Because a thread is smaller than a process, thread creation typically uses fewer resources than process creation. Creating a process requires allocating a process control block (PCB), a rather large data structure. The PCB includes a memory map, a list of open files, and environment variables. Allocating and managing the memory map is typically the most time-consuming activity. Creating either a user thread or a kernel thread involves allocating a small data structure to hold a register set, stack, and priority.