[puzzle]

The Amiga had a feature in the Exec kernel since version 3.0 that let applications register so-called memory handlers, which the system could call to purge e.g. thumbnail caches owned by the userland process, etc. Of course the Amiga had a single address space and no memory protection or paging, so things were easier.

[egorfine]

iOS has a mechanism like that. The application receives notification on memory pressure and can release some memory in order to not being killed.