Measuring Enigma's performance: A paradox?

[Andreas Lochmann]

Hi everyone,

I'm currently performing some experiments to improve Enigma's 
performance in the graphics department. For this, I measure the CPU time 
used to solve certain self-solving levels, particularly one with smooth 
scrolling, because this is our Achilles' heel right now.

I noticed that Enigma uses less CPU time when something else runs in the 
background, like a web video. This is easily explained by the CPU 
frequency stepping up. However, it seems like this effect appears even 
when I activate/deactivate the Enigma's own status bar (the one counting 
up the time and displaying the level title) and full utilisation. Let me 
explain:

I first launched several prime generators in the background, so my cores 
were 100% utilised and CPU frequency on its maximum. With status bar 
activated, Enigma uses on average 13.05 CPU-seconds for a specific task. 
When I completely deactivate the status bar, the same task takes about 
13.87 CPU-seconds -- and this although drawing the status bar itself has 
to be done within the same 13.05 CPU-seconds. (This is not a statistical 
fluke. For the average I used four runs each time, and all four runs 
WITH status bar needed consistently less time than WITHOUT status bar. 
And I did similar, slightly different experiments before, all showing 
the same paradoxical behaviour.)

How can it be that drawing the status bar still leads to less CPU time 
used? Is "video memory cache in CPU" a thing? (Remember that Enigma 
relies on software acceleration for graphics.)

Maybe someone of you knows this and can help me out?

Because: If this turns out to be real, it might make Enigma faster if it 
has to draw more on each time step. After all, we have lots of code 
trying to reduce the blit count.

Cheers
Andreas


(Sidenote:
Turns out that "killall primes" does not make the universe implode.)