the UIImages created in these blocks of code.
As far as I understand, the Image IO or GC raster data labeled chunks of memory are really just the underlying data of your images (UIImage being a UIKit wrapper on top of these). So to release the memory, release the image.
I tested this by creating a bunch of UIImages using your code, simulating a memory warning which released all of the created images:
The images speak for themselves. Releasing my UIImages (at ~00:08) removed the big GC raster data chunk from resident memory.
Because removing completely an image from your UI may not be the best solution for user experience, maybe you could try to downsize your largest images when receiving a memory warning, a poorer resolution resulting in a smaller memory footprint. Another idea (again this depends on what your images are used for) could be to dump the images to disk, and load them latter, when needed.
Hope that helps.