I should also point out that Bacon-Rajan-style concurrent reference counting with cycle detection[0] can in some cases dramatically out-perform tracing GCs as f

I should also point out that Bacon-Rajan-style concurrent reference counting with cycle detection[0] can in some cases dramatically out-perform tracing GCs as found in Oracle's JVM, .NET, and most JavaScript engines. The cost of tracing GC is proportional to the rate of object creation times the number of live objects. The cost of reference counting is proportional to the rate of reference destruction (with batching optimizations to reduce the cost of a small number of objects having lots of references created and destroyed.) If your programs create a lot of long-lived objects that rarely have their reference counts change, then cycle-detecting reference counting can be a big win. [0] https://researcher.watson.ibm.com/researcher/files/us-bacon/Bacon01Concurrent.pdf https://researcher.watson.ibm.com/researcher/files/us-bacon/...