Thursday, September 1, 2011

What is "WCHAN" attribute at "ps -alwww" on linux ?

■ Requirement : WCHAN value in output of  "ps -alwww" 
■ OS Environment : Linux, RHEL, Centos
■ Resolution  : 

        WCHAN : Name of the kernel function in which the process is sleeping, a "-" if the process is running, or a "*" if the process is multi-threaded and ps is not displaying threads.

Name of kernel functions are :

Proccess Kernel States (wchan=wait channel in ps -l ) :

biord: block on io read.
futex: [Linux emulation] process is waiting until a futex is released (see fast userspace mutex)
getblk: get block (seems to be generated often by tar)
nanoslp: process is sleeping for some number of nanoseconds (see nanosleep(2))
pause: process is waiting for a signal (see pause(3))
pcmwrv: waiting for audio samples to be played
piperd: read(2) from a pipe
pipewr: write(2) to a pipe
physrd: reading from a HDD
runnable: process is ready to run on the CPU
running: currently on CPU
sbwait: wait for socket to return data (see uipc_sockbuf.c)
swread: read in from swap
stopev: process is stopped because of a debugging event (see sys_process.c; relates to ptrace(2))
tttout: write(2) to a tty
ttyin: read(2) from a tty
ucond: a proccess is blocked until a pthreads mutex is released
vnread: part of the pager (see vnode_pager.c)
wait: wait(2) for a child process
wdrain: write drain. On a device mounted with the async option (or soft-updates) wait until all the previous writes have been completed. (see vfs_bio.c)
zombie: a process died but its parent did not wait(2) for it.

There are other syscalls that are similar to the ones mentioned above (such as readv(2) instead of read(2), and waitpid(2) instead of wait(2)) which will end up with the same wchans.


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)
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


  1. Isnt this a copy of ?