NSCache is not evicting data

NSCache is a rarely used tool which actually looks quite useful. I created a simple experiment to see how it works and it looks like it does not auto-evict data in low memory situations (or I am doing something wrong!)

- (void)viewDidLoad
{
    _testCache = [[NSCache alloc] init];

    // Allocate 600 MB of zeros and save to NSCache
    NSMutableData* largeData = [[NSMutableData alloc] init];
    [largeData setLength:1024 * 1024 * 600]; 
    [_testCache setObject:largeData forKey:@"original_Data"];
}

- (IBAction)buttonWasTapped:(id)sender {

    // Allocate & save to cache 300 MB each time the button is pressed
    NSMutableData* largeData = [[NSMutableData alloc] init];
    [largeData setLength:1024 * 1024 * 300]; 
    static int count = 2;
    NSString* key = [NSString stringWithFormat:@"test_data_%d", count++];
    [_testCache setObject:largeData forKey:key];

    NSMutableData* dataRecoveredFromCache = [_testCache objectForKey:@"original_Data"];

    if (dataRecoveredFromCache) {
        NSLog(@"Original data is ok");
    } else {
        NSLog(@"Original data is missing (purged from cache)");
    }
}

So I ran the app in the simulator, and taped the button a few times however no items were evicted... The app eventually crashed:

2012-07-17 14:19:36.877 NSCacheTest[15302:f803] Data is ok
2012-07-17 14:19:37.365 NSCacheTest[15302:f803] Data is ok
2012-07-17 14:19:37.861 NSCacheTest[15302:f803] Data is ok
2012-07-17 14:19:38.341 NSCacheTest[15302:f803] Data is ok
2012-07-17 14:19:38.821 NSCacheTest[15302:f803] Data is ok
NSCacheTest(15302,0xac0942c0) malloc: *** mmap(size=393216000) failed (error code=12)
*** error: can't allocate region