Thank you for the suggestion to respond to applicationDidReceiveMemoryWarning and for pointing out the different memory behavior of debug vs. release. Ideally I would want to just trust the Swift cooperative thread pool and not come up with my own arbitrary limit for concurrent tasks. It’s difficult to imagine a clean way to handle older devices with few cores and little RAM versus much more capable newer devices, and I wonder if the Swift thread pool already has some magic built in to recognize things like memory pressure and to put the brakes on concurrency. Manually responding to memory warnings almost feels too late. But I don’t know what else I should do. To provide a little more clarity about my case, it’s rendering PDF pages to images in the background and persisting those images to disk (no immediate presentation in the user interface). Thank you for any additional insights or tips.