Using the ps command

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The ps command is a flexible tool for identifying the programs that are running on the system and the resources they are using. It displays statistics and status information about processes on the system, such as process or thread ID, I/O activity, CPU, and memory utilization.

In this section the options and output fields that are relevant for the CPU are discussed.

Three of the possible ps output columns report CPU usage, each in a different way.

Column: Value Is:
C: Recently used CPU time for the process (in units of clock ticks).
TIME: Total CPU time used by the process since it started (in units of minutes and seconds).
%CPU: Total CPU time used by the process since it started, divided by the elapsed time since the process started. This time is a measure of the CPU dependence of the program.

CPU intensive

The following shell script:

# ps -ef | egrep -v "STIME|$LOGNAME" | sort +3 -r | head -n 15

is a tool for focusing on the highest recently used CPU-intensive user processes in the system (the header line is reinserted for clarity):

     UID   PID  PPID   C    STIME    TTY  TIME CMD
    mary 45742 54702 120 15:19:05 pts/29  0:02 ./looper
    root 52122     1  11 15:32:33 pts/31 58:39 xhogger
    root  4250     1   3 15:32:33 pts/31 26:03 xmconsole allcon
    root 38812  4250   1 15:32:34 pts/31  8:58 xmconstats 0 3 30
    root 27036  6864   1 15:18:35      -  0:00 rlogind
    root 47418 25926   0 17:04:26      -  0:00 coelogin <d29dbms:0>
    bick 37652 43538   0 16:58:40  pts/4  0:00 /bin/ksh
    bick 43538     1   0 16:58:38      -  0:07 aixterm
     luc 60062 27036   0 15:18:35 pts/18  0:00 -ksh

The column (C) indicates the recently used CPU. The process of the looping program leads the list. The C value can minimize the CPU usage of the looping process because the scheduler stops counting at 120. For a multithreaded process, this field indicates the sum of CP listed for all the threads within that process.

The following example shows a simple five-thread program with all the threads in an infinite looping program:

ps -lmo THREAD -p 8060956
    USER     PID    PPID       TID ST  CP PRI SC    WCHAN        F     TT BND COMMAND
    root 8060956 6815882         - A  720 120  0        -   200001  pts/0   - ./a.out
       -       -       -   8716483 R  120 120  0        -   400000      -   - -
       -       -       -  17105017 R  120 120  0        -   400000      -   - -
       -       -       -  24182849 R  120 120  0        -   400000      -   - -
       -       -       -  24510589 R  120 120  0        -   400000      -   - -
       -       -       -  30277829 R  120 120  0        -   400000      -   - -
       -       -       -  35913767 R  120 120  0        -   400000      -   - -

In the CP column, the value 720 indicates the sum of individual threads listed below this value, that is: (5 * 120) + (120).

CPU time ratio

The ps command, run periodically, displays the CPU time under the TIME column and the ratio of CPU time to real time under the %CPU column. Look for the processes that dominate usage. The au and v options give similar information on user processes. The options aux and vg display both user and system processes.

The following example is taken from a four-way SMP system:

# ps au
USER       PID %CPU %MEM   SZ  RSS    TTY STAT    STIME TIME COMMAND
root     19048 24.6  0.0   28   44  pts/1 A    13:53:00  2:16 /tmp/cpubound
root     19388  0.0  0.0  372  460  pts/1 A      Feb 20  0:02 -ksh
root     15348  0.0  0.0  372  460  pts/4 A      Feb 20  0:01 -ksh
root     20418  0.0  0.0  368  452  pts/3 A      Feb 20  0:01 -ksh
root     16178  0.0  0.0  292  364      0 A      Feb 19  0:00 /usr/sbin/getty
root     16780  0.0  0.0  364  392  pts/2 A      Feb 19  0:00 -ksh
root     18516  0.0  0.0  360  412  pts/0 A      Feb 20  0:00 -ksh
root     15746  0.0  0.0  212  268  pts/1 A    13:55:18  0:00 ps au

The %CPU is the percentage of CPU time that has been allocated to that process since the process was started. It is calculated as follows:

(process CPU time / process duration) * 100

Imagine two processes: The first starts and runs five seconds, but does not finish; then the second starts and runs five-seconds but does not finish. The ps command would now show 50 percent %CPU for the first process (five-seconds CPU for 10 seconds of elapsed time) and 100 percent for the second (five-seconds CPU for five seconds of elapsed time).

On an SMP, this value is divided by the number of available CPUs on the system. Looking back at the previous example, this is the reason why the %CPU value for the cpubound process never exceeds 25, because the example is run on a four-way processor system. The cpubound process uses 100 percent of one processor, but the %CPU value is divided by the number of available CPUs.

The THREAD option

The ps command can display threads and the CPUs that threads or processes are bound to by using the ps -mo THREAD command. The following is an example:

# ps -mo THREAD
USER PID   PPID  TID   ST CP PRI SC WCHAN F      TT    BND COMMAND
root 20918 20660 -     A  0  60  1  -     240001 pts/1 -   -ksh
-    -     -     20005 S  0  60  1  -     400    -     -   -

The TID column shows the thread ID, the BND column shows processes and threads bound to a processor.

It is normal to see a process named kproc (PID of 516 in operating system version 4) using CPU time. When there are no threads that can be run during a time slice, the scheduler assigns the CPU time for that time slice to this kernel process (kproc), which is known as the idle or wait kproc. SMP systems has an idle kproc for each processor.

For complete details about the ps command, see in Commands Reference.