Processes in D state
If there are a lot of processes in D state in your system, then something is wrong with it.
What is D state and a process state in general? A process in Linux can be in several states: running, sleeping, etc. Running process runs on a CPU just now, sleeping process waits for its turn on CPU or for some other event. Using ps command you can obtain information about state of each process in the system: STAT column shows that:
# ps a PID TTY STAT TIME COMMAND 4975 tty1 Ss+ 0:00 /sbin/mingetty tty1 4976 tty2 Ss+ 0:00 /sbin/mingetty tty2 6202 pts/0 Ss+ 0:01 -bash 10312 pts/1 Ss 0:00 -bash 639 pts/1 R+ 0:00 ps a
First big S stands for Sleeping, R stands for running ("+" means that the process is foreground and small "s" means that the process is session leader, but it is not relevant for this article).
D state occurs then the process is in uninterruptible sleep. This state is bad, because you can't do anything with the process in D state. Fortunately, process normally remains in such state not for so long. But if you have a heap of D state processes then some logic in system is disrupt. If that is happening, the very important thing is to determine where this unlucky sleep occurs. It is easy to do with ps command with l option. WCHAN column shows the name of the kernel function where the process is sleeping:
# ps axl | awk '$10 ~ /D/' F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND vass 13478 7.2 0.0 1732 624 pts/1 D+ 17:36 0:00 find ./
|Note: In case you have problem with the D state processes, always attach this information to the report you send to developers.|
 describes the various states.
Process waiting for its turn on CPU is in R state.