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Measuring Enigma's performance: A paradox?

From: Andreas Lochmann
Subject: Measuring Enigma's performance: A paradox?
Date: Sat, 10 Apr 2021 18:23:55 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1

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.


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

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