Why is memory not being released when I clear my array?

Question

This code tells me the amount of memory I'm currently consuming:

-(u_int)memoryUsage {
    u_int result = 0;
    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 ) {
        result = (info.resident_size / 1024) / 1024;
    } else {
        NSLog(@"Error with task_info(): %s", mach_error_string(kerr));
    }
    return result;
}

To test this, I wrote this:

NSLog(@"USAGE: %u mb",[self memoryUsage]);

NSMutableArray *array = [NSMutableArray array];
for (int i = 0; i < 1000000; ++i) {
   [array addObject:@"Hello!"];
}

NSLog(@"USAGE: %u mb",[self memoryUsage]);
[array removeAllObjects];

NSLog(@"USAGE: %u mb",[self memoryUsage]);

The output is:

5 mb
10 mb
10 mb

After making an array with 1000000 strings, memory was increased by 5 mb. But after cleaning it up, memory usage is not modified.

Why is that?

That function does not show how much memory your app is using. It shows how much address space the kernel has made available for a certain portion of your app. The connection between that number and object allocations is highly specious, at best. — bbum, Jun 23 '13 at 23:06

score 7 · Accepted Answer · answered Jun 23 '13 at 22:22

7

In general, freeing memory allocations from your program doesn't return that memory to the operating system. The memory allocation system keeps the freed memory around for use in later allocations.

answered Jun 23 '13 at 22:22

Carl Norum

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Why is memory not being released when I clear my array?

1 Answers1