Home > Apple > Size matters: an exploration of virtual memory on iOS

Size matters: an exploration of virtual memory on iOS

Apple 1 Comment

I ran into an odd out-of-memory problem the other day when attempting to debug an iOS app on device. The app consistently crashed shortly after launch, preventing me from investigating the bug. To unblock myself, I learned a lot about the iOS virtual memory implementation and journaled my findings (including a fix!) here.

Nothing to add – exactly what it says on the tin.

About The Author

Thom Holwerda

Follow me on Mastodon @[email protected]

One Response

  1. 2023-04-13 6:00 pm
    Alfman verbose=1

    I didn’t realize is that IOS doesn’t support using virtual memory for application swapping…

    Unlike macOS (and most operating systems with virtual memory), iOS does not page out dirty memory to disk. (The term dirty refers to memory that has been written to by the process.) iOS only discards read-only pages of a memory-mapped file, which can be paged in again as necessary. The main executable, shared libraries, and some types of resources are typically memory-mapped into the process address space and are therefore candidates for eviction from RAM if the system is low on physical memory.

    I’m guessing this has to do with apple’s aggressive stance against background applications on IOS. Any time swapping would be beneficial the application gets killed instead, nullifying the need to swap.

    https://forums.macrumors.com/threads/anyone-notice-ios-15-is-more-murderous-with-background-apps.2315154/
    https://discussions.apple.com/thread/253608967
    https://discussions.apple.com/thread/251321971

    Anyway, the author’s problem had more to do with IOS address space limits than available memory. He solved his application’s allocation error by granting his application “Extended Virtual Addressing” permission, which overrides the default IOS virtual memory limits (ie 7.3GB usable on iphone xs/13).