How to make the Ubuntu Linux 16.10 System Monitor % CPU utilization agree with ps or top for a Firefox process on quad core CPU?

Question

I would like to make the Ubuntu Linux 16.10 System Monitor % CPU utilization measurement basically agree with the ps "pcpu", % CPU utilization for an individual Firefox browser process running on a Lenovo Thinkstation desktop with quad cores.

The Ubuntu Linux 16.10 System Monitor Preferences are currently set at:

Update interval in seconds: 3.00
   Enable smooth refresh: Yes
   Alert before ending or killing processes Yes
   Divide CPU usage by CPU count : Yes

The ps output for the Firefox process is:

$ ps -eo pid,rss,c,pcpu,cputime,cmd | grep firefox
2848 726024  3 3.5 00:50:23 /usr/lib/firefox/firefox -new-window

After reading Linux : See CPU usage by a process for the last second, I learned how to view cpu usage for the last second. The top output for the Firefox process is:

$ top -b -d 1 | grep -in "firefox"
8: 2848 ratio    20   0 1980240 641516 115240 S  18.8 15.9  73:10.86 firefox

For the Firefox browser app, I get 3.5% pcpu from ps -eo pid,rss,c,pcpu,cputime,cmd while for the same Firefox browser app, the GUI Application Ubuntu System Monitor shows 5% CPU utilization. In addition, I get 15.9% CPU usage from top -b -d 1 | grep -in "firefox". When that is divided by 4 for the 4 cores used by our CPU, I get 4.0% for top's output.

How can I get the ps or top outputs and System Monitor values to agree? Should I enable the Divide CPU usage by CPU count checkbox?

Answer 1

Quick answer: no, that's not really possible.

The problem here is first with the concept of CPU "percentage". Strictly speaking, that doesn't actually mean anything. A CPU only has two states: it is either working on something (100%) or it is waiting.

So, when a CPU usage percentage of 50% is reported for a process, that doesn't mean that only 50% of the CPU is being used. It means that over a specific time period, the CPU spent 50% of its time working on the process. For example, if the refresh interval is set to 1 second and the CPU spent half a second working on process N, then the CPU% for process N will be shown as 50%.

With that in mind, we can have a look at the various ways of checking CPU%. For ps, this is measured as (from man ps):

CPU usage is currently expressed as the percentage of time spent
running during the entire lifetime of a process.  This is not ideal,
and it does not conform to the standards that ps otherwise conforms to.
CPU usage is unlikely to add up to exactly 100%.

So, the percentage as reported by ps is actually the average over the entire lifetime of the process. By contrast, top reports:

The task's share of the elapsed CPU time since the last screen
update, expressed as a percentage of total CPU time.

I assume gnome-system-monitor works in a similar way and its CPU % is also the percentage of time the CPU spent on a given process since the last refresh. My C++ knowledge is essentially non-existant but the following lines from proctable.cpp (part of the gnome-system-monitor source code) suggest that it is indeed taking the average since the last refresh:

/* FIXME: total cpu time elapsed should be calculated on an individual basis here
** should probably have a total_time_last gint in the ProcInfo structure */
glibtop_get_cpu (&cpu);
app->cpu_total_time = MAX(cpu.total - app->cpu_total_time_last, 1);
app->cpu_total_time_last = cpu.total;

So no, you can't get ps and gnome-system-monitor to agree because ps shows the percentage of time the CPU spent on a process since the process was started and gnome-system-monitor shows the percentage since the last refresh.

There is also no reason I can imagine why you would need to get those two values to agree. I think what we have here is some sort of XY problem. You might want to ask a new question describing what your final objective is, what problem you are trying to solve by making the two values agree.