What do the elements of the Parallel Kernel Status screen mean?

Question

I just managed to get my multiple kernels up and humming on a new multi-core machine, which I am quite happy about.

I am slightly at a loss, though, about the meaning of the different parts of the Parallel Kernel Status monitoring screen, as the official documentation on it is quite sparse.

This screen is accessible from the Evaluation menu. Clicking on Select Columns... goes some way into clarifying some of the more mysterious elements:

In particular

• CPU gives the CPU time used so far by that kernel, and
• RAM gives the RAM it's currently using.
• The dots change color from green to red when a kernel starts calculation and back to green when it finishes.

However, I have a few specific questions:

• What does the Time column measure?
• What does the Elapsed dialog measure?
• What exactly is the speedup? What is it comparing with?
• What are the green bars? Upon mouseover they grow tooltips with percentages with no indication of what they are. For the ones above I imagine the kernels are not really being used to their full potential; is that the case? (And, if so, is there a good resource for ideas on fixing that?)

Answer 1

• What does the Time column measure?

For each kernel, it measures the CPU time used, or the difference in TimeUsed[] since the beginning of the current computation.

• What does the Elapsed dialog measure?

The elapsed (wall-clock) time, or the difference in AbsoluteTime[] since the beginning of the current computation.

• What exactly is the speedup? What is it comparing with?

It is the ratio of the CPU time used by all the slave kernels to the elapsed time. For example, using the timings shown in the screenshot, the speedup would be

cpus = {13135.2, 11318., 9735.5, 8317.07, 7077.85, 5873.04, 4576.86, 3539.84};
master = 1822.09;
abs = 13382.16;

Round[Total[cpus]/abs, .01]

(* 4.75 *)

• What are the green bars? Upon mouseover they grow tooltips with percentages with no indication of what they are.

They are load indicators, displaying the percentages of CPU time used by each kernel (whether master or slave) as a fraction of the elapsed time. Again using the timings from the screenshot, the green bars resemble the following

With[{per = Reverse[Join[{master}, cpus]]/abs}, 
  BarChart[per, ChartLabels -> Placed[Round[100 per, .1], After], 
    Axes -> False, BarOrigin -> Left, BarSpacing -> 1, ChartStyle -> Green]]