why ps aux is stuck?

I wonder on my staging server, whenever I run ps aux, it'll stuck. It outputs a list of process and stop responding. I checked there is enough RAM (1GB).

When I run the top command, it looks ok to me, but I wonder there is one zombie process. What's that? Anyone can explain?

top - 11:00:29 up  3:53,  2 users,  load average: 51.75, 50.52, 45.38
Tasks:  79 total,   1 running,  77 sleeping,   0 stopped,   1 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1747660k total,   603572k used,  1144088k free,    12644k buffers
Swap:   917496k total,        0k used,   917496k free,    97732k cached

Thanks

From serverfault Chamnap

• ps aux outputs the list of processes once and quits. It does not periodically update like top does.

  A "zombie" process is a process that has completed execution but still has an entry in the process table. This entry is still needed to allow the process that started the (now zombie) process to read its exit status.

  Chamnap : "ps aux" usually does this, but now it was stuck.

  Delan Azabani : What do you mean exactly by 'stuck'? Does the program no longer return to the shell prompt after outputting the process list?

  Chamnap : yes, it only outputs about more than 10 processes, and it stops responding.

  Chamnap : It seems there are some other processes were stucking. I cannot restart mysql or hudson service at all.
  From Delan Azabani

• If on Linux, run strace on ps to see what system calls it calls, and you may potentially see on which one it's hanging on:

  $ strace ps aux

  If you're on a different Unix-y system you'd use truss or dtruss.

  Chamnap : Thanks. I'll try next time. I just restart my staging server.

  Chamnap : it seems the problem causes from hudson when i ran strace ps aux.
  From DAM

• Wow, from your top output, your load is ridiculously high. Unless you have a 50+ CPU system, that is way too high. The CPUs though are not doing anything:

  Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
  

  This sounds like you have a hardware problem. What does the output of dmesg tell you?

  Chamnap : Thanks. I'll try next time. I just restart my staging server.
  From vmfarms