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I'm definitely confused on this point.

I have an iPad application that shows 'Live Bytes' usage of 6-12mb in the object allocation instrument. If I pull up the memory monitor or activity monitor, the 'Real Memory' Column consistently climbs to around 80-90mb after some serious usage.

So do I have a normal memory footprint or a high one?

This answer and this answer claim you should watch 'Live Bytes' as the 'Real Memory' column shows memory blocks that have been released, but the OS hasn't yet reclaimed it.

On the other hand, this answer claims you need to pay attention to that memory monitor, as the 'Live Bytes' doesn't include things like interface elements.

What is the deal with iOS memory footprint!? :)

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Bob Spryn
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3 Answers3

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Those are simply two different metrics for measuring memory use. Which one is the "right" one depends on what question you're trying to answer.

In a nutshell, the difference between "live bytes" and "real memory" is the difference between the amount of memory currently used for stuff that your app has created and the total amount of physical memory currently attributed to your app. There are at least two reasons that those are different:

  • code: Your app's code has to be loaded into memory, of course, and the virtual memory system surely attributes that to your app even though it's not memory that your app allocated.

  • memory pools: Most allocators work by maintaining one or more pools of memory from which they can carve off pieces for individual objects or allocated memory blocks. Most implementations of malloc work that way, and I expect that the object allocator does too. These pools aren't automatically resized downward when an object is deallocated -- the memory is just marked 'free' in the pool, but the whole pool will still be attributed to your app.

There may be other ways that memory is attributed to your app without being directly allocated by your code, too.

So, what are you trying to learn about your application? If you're trying to figure out why your app crashed due to low memory, look at both "live bytes" (to see what your app is using now) and "real memory" (to see how much memory the VM system says your app is using). If you're trying to improve your app's memory performance, looking at "live bytes" or "live objects" is more likely to help, since that's the memory that you can do something about.

Caleb
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  • Well I guess first I'm trying to determine if that's even an accurate depiction of those metrics. I'm not convinced it is (with no offense intended.) There is also the 'overall bytes' column in the allocation instrument, which continues to increase over time -- which makes sense since its keeping track of the full history of allocations. But it varies wildly from the 'Real Memory' number. Also the 'Real Memory' number doesn't continue to increase forever, but seems to settle in at a much higher number than 'live bytes'. First I'm looking to understand what is what. – Bob Spryn Mar 12 '12 at 04:50
  • @BobSpryn I rewrote my answer -- hopefully it'll be more useful for you and future readers. – Caleb Mar 12 '12 at 07:03
  • Seems like a reasonable enough explanation. Thanks for the details! – Bob Spryn Mar 12 '12 at 07:27
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Seeing as how I wrote the last answer you linked to, I'll have to stand by that. If you want a total, accurate count of the current memory usage for your application, use the Memory Monitor instrument.

For reasons that I describe in this answer, Allocations hides the memory sizes of certain elements, meaning that its memory usage totals are significantly lower than your application's in-memory size. Many people find this out the hard way when they try to get their application functional on older iOS devices. On the older hardware, you had a hard memory ceiling of ~30 MB, where if you exceeded that your application was hard-killed.

Many developers (myself included) saw that we only had ~1-2 MB of live bytes in Allocations and thought we were good, until our applications started receiving memory warnings and early terminations. If you looked at Memory Monitor, you could see the true in-memory size of these applications being >20 MB, and you could see the applications being terminated the instant they crossed the 30 MB barrier in Memory Monitor.

Therefore, if you want an accurate assessment of your total application memory usage, use Memory Monitor. Allocations is great to find out the specific objects that are in memory, particularly when you use the heap shots to find things that might be accumulating (as leaks, retain cycles, or for other reasons). Just don't trust it when determining your application's actual size in memory.

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Brad Larson
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'Live bytes' means memory allocated by your code (for example by malloc), so you have access to this memory. 'Real memory' shows physical amount of memory used by your app. This include also OpenGL textures, (possibly) sounds from Open AL...

Live bytes is useful to check when you allocate and release memory in your code. Real memory is good indicator for memory optimization efficiency. And it's overhead causes 'low memory' warnings.

brigadir
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