Memory management and garbage collectors

[bearophile]

JMRyan:
> In theory, garbage collectors make memory leaks a thing of the past.

Even with a perfect GC you may leave around references that keep alive some data that you will never need to use. This is a kind of memory leak.

And the current D GC is not fully precise, this means that sometimes it sees as possible pointers even stuff that is not a pointer, so noise may keep alive memory blocks that are not really referenced.


> So take a for-
> instance.  Consider a circular linked list that we are done with.  Each 
> node has a reference to it (from another node in the list).  But, being 
> done with the list, we imagine that there are no references from outside 
> the list to any node.  Is D's garbage collector smart enough to deal 
> recognize the circle of nodes as garbage?

It's not a matter of being smart, it's a matter of how its algorithm works. In a GC based on reference counts, you keep an object alive if its count is nonzero, this means if one or more references point to it.

In a GC like the D one (well, in a basic implementation of its idea), you have some starting pointers, the roots, and starting form them you explore the graph of the references, and along the way you mark the visited objects. At the end you scan the list of all the objects and delete the ones with no mark. So the presence of a cycle is not a problem. A cycle is recycled if no external pointers point to it.

CPython GC is a reference counter, but they have added extra logic to recognize and recycle cycles too.

I leave other of your questions to other people.

Bye,
bearophile