Using Instruments to Work Through Low Memory Warnings

Question

I am trying to work through some low memory conditions using instruments. I can watch memory consumption in the Physical Memory Free monitor drop down to a couple of MB, even though Allocations shows that All Allocations is about 3 MB and Overall Bytes is 34 MB.

I have started to experience crashing since I moved some operations to a separate thread with an NSOperationQueue. But I wasn't using instruments before the change. Nevertheless, I'm betting I did something that I can undo to stop the crashes.

By the way, it is much more stable without instruments or the debugger connected.

I have the leaks down to almost none (maybe a hundred bytes max before a crash).

When I look at Allocations, I only see very primitive objects. And the total memory reported by it is also very low. So I cant see how my app is causing these low memory warnings.

When I look at Heap Shots from the start up, I don't see more than about 3 MB there, between the baseline and the sum of all the heap growth values.

What should I be looking at to find where the problem is? Can I isolate it to one of my view controller instances, for example? Or to one of my other instances?

What I have done: I powered the device off and back on, and this made a significant improvement. Instruments is not reporting a low memory warning. Also, I noticed that Physical Free Memory at start up was only about 7 MB before restarting, and its about 60 MB after restarting.

However, I am seeing a very regular (periodic) drop in Physical Free Memory, dropping from 43 MB to 6 MB (an then back up to 43 MB). I would like to knwo what it causing that. I don't have any timers running in this app. (I do have some performSelector:afterDelay:, but those aren't active during these tests.)

I am not using ARC.

Answer 1

The allocations and the leaks instruments only show what the objects actually take, but not what their underlaying non-object structures (the backing stores) are taking. For example, for UIImages it will show you have a few allocated bytes. This is because a UIImage object only takes those bytes, but the CGImageRef that actually contains the image data is not an object, and it is not taken into account in these instruments.

If you are not doing it already, try running the VM Tracker at the same time you run the allocations instrument. It will give you an idea of the type memory that is being allocated. For iOS the "Dirty Memory", shown by this instrument, is what normally triggers the memory warnings. Dirty memory is memory that cannot be automatically discarded by the VM system. If you see lots of CGImages, images might be your problem.

Another important concept is abandoned memory. This is memory that was allocated, it is still referenced somewhere (and as such not a leak), but not used. An example of this type of memory is a cache of some sort, which is not freeing up upon memory warning. A way to find this out is to use the heap shot analysis. Press the "Mark Heap" button of the allocations instrument, do some operation, return to the previous point in the app and press "Mark Heap" again. The second heap shot should show you what new objects have been allocated between those two moments, and might shed some light on the mystery. You could also repeat the operation simulating a memory warning to see if that behaviour changes.

Finally, I recommend you to read this article, which explains how all this works: http://liam.flookes.com/wp/2012/05/03/finding-ios-memory/.

Answer 2

The difference between physical memory from VM Tracker and allocated memory from "Allocations" is due to the major differences of how these instruments work:

• Allocations traces what your app does by installing a tap in the functions that allocate memory (malloc, NSAllocateObject, ...). This method yields very precise information about each allocation, like position in code (stack), amount, time, type. The downside is that if you don't trace every function (like vm_allocate) that somehow allocates memory, you lose this information.

• VM Tracker samples the state of the system's virtual memory in regular intervals. This is a much less precise method, as it just gives you an overall view of the current state. It operates at a low frequency (usually something like every three seconds) and you get no idea of how this state was reached.

A known culprit of invisible allocations is CoreGraphics: It uses a lot of memory when decompressing images, drawing bitmap contexts and the like. This memory is usually invisible in the Allocations instrument. So if your app handles a lot of images it is likely that you see a big difference between the amount of physical memory and the overall allocated size.

Spikes in physical memory might result from big images being decompressed, downsized and then only used in screen resolution in some view's or layer's contents. All this might happen automatically in UIKit without your code being involved.

Answer 3

I have the leaks down to almost none (maybe a hundred bytes max before a crash).

In my experience, also very small leaks are "dangerous" sign. In fact, I have never seen a leak larger than 4K, and leaks I usually see are a couple hundreds of bytes. Still, they usually "hide" behind themselves a much larger memory which is lost.

So, my first suggestion is: get rid of those leaks, even though they seem small and insignificant -- they are not.

I have started to experience crashing since I moved some operations to a separate thread with an NSOperationQueue.

Is there a chance that the operation you moved to the thread is the responsible for the pulsing peak? Could it be spawned more than once at a time?

As to the peaks, I see two ways you can go about them:

1. use the Time Profiler in Instruments and try to understand what code is executing while you see the peak rising;

2. selectively comment out portions of your code (I mean: entire parts of your app -- e.g., replace a "real" controller with a basic/empty UIViewController, etc) and see if you can identify the culprit this way.

I have never seen such a pulsating behaviour, so I assume it depends on your app or on your device. Have you tried with a different device? What happens in the simulator (do you see the peak)?

Answer 4

When I'm reading your text, I have the impression that you might have some hidden leaks. I could be wrong but, are you 100% sure that you have check all leaks?

I remember one particular project I was doing few month ago, I had the same kind of issue, and no leaks in Instruments. My memory kept growing up and I get memory warnings... I start to log on some important dealloc method. And I've seen that some objects, subviews (UIView) were "leaking". But they were not seen by Instruments because they were still attached to a main view.

Hope this was helpful.

Answer 5

In the Allocations Instrument make sure you have "Only Track Active Allocations" checked. See Image Below. I think this makes it easier to see what is actually happening.

Answer 6

1. Have you run Analyze on the project? If there's any analyze warnings, fix them first.

2. Are you using any CoreFoundation stuff? Some of the CF methods have ... strange ... interactions with the ObjC runtime and mem management (they shouldn't do, AFAICS, but I've seen some odd behaviour with the low-level image and AV manipulations where it seems like mem is being used outside the core app process - maybe the OS calls being used by Apple?)

... NB: there have also, in previous versions of iOS, been a few mem-leaks inside Apple's CF methods. IIRC the last of those was fixed in iOS 5.0.

1. (StackOVerflow's parser sucks: I typed "3" not "1") Are you doing something with a large number of / large-sized CALayer instances (or UIView's with CG* methods, e.g. a custom drawRect method in a UIView?)

... NB: I have seen the exact behaviour you describe caused by 2 and 3 above, either in the CF libraries, or in the Apple windowing system when it tries to work with image data that was originally generated inside CF libraries - or which found its way into CALayers.

It seems that Instruments DOES NOT CORRECTLY TRACK memory usage inside the CA / CG system; this area is a bit complex since Apple is shuffling back and forth between CPU and GPU ram, but it's disappointing that the mem usage seems to simply "disappear" when it clearly is still being used!

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Final thought (4. -- but SO won't let me type that) - are you using the invisible RHS of Instruments?

Apple hardcoded Instruments to always disable itself everytime you run it (so you have to keep manually opening it). This is stupid, since some of the core information only exists in the RHS bar. But I've worked with several people who didn't even know it existed :)