PROCESS STATE CODES
Here are the different values that the s, stat and state output specifiers
(header “STAT” or “S”) will display to describe the state of a process.
D Uninterruptible sleep (usually IO)
不可中断的深度睡眠，一般由IO引起，同步IO在做读或写操作时，cpu不能做其它事情，只能等待，这时进程处于这种状态，如果程序采用异步IO，这种状态应该就很少见到了
R Running or runnable (on run queue)
进程处于运行或就绪状态
S Interruptible sleep (waiting for an event to complete)
可接收信号的睡眠状态，sleep函数可演示这种状态
T Stopped, either by a job control signal or because it is being traced.
被ctrl+z中断或被trace
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
进程已经完全死掉，不可能看见这种状态的
Z Defunct (“zombie”) process, terminated but not reaped by its parent.
进程已经终止，但是其父进程没有来及处理它，多进程写不好的话，这种状态是常见的
For BSD formats and when the stat keyword is used, additional characters may
be displayed:
< high-priority (not nice to other users)
N low-priority (nice to other users)
L has pages locked into memory (for real-time and custom IO)
s is a session leader
进程组组长
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group