With UseG1GC, specifying -XX:ParallelGCThreads=0 is no longer allowed. Previously, with -XX:ParallelGCThreads=0, G1 would execute some tasks using serial code executed by the VM thread. The closest approximation of this behavior is to specify -XX:ParallelGCThreads=1, which causes parallel tasks to be executed by a single GC worker thread using parallel code.