A ‘Stop-The-World’ garbage collector makes it almost impossible to get performance estimates right. Given the vast combination of parameters that will influence actual deployments, we can’t predict when the collector will run. A reference counted real-time collector is simpler to implement and provides room for making better runtime estimates. But the VM should still keep track of allocated objects for two reasons. One is for doing a full sweep to break cyclic references. (Memory exhaustion caused by cyclic references are extremely rare. So in actual practice this collector will almost never run and memory management could be considered ‘real-time’). A record of allocated objects is also needed for providing data required by profilers. The Limbo programming language implements this scheme.