I have an Ubuntu 12.04 VM running in VirtualBox that has a whole MongoDB cluster on it (4 data nodes, one config node, and a mongos router). When I look at "top", I see that each of those processes takes 5-15% CPU time each. But when I look at the total CPU % used at the top, it says 98% idle. The VM has 8 CPUs cores assigned, but even so, how does this output add up?
top - 20:53:31 up 9 min, 2 users, load average: 0.46, 0.76, 0.49
Tasks: 190 total, 1 running, 189 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.1%us, 1.9%sy, 0.0%ni, 98.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8177528k total, 3517632k used, 4659896k free, 84696k buffers
Swap: 4191228k total, 0k used, 4191228k free, 1578544k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1709 root 20 0 7692m 47m 19m S 16 0.6 0:45.53 mongod
4215 ben 20 0 83964 3340 1840 S 9 0.0 0:32.92 mongos
1704 root 20 0 6672m 47m 19m S 8 0.6 0:46.89 mongod
1693 root 20 0 6717m 46m 19m S 6 0.6 0:45.33 mongod
1695 root 20 0 6717m 46m 19m S 6 0.6 0:43.44 mongod
1696 root 20 0 230m 46m 19m S 5 0.6 0:43.47 mongod
16 + 9 + 8 + 6 + 6 + 5 = 50% of one CPU core. 50% / 8 cores = 6% of the total CPU time. And that's not counting a number of additional processes taking 1-2% each. How can it be saying 98% idle?
Best Answer
I think that it's connected with the representation of top. E.g., values in %CPU column are based on elapsed time (as written in top's man page). See also this question.