Measuring load – %CPU in top is different than %us-Collection of common programming errors

11 years ago
admin
2 minutes

When I run TOP on a remote server, I see a very small system utilization (3.8%us). However, for the actual process I am seeing 29.6% of the CPU.

Since this server has 8 CPU cores, is the %CPU the utilization of only 1 CPU, while the us up top is over the 8 CPUs?

When I divide 29.6 by 8, I get 3.7 which looks to validate my guess. So questions:

  • The low utilization I am seeing is actually over 8 CPUs while the %CPU is for the CPU the process is using?
  • What happens for processes that are utilizing more than one CPU?
  • Does tomcat use more than one CPU if required? Or will tomcat crash if the load is increased but we will never be informed of high load

Top Output

    top - 12:17:40 up 9 days, 21:51,  2 users,  load average: 0.32, 0.28, 0.27
    Tasks: 201 total,   2 running, 199 sleeping,   0 stopped,   0 zombie
    Cpu(s):  3.8%us,  0.1%sy,  0.0%ni, 96.1%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
    Mem:  16229072k total,  5617572k used, 10611500k free,   156480k buffers
    Swap:  8388588k total,        0k used,  8388588k free,  2632452k cached

      PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
    32676 tomcat    20   0 12.4g 2.3g  10m S 29.6 14.6   2530:56 java

I have checked sources such as http://www.kernelhardware.org/linux-top-command/ , but not one of them makes the distinction between %us and %CPU .

Only some ambiguous information such as: %CPU - Percentage of CPU used

  1. The CPU percentages shown in top are for all CPUs combined. To see per-CPU statistics, press 1 while top is running.

    The percentages in the process list, on the other hand, are a percentage of a single CPU core, and thus may exceed 100%.