Any GC that walks the graph, and isn't mostly concurrent. You will know when the graph becomes complex enough, because thereafter the GC will not let you ignore it. In my experience, as few as several hundred thousand objects can become a problem. Imagine trying to write a responsive 3D modeling app with vertices, edges, and faces all bidirectionally pointing to each other. You the programmer would think very carefully before fully traversing such a data structure (much of the point of having thorough internal references is avoiding doing much traversal!), and yet the runtime helpfully does it for you, automatically, and there's nothing you can do to stop it.

https://samsaffron.com/archive/2011/10/28/in-managed-code-we...

http://stackoverflow.com/a/14277068