On Scheme

Thoughts on Scheme and teaching Scheme

Progress On Garbage Collection

Posted by Peter on April 15, 2006

I know I haven’t posted for a while, and I blame my persistent illness for making me too lazy to work seriously. I have been working on my own brand of asynchronous copying garbage collection. Unfortunately I now realize that the method I have been developing is too inefficient, and needs to be re-done.

This leaves me with basically three choices:
The write barrier method (with an asynchronous copying collector): The write barrier method has the advantage of being able to use plain pointers for access. Unfortunately it has the unattractive problem of needed to update the addresses of pointers (since it is a copying collector), which means the collector needs to be able to safely tamper with the stack and global data in the C source to update pointer values. This can be a bit complicated, and is one of the problems my own solution was struggling with.

The handle method (with an asynchronous copying collector): In this method you don’t deal with pointers but with unique handles that you can “lock” in order to get at the memory they currently reference. This method has the advantage of being relatively simple, and safe. Unfortunately it requires frequent hash lookups of handles, which means that it is effectively both a read and write barrier method, which means it is probably too slow.

Asynchronous reference counting: In this method you push the tasks of referencing, dereferencing, and checking for cycles into their own thread, which improves speed in the mutator section of the code. The big disadvantage of this method is that it is not a copying solution.

p.s. Don’t know what a write barrier / read barrier is? Go here for a quick definition.

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