linux中pid＝0的是什么进程？？？

top - 22:25:56 up 41 min, 2 users, load average: 0.23, 0.23, 0.20
Tasks: 112 total, 3 running, 108 sleeping, 0 stopped, 1 zombie
Cpu(s): 12.0%us, 1.0%sy, 0.0%ni, 87.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 1295272k total, 774012k used, 521260k free, 44508k buffers
Swap: 3229024k total, 0k used, 3229024k free, 426160k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6196 kiyo 20 0 265m 112m 29m R 8.3 8.9 3:41.96 firefox
5525 root 20 0 50432 38m 9108 S 3.3 3.1 0:54.68 Xorg
6691 kiyo 20 0 63604 15m 10m R 2.0 1.2 0:00.58 gnome-terminal
6711 kiyo 20 0 2308 1108 852 R 0.7 0.1 0:00.06 top
1 root 20 0 2844 1692 544 S 0.0 0.1 0:01.08 init
2 root 15 -5 0 0 0 S 0.0 0.0 0:00.00 kthreadd