SDL 2.0.5 on Gnome 3.22.2, regular draw time spike

Stan · June 1, 2017, 11:11am

Hi all,

I’m hoping someone could help me with a problem.

I’ve made a simple application, a few thousand particles flying around bouncing off the screen borders, drawn with the SDL renderer. The drawing is straightforward immediate mode: SDL_RenderClear() followed by a loop of SDL_SetRenderDrawColor()/SDL_RenderFillRect() calls, and finally SDL_RenderPresent(). I’ve noticed a strange regular frame stutter, so I added a frame time plot, and this is what I observe:

(horizontal is frames, vertical is frame time, full bar height is 16ms)

As you can see, the frame time jitters just short of 8ms, except there’s a pretty steady beat of frames longer than 12 or even 16ms. So I’ve made my built-in profiler dump stats whenever there’s a spike > 10ms:

---
2017-Jun-01 12:04:48.4670619
src/entry.d(268) [Frame]: 18 ms, 229 μs, and 6 hnsecs
  src/entry.d(299) [SDL events]: 7 μs and 3 hnsecs
  src/entry.d(385) [App update]: 18 ms, 174 μs, and 3 hnsecs
    src/app.d(200) [update before draw]: 44 μs and 7 hnsecs
    src/app.d(246) [draw with present]: 18 ms and 129 μs
      src/app.d(248) [draw only]: 3 ms, 461 μs, and 9 hnsecs
---
2017-Jun-01 12:04:53.454799
src/entry.d(268) [Frame]: 17 ms and 598 μs
  src/entry.d(299) [SDL events]: 6 μs and 3 hnsecs
  src/entry.d(385) [App update]: 17 ms, 540 μs, and 4 hnsecs
    src/app.d(200) [update before draw]: 49 μs and 7 hnsecs
    src/app.d(246) [draw with present]: 17 ms, 490 μs, and 1 hnsec
      src/app.d(248) [draw only]: 3 ms, 474 μs, and 8 hnsecs
---
2017-Jun-01 12:04:58.4590516
src/entry.d(268) [Frame]: 18 ms, 151 μs, and 4 hnsecs
  src/entry.d(299) [SDL events]: 5 μs and 8 hnsecs
  src/entry.d(385) [App update]: 18 ms, 97 μs, and 9 hnsecs
    src/app.d(200) [update before draw]: 46 μs and 2 hnsecs
    src/app.d(246) [draw with present]: 18 ms, 51 μs, and 1 hnsec
      src/app.d(248) [draw only]: 18 ms, 36 μs, and 4 hnsecs
---
2017-Jun-01 12:05:03.4552663
src/entry.d(268) [Frame]: 13 ms, 938 μs, and 3 hnsecs
  src/entry.d(299) [SDL events]: 6 μs and 8 hnsecs
  src/entry.d(385) [App update]: 13 ms, 881 μs, and 5 hnsecs
    src/app.d(200) [update before draw]: 52 μs and 4 hnsecs
    src/app.d(246) [draw with present]: 13 ms, 828 μs, and 2 hnsecs
      src/app.d(248) [draw only]: 3 ms, 471 μs, and 3 hnsecs
---
2017-Jun-01 12:05:08.4643558
src/entry.d(268) [Frame]: 19 ms, 431 μs, and 1 hnsec
  src/entry.d(299) [SDL events]: 6 μs and 1 hnsec
  src/entry.d(385) [App update]: 19 ms, 376 μs, and 6 hnsecs
    src/app.d(200) [update before draw]: 48 μs and 3 hnsecs
    src/app.d(246) [draw with present]: 19 ms, 327 μs, and 7 hnsecs
      src/app.d(248) [draw only]: 3 ms and 493 μs
---
2017-Jun-01 12:05:13.4659313
src/entry.d(268) [Frame]: 12 ms and 870 μs
  src/entry.d(299) [SDL events]: 6 μs and 3 hnsecs
  src/entry.d(385) [App update]: 12 ms, 816 μs, and 7 hnsecs
    src/app.d(200) [update before draw]: 50 μs and 2 hnsecs
    src/app.d(246) [draw with present]: 12 ms and 766 μs
      src/app.d(248) [draw only]: 3 ms, 474 μs, and 7 hnsecs
---
2017-Jun-01 12:05:18.4539787
src/entry.d(268) [Frame]: 20 ms, 782 μs, and 2 hnsecs
  src/entry.d(299) [SDL events]: 6 μs
  src/entry.d(385) [App update]: 20 ms, 726 μs, and 5 hnsecs
    src/app.d(200) [update before draw]: 53 μs and 7 hnsecs
    src/app.d(246) [draw with present]: 20 ms, 672 μs, and 2 hnsecs
      src/app.d(248) [draw only]: 3 ms, 477 μs, and 2 hnsecs

(It’s indented by scope, outer scopes include timings of inner scopes.)

It turns out that every 5 seconds a frame takes much longer either to SDL_RenderPresent(), or even draw (see entry #3, 12:04:58). What’s interesting about those 5 seconds is that the timing is consistent, i.e. in this case it spikes at (real-time seconds) 03, 08, 13, 18…, and this timing remains the same between application runs. Only after I re-login to my Gnome session may I see a different timing (i.e. 01, 06, 11…).
This persists regardless of:

windowed mode
SDL_WINDOW_FULLSCREEN_DESKTOP
SDL_WINDOW_FULLSCREEN
SDL_RENDERER_ACCELERATED
SDL_RENDERER_SOFTWARE
actually calling SDL_RenderPresent(), i.e. issuing draw calls is enough. However, if I comment out my draw() call altogether (the [draw with present] block), the spikes seem to disappear, though I’m yet to make an external plot to verify.

I’ve tested the app on a bare X session with just an xterm running, and there’s no such beat there.

The steady 5 second intervals suggest this could be _NET_WM_PING, but I am at a loss as to why the spike happens while drawing, since, if I understand correctly, the ping is taken care of by SDL_PumpEvents(), which I call in the [SDL events] block (I’m using the SDL_PumpEvents()/SDL_PeepEvents() loop).

Another thought I had was this could have something to do with compositor, but it seems unusual for it to produce such regular and strictly-timed intervals, and that it persists regardless of what window mode I choose.

I’m having a hard time understanding where this stutter is coming from or how to combat it, and thus I’ll appreciate any and all suggestions.

Thanks in advance for your time and any advice!

The specs just in case:
i7-6700K, nVidia GTX 980 Ti (driver version 381.22), Gentoo Linux (kernel 4.9.16), Gnome 3.22.2, SDL 2.0.5.

Stan · June 2, 2017, 4:06am

I’ve checked a couple of other applications (glxgears, Vulkan cube demo), and they also seem to have this periodic stutter on Gnome. So I guess this isn’t really SDL-related. Probably just another Gnome+nVidia issue.

Stan · June 2, 2017, 6:54am

…And yes, just tested on Plasma. As with bare X session, no issues there. Good bye Gnome.

daPhie79 · April 1, 2019, 3:14am

i know i’m necro-ing this post (and it’s my first post), but i’m having this extremely weird problem with SDL 2.0.9, today, on Windows 10 1809.

i’ve tried building with mingw and with vc versions of the dev kit, and both exhibits the exact same behaviour.
i tried running the app in safe mode, it does the same thing.
i am running the frametime.cpp example code that is very simple and meant to test frametime (pretty sure i’ve found it in another thread here), and it does the same thing as my project.
i tried closing all my running apps, of course, no different.
i tried software rendering, it still does it.
full screen as well.
with and without v-sync, no influence.

i don’t have this issue in any other games or software on my pc. i have an nvidia rtx 2060 with up to date drivers.

here’s what the framegraph looks like:

Daniel_Gibson · April 1, 2019, 3:43am

try passing SDL_INIT_JOYSTICK to SDL_Init(), yes, it’s a bug, it’s fixed in hg and will be in release 2.0.10

daPhie79 · April 1, 2019, 4:01am

thanks for the reply! i JUST saw a thread about this and tried it, and it indeed works. i thought i was going crazy, because back when i was working with SDL2 in my little game, i wasn’t havint this issue. i went ahead and reinstalled my old libs (2.0.5) and yep, this bug wasn’t there. i tried 2.0.8 though and it also had the bug i think, albeit a bit less severe.