Process state can be observed by ps command
root@gerrit01:/home/vagrant# ps auxf
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 2 0.0 0.0 0 0 ? S Mar19 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? S Mar19 0:01 \_ [ksoftirqd/0]
root 5 0.0 0.0 0 0 ? S< Mar19 0:00 \_ [kworker/0:0H]
Process state can be one of the following
PROCESS STATE CODES
D uninterruptible sleep (usually IO)
R running or runnable (on run queue)
S interruptible sleep (waiting for an event to complete)
T stopped, either by a job control signal or because it is being traced
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z defunct ("zombie") process, terminated but not reaped by its parent
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
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group
The most interesting is uninterruptible sleep D, which can be watched using
$while true; do date; ps auxf | awk '{if($8=="D") print $0;}'; sleep 1; done
Process in in uninterruptible sleep, means that it runs in kernel mode and can't be preempted (see preemption). The CPU is not used but can't be reused to run other tasks because kernel either is holding lock or its data structure is not protected. It is a sign of deficiency of certain parts of the kernel and should be avoided as much as possible.
References