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How can I accurately graph the average CPU usage... not just load average?

Posted on 2004-04-25
Medium Priority
Last Modified: 2012-08-13
I am writing a perl script that does some forking work, and I would like to monitor the loads of the computer running it.  The load averages are typically around 3 - 6 over 1, 5, 15 minutes, but I seek more information than that.

I already watch the CPU usage on a 10 second interval, as presented by "ps -axo %cpu", but I don't think I am getting the whole picture.

When I use "top" it shows % User, % System, % Nice, % Interrupt, % Idle, but I don't think that "ps -axo %cpu" shows anything but the % User.  How can I get a more accurate picture of how much load the system is under?

I haven't been able to find a perl module that does this.

Thank you.
Question by:vancetech
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LVL 48

Expert Comment

ID: 10914597
You could try

time perlscript
LVL 20

Expert Comment

ID: 10915004
There are multiple meanings for the phrase "how much load the system is under". Since it sounds like you're trying to get a more comprehensive picture rather than a single "number", you might like a utility like 'vmstat' or combine it with 'iostat'.

Within your application, you can use the 'times' operator to get information similar to what is reported by the command-line utility 'time'.

Expert Comment

ID: 10915675
I have a suggestion for you.

Once you have chosen your metric, you could very easily write a perl script that call "top" periodically, pick up the relevant information and print it in a file. Say you chose, the "memory usage". You call top every second and pick up the "memory usage" of your script. Now print all this in a file.

After you are done with the script, you could use "gnuplot" to plot a graph of this data which gives you a broader picture or the trend of memory usage by your script.

Of course, once you have such a monitoring script in place, you could make it look for multiple parameters at a time.
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LVL 12

Accepted Solution

stefan73 earned 252 total points
ID: 10916716
Hi PerlKing,
> call "top" periodically

You can simply open the output of top, like:

open(TOP,"top -b -s10 -d15 -u 0|") or die "No top";

And then just parse the output. An empty line ends a single output.

(check top man pages for the options used)


Expert Comment

ID: 10916736
Sorry I was not clear with my wordings, but that is exactly what I meant too!
Thanks for clarifying it stefan.
LVL 18

Assisted Solution

kandura earned 248 total points
ID: 10917100
try parsing the contents of /proc/stat yourself instead of using top.
the first line will give you counters for overall cpu usage.
A sample:

cpu  3028583 89870834 2870091 9335

Then use something like:
         ($cpu[$r], $usr[$r], $nice[$r], $sys[$r], $idle[$r])= split / +/,$line;

when you take a new measurement. flip $r
         $r = 1 - $r
and convert the last two measurements to a percentage:

      $usr_100  =rate_prcnt($usr [$r],$usr [1-$r],$total[$r],$total[1-$r]);
      $nice_100 =rate_prcnt($nice[$r],$nice[1-$r],$total[$r],$total[1-$r]);
      $sys_100  =rate_prcnt($sys [$r],$sys [1-$r],$total[$r],$total[1-$r]);
      $idle_100  =rate_prcnt($idle[$r],$idle[1-$r],$total[$r],$total[1-$r]);

These four numbers give you the average cpu usage during the time between the two measurements.

sub rate_prcnt {
   my ($a1,$a2,$b1,$b2) = @_;
   if (!($a1&&$b1&&$a2&&$b2)) {
      return 0;

   return (abs($b1-$b2)==0)?0:sprintf ("%.2f",100*abs($a1-$a2)/abs($b1-$b2));

Note that I grabbed the code for this from, which is a contribution to Orca, which is a systems monitoring and graphing package using RRD.

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