GCMemoryManager's field `_num_gc_threads` is initialized to 1. It is later updated to whatever is correct at `MemoryService::set_universe_heap(CollectedHeap* heap)` during startup, and is *never* updated after that point. My guess is that the information is not of much use, but it should be correct.

For example, -XX:ConcGCThreads=128 gives _num_gc_threads the value of 6 in `set_universe_heap` and it will not be updated after.