CPU Utilization high for sleeping processes
I have a process that appears to be deadlocked:
# strace -p 5075
Process 5075 attached - interrupt to quit
futex(0x419cf9d0, FUTEX_WAIT, 5095, NULL
It is sitting on the "futex" system call, and seems to be indefinitely waiting on a lock. The process is shown to be consuming a large amount of CPU when "top" is run:
# top -b -n 1
top - 23:13:18 up 113 days, 4:19, 1 user, load average: 1.69, 1.74, 1.72
Tasks: 269 total, 1 running, 268 sleeping, 0 stopped, 0 zombie
Cpu(s): 8.1%us, 0.1%sy, 0.0%ni, 91.8%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 12165696k total, 3810476k used, 8355220k free, 29440k buffers
Swap: 8388600k total, 43312k used, 8345288k free, 879988k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5075 omdb 18 0 2373m 1.7g 26m S 199.7 14.9 102804:11 java
The process is also shown to be in a "S" - Sleep state, which makes sense if it's waiting on some resource. However, I don't understand why CPU utilization would be close to 200% if the process is in the sleep state. Why does top report such high CPU utilization on a sleeping process? Shouldn't its CPU utilization be zero?
Answer
There is no correlation between CPU usage as reported by top and process state. The man page says (emphasis mine):
%CPU -- CPU usage
The task's share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time.
So, your process indeed used a huge amount of processor time since the last screen update. It is sleeping, yes, but that's because the currently running process is top itself (which makes sense, since it's currently updating the screen).