A method of garbage collection of shared data in a system heap within a data processing system, the method comprising the steps of: (a) determining if the contents of the system heap has reached a predetermined threshold, the system heap storing shared objects which are shared amongst a plurality of processes running in a data processing system; (b) in a table identifying each of the plurality of processes with a status field, setting the status field of a process which has carried out determining step (a) to a mark initiator value; (c) triggering remaining processes to mark all objects in the system heap that each process is currently using; (d) determining that each of the remaining processes has completed the marking, at step (c); (e) releasing any unmarked objects in the system heap to an object pool.