At Mental Canvas, Dave Burke and I recently resolved a memory leak bug in our iOS app using the Metal graphics libraries. Memory management on Metal has had a few good WWDC talks but has surprisingly little chatter on the web, so I thought I’d share our process and findings.

1. The issue: a transient memory spike
2. iOS / iPadOS memory limits
3. Forming hypotheses
4. Tools at hand
5. Findings
6. Conclusion
7. References

The issue: a transient memory spike

Our sketching app does a lot of rendering to intermediate, offscreen textures. In the process, we generate (and quickly discard) a lot of intermediate textures. A lot of that rendering happens in a continuous batch on a dedicated thread, without yielding execution back to the operating system.

A consequence of that behavior: our memory footprint grows rapidly until we hit the critical threshold — on a 3 GB iPad Air, about 1.8 GB — and then iOS summarily kills the app. From our app’s perspective, memory footprint is only growing gradually, as most of the memory usage is temporary intermediate rendertargets; they should be freed and available for reuse. But from the operating system’s perspective, it considers all of those temporaries to still be in use.

Which led us to the question: why? And how do we figure out which temporaries are leaking, and which Metal flag / call will allow us to return that data to the operating system?