Question

I have been developing an application for iPad that is very graphically intensive. I have been able to squeeze quite a bit of performance out already on the iPad 2, but the @2x graphics for the new iPad are packing quite the punch in the memory department. Using the Activity Monitor in Instruments I am able to see the size of my application mushrooming into the 300MB-400MB range but I do not receive any low memory notifications. I am using a UINavigationController to manage my views, so getting down into the stack has a cumulative effect on memory that ends in its eventual termination. I do not experience this problem on the iPad 2, where I receive low memory notifications as expected. My app has been coded to clean up as much as possible and performs very well on that device.

I have read a number of similar questions asked:

IOS app killed for Low Memory but no Memory Warning received
iPhone app uses 150 MB memory and still no low memory warning!

None of the suggestions seem to help.

I have inserted code to force a low-memory notification to be sent:

[[UIApplication sharedApplication] _performMemoryWarning];

This does cause the inactive views to unload as expected and returns memory consumption to normal. This uses a private API and is hack, so for practical reasons is not a solution. How do I get my device to properly respond to low memory conditions and let my app know that it needs to clean up??

Answer 1

This problem was fixed in iOS 5.1.1. For those users who are on 5.1, I have implemented my own memory watchdog and sending out a notification similar to the one used when a real memory warning is issued.

I first created a category on UIApplication. This posts a notification that UIImage listens for (or whatever its backing cache is) to unload cached images.

.h

@interface UIApplication (ForceLowMemory)

+ (void) forceUnload;

@end

.m

#import "UIApplication+ForceLowMemory.h"

@implementation UIApplication (ForceLowMemory)

+ (void)forceUnload {
    [[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidReceiveMemoryWarningNotification 
                                                        object:[UIApplication sharedApplication]];
}

@end

Next, I created a memory manager watchdog like the following:

.h

@interface ELMemoryManager : NSObject

- (void)startObserving;
- (void)stopObserving;

+ (ELMemoryManager*) sharedInstance;

@end

.m

#import "ELMemoryManager.h"
#import "UIApplication+ForceLowMemory.h"
#import <mach/mach.h>

@interface ELMemoryManager()

@property (nonatomic, retain) NSTimer *timer;
uint report_memory(void);

@end

#define MAX_MEM_SIZE 475000000

@implementation ELMemoryManager
@synthesize timer = timer_;

static ELMemoryManager* manager;

#pragma mark - Singleton

+ (void) initialize {
    if (manager == nil) {
        manager = [[ELMemoryManager alloc] init];
    }
}

+ (ELMemoryManager*) sharedInstance {
    return manager;
}

#pragma mark - Instance Methods

uint report_memory(void) {
    struct task_basic_info info;
    mach_msg_type_number_t size = sizeof(info);
    kern_return_t kerr = task_info(mach_task_self(),
                                   TASK_BASIC_INFO,
                                   (task_info_t)&info,
                                   &size);
    if( kerr == KERN_SUCCESS ) {
        return info.resident_size;
    } else {
        return 0;
    }
}

- (void)startObserving {
    if (!self.timer) {
        NSTimer* timer = [NSTimer scheduledTimerWithTimeInterval:5.0f target:self selector:@selector(checkMemory:) userInfo:nil repeats:YES];
        self.timer = timer;
    }
    [self.timer fire];
}

- (void)stopObserving {
    [self.timer invalidate];
    self.timer = nil;
}

- (void)checkMemory:(id)sender {
    uint size = report_memory();
    if (size > MAX_MEM_SIZE) {
        NSLog(@"we've busted the upper limit!!!");
        [UIApplication forceUnload];
    }
}

#pragma mark - Memory Management
- (void)dealloc {
    [self.timer invalidate];
    [timer_ release];
    [super dealloc];
}

@end