The curious case of FPS jitter.

I tried to record a video of my game this morning, and it bugged me that it wasn't 100% smooth. The game did report 60fps though, so let's find out what is going on.

First order of business, is to graph the delta-time for each frame, instead of just reporting the frames-per-second. And sure enough, I would see jitter in the signal: a slow frame followed by a fast frame.

What was really puzzling, was that I could induce this jitter by pressing a key on the keyboard. Even if this key has no game functionality behind it, the frame time would jitter: slow+fast, for each and every press. And also for the auto-repeat events.

In the picture above, the three red lines are at values 1/60, 1/30 and 1/20 seconds. The green marks are the measured frame times. The jitter shows up for every key I press.

So, perf and FlameGraph to the rescue.

To my surprise, I notice that SDL_IBus_UpdateTextRect() shows up in the profile. Why is SDL updating text rectangles? I'm not doing any text related things. I just render to an OpenGL window. Notice how a single key press leads to an avalanche of computation and communication, with a call depth of 34 functions deep no less!

Frogtoss told me to look into SDL's Text Input system. My code never started a text input cycle, but to be sure, I called SDL_IsTextInputActive() to check. And sure enough: Text Input is active by default! Adding a SDL_StopTextInput() fixed the jitter.

Judging from the flamegraph, a key press when Text Input is active, is incredibly costly, as it involves computation, communication with the X server, polling, waking up stuff, and more. An avalanche of IO happens for every press. So for games, it's best to turn it off as soon as you have initialized SDL.

Executive Summary: after launching your SDL2 based game, call SDL_StopTextInput() for a smoother frame rate.

 if ( SDL_IsTextInputActive() )
  SDL_StopTextInput();

Post Sctript: I will try recording that video again. If it still isn't smooth, at least it is not because of this.

Test specs:
Ubuntu 18.04
SDL 2.0.8