Get CPU usage in shell script?

Question

I'm running some JMeter tests against a Java process to determine how responsive a web application is under load (500+ users). JMeter will give the response time for each web request, and I've written a script to ping the Tomcat Manager every X seconds which will get me the current size of the JVM heap.

I'd like to collect stats on the server of the % of CPU being used by Tomcat. I tried to do it in a shell script using ps like this:

PS_RESULTS=`ps -o pcpu,pmem,nlwp -p $PID`

...running the command every X seconds and appending the results to a text file. (for anyone wondering, pmem = % mem usage and nlwp is number of threads)

However I've found that this gives a different definition of "% of CPU Utilization" than I'd like - according to the manpages for ps, pcpu is defined as:

cpu utilization of the process in "##.#" format. It is the CPU time used divided by the time the process has been running (cputime/realtime ratio), expressed as a percentage.

In other words, pcpu gives me the % CPU utilization for the process for the lifetime of the process.

Since I want to take a sample every X seconds, I'd like to be collecting the CPU utilization of the process at the current time only - similar to what top would give me (CPU utilization of the process since the last update).

How can I collect this from within a shell script?

Answer 1

Use top -b (and other switches if you want different outputs). It will just dump to stdout instead of jumping into a curses window.

Answer 2

Off the top of my head, I'd use the /proc filesystem view of the system state - Look at man 5 proc to see the format of the entry for /proc/PID/stat, which contains total CPU usage information, and use /proc/stat to get global system information. To obtain "current time" usage, you probably really mean "CPU used in the last N seconds"; take two samples a short distance apart to see the current rate of CPU consumption. You can then munge these values into something useful. Really though, this is probably more a Perl/Ruby/Python job than a pure shell script.

You might be able to get the rough data you're after with /proc/PID/status, which gives a Sleep average for the process. Pretty coarse data though.

Answer 3

The most useful tool I've found for monitoring a server while performing a test such as JMeter on it is dstat. It not only gives you a range of stats from the server, it outputs to csv for easy import into a spreadsheet and lets you extend the tool with modules written in Python.

Answer 4

@Tim - dstat is awesome! Thank you.