Precise means that on a GC cycle, only true pointers to heap allocated objects are identified as roots, which means that an unreferenced object cannot be kept alive by a value that happens to have a similar bit pattern as a heap pointer. This guarantees that unreferenced objects are eventually cleaned up, but not that this cleanup happens immediately, certainly not as part of a busy loop. (Unless the system runs out of memory, but in that case, a GC iteration is required anyway.)