Create a progress bar for a selected process' CPU value

[LauraAubin]

What does this do?

Part of #6

This creates a progress bar for the selected process' CPU usage. The progress bar is out of 100%.

How is this accomplished?

• Value will update every 5 seconds

This makes use of the popen command. We use this to open a process command and read the values that it returns.

How to parse the output of a Linux command.

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The following code snippet is from Chromium, an open source web browser provided by Google:

TimeTicks time = TimeTicks::Now();
int64_t task_time = TimeValToMicroseconds(task_timeval);

if (last_system_time_ == 0) {
    // First call, just set the last values.
    last_cpu_time_ = time;
    last_system_time_ = task_time;
    return 0;
}

int64_t system_time_delta = task_time - last_system_time_;
int64_t time_delta = (time - last_cpu_time_).InMicroseconds();
DCHECK_NE(0U, time_delta);

if (time_delta == 0)
    return 0;

    last_cpu_time_ = time;
    last_system_time_ = task_time;

   return static_cast<double>(system_time_delta * 100.0) / time_delta;
}

Sketch of the math:



Basically, we calculated the CPU percentage based on how long the process has been running. This is how long it has been running on the CPU, not as if the program was just opened now. We found this value by using the top command. To use an example:

Imagine that a process has been running for 30 seconds. If we check it again 1 second later and the process has been running on the CPU for 30.2 seconds, then we subtract the previous time from the current time:

30.2 - 30 = 0.2 seconds running on the CPU in 1 second.

0.2 * 100 = 20%.

This means that the process is taking up 20% of the CPU. In our case, we're checking on intervals of 5 seconds, so we subtract the final number by 5.

What did I learn?

ps doesn't actually return the current CPU percentage for a running process. While top is monitoring, ps gives you the average percentage over the life of the program. More context here.

[LauraAubin]

Need to reset these values when you click on a new process from the list