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Why would Xcode show MUCH more memory use than Instruments for SceneKit app?

I'm trying to debug why our SceneKit-based app is using so much memory but Xcode and Instruments / Allocations seem to have very different values for the amount of memory being used. When I look in Xcode I see something like 600 MB but when I transfer the same running session over to Instruments / Allocations, I see a very different number for persistent bytes, like 150 MB.

Which one is correct? Why the difference? Are they measuring different things?

(Regardless of whether I Transfer an Xcode debug session or start fresh in Instruments, it doesn't seem to make much difference.)

Xcode screenshotInstruments screenshot

The reason that I care is that iOS is killing the app for excessive memory use (according to Xcode) but I can't seem to find the problem via Instruments.

I've tried turning off all GPU and Metal debug options but they don't seem to make a difference.

Xcode debug scheme optionsenter image description here

Solution

  • Which one is correct?

    My intuition is: Instruments. It uses Dtrace to (sorry) instrument your code and watch actual allocations and deallocations as they happen, at the expense of performance. The Xcode debug navigator memory graph is more of an outside view designed to give a very general sense of what’s happening. That is exactly why the latter offers you a way to switch to the former — because that (Instruments) is where you’re going to get real measurements.

    (However, let’s keep in mind that Instruments may fail to include in the total you’re seeing some virtual memory backing stores for graphics. There are plenty of WWDC videos discussing this topic in more detail. )