CrashyCartman

OpenGL Low framerate during first seconds

Recommended Posts

CrashyCartman    605

Hi gamedev !

I'm facing a really strange performance issue occurring during the first seconds of my app.
When launching the app, three scenarios:

  • Some erratic frames at the very beginning, then 70+fps.
  • Some erratic frames at the very beginning, then ~12fps, then, after a few seconds, BAM, 70+fps.
  • Some erratic frames at the very beginning, then stuck at ~12fps.


Camera is static, so is the scene.

I used nsight to try figure out what's happening, especially in case 2 where I can see what are the differences.
When it's rendering at ~12fps, some glDrawRangeElementsBaseVertex/glDrawArrays calls takes a lot of time, some SwapBuffers calls too, and after switching to "cruise speed", those calls are taking a more moderate amount of time:

Here is a capture of a nsight session timeline:


timeline.thumb.jpg.446f9edf22d15f37e50044c67d519cbe.jpg

1->erratic framerate at the beginning

2->KICKSTART !

3->stable and high framerate

Memory usage is the same in all cases.
CPU Usage is a bit higher when stuck at ~12fps, but nothing remarkable.

My scene is really simple (only a few cubes), but I use a lot of post process (SSAO, SSR). Reducing the framebuffer size to a smaller one doesn't change anything (although top speed is faster).
I also tried to disable all my post process, issue is still there but case 1 is the most common.

it is really similar to what's described in this post, although I'm not using vsync at all (double checked code, and also forced vsync off with the nvidia control panel) nor SDL.
https://www.opengl.org/discussion_bo...-slow-on-start

I also tried to put some glFlush()/glFinish() here and there in my code, without success.

A few specifications:
OS: Windows 7
CPU: Core I7 6700
GPU: Nvidia 560Ti
Drivers: 385.41
I'm stuck with this issue for months now, it's driving me crazysick.gif

Thanks for any help/advice/question/test to make

Edited by CrashyCartman

Share this post


Link to post
Share on other sites
TheChubu    9454

Throwing a dart in the dark here...

When you launch your app, everything is cold. Driver has to setup who knows what for your application in the background, compile your shaders, pass stuff to the GPU, etc. Once all of this stabilizes (say, you've used all your shaders at least once for actual drawing), everything runs as expected.

Drivers do a lot of lazy loading, specially for OpenGL where there are tons of paths to do the same thing and they might require slightly different things. You might ask the driver to compile a shader, and bind a bunch of state, but the driver might only actually do that work when you actually draw with any of them set, until that happens, the driver can just "pretend" it did what you told it to do. It saves up work if you say, change the raster state 3 or 4 times until you actually draw with it. Driver makers prefer to assume you're stupid rather than taking the backlash of XYZ big game that doesn't works as fast as the competition.

Also if you're using any kind of "managed language" (something that runs on top of the CLR or a JVM for instance), those have to do a lot of setup work too, usually concurrently with your application (say, HotSpot, a JVM, can run your code in interpreted mode, while it is compiling it at the same time, then later replace it with the compiled code when it finishes).

This is also true of native languages to a lesser extent for different reasons, big exe/dll/so/dylib/etc might take a while to load and the important bits might take a while until  they hit the CPU caches.

Share this post


Link to post
Share on other sites
CrashyCartman    605

Thanks for the answer:)

I know driver must be busy after resource loading, and could accept having some long frames at start, but in this case, 33% of time I've got a game completely unplayable.

I also made tests removing all of my threads, just to see if it was a concurrency issue, and unfortunately it didn't change anything.

Everything is in c++, no managed code.

I may add that if I run the exe by doing "Nsight/Start Graphics Debugging", everything is ok, of course I have some hiccups at start but then, game runs smooth every time.

 

Share this post


Link to post
Share on other sites
TheChubu    9454

Ah right, I misunderstood the problem. Just in case, hows GPU memory usage? Most 560 have 1Gb, you might be threading in a line where sometimes you're running out of GPU memory and sometimes you don't, for whatever reason.

You can check it in a nice graph with something like MSI Afterburner or GPU-Z.

Check system memory too just in case.

Edited by TheChubu

Share this post


Link to post
Share on other sites
CrashyCartman    605

Alright here are the results of my quick benchmarks made using GPU-Z to track gpu memory.

During loading and the first erratic frames, gpu memory is full (1000MB).

After that, it falls down to around 800 MB, framerate is stable but low.

And if after a few seconds framerate increases, GPU memory goes to around 950MB, if not, it stays at 800MB/low fps forever.

So there is a real correlation between both memory usage & performance.

I tried changing the format of the few FLOAT16 RGBA  FBOs I use to RGBA8 in order to reduce size, but it didn't change anything to memory usage, which is strange.

System memory shows the same relationship between memory & framerate.

systemmemory.jpg.33a612cab7207080ff0e870e6bae7005.jpg

1: first run, app runs fine at 70 fps

2: second run, app is stuck at 12 fps.

3: third run, same as 1

As you can see, the difference is small (100 MB) but noticeable.

Thanks for trying to help me :)

Share this post


Link to post
Share on other sites
TheChubu    9454

Are you running in fullscreen at 4k or something? Say, on my Windows 7 desktop I get around 100Mb of GPU memory used constantly, the rest is available. So if you have anything else running that hogs VRAM, close it (games, videos, chrome, whatever). See how much your application is actually taking. Try running it in a 800x600 window.

Share this post


Link to post
Share on other sites
CrashyCartman    605

Running in 720p, and yes, I find this crazy to have so much vram used, should have tracked that before. I use only a few small textures for the moment, no reason to have so much memory used.

Thanks for your help, I'll check what's wront and post the solution here :)

Share this post


Link to post
Share on other sites
CrashyCartman    605

Alright, here are the various causes :)

  1. I had some extra large (8k) textures I used to generate small sdf font textures, but never removed them after sdf generation.
  2. My shadow maps are packed into a single big GL_R32F atlas, and the depth buffer of this FBO is as large as the atlas, so it's a huge amount of memory. Correct me if I'm wrong, but I think if I use GL_DEPTH_COMPONENT32F as a format I'll reduce memory used by 2.

Thanks again for pointing me in the right direction.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

    • By pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
       
      Thanks
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
       
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      http://www.informit.com/articles/article.aspx?p=2120983
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
      Thanks!
  • Popular Now